diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..316a072
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,29 @@
+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+*.so
+*.egg-info/
+.dist-info/
+.eggs/
+
+# Virtual environments
+.venv/
+venv/
+env/
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+
+# Testing
+.pytest_cache/
+.coverage
+htmlcov/
+
+# Distribution
+dist/
+build/
+*.tar.gz
diff --git a/README.md b/README.md
index 2bec551..2ef65a1 100644
--- a/README.md
+++ b/README.md
@@ -1,26 +1,84 @@
-# Transmutate
+# Transmutate (Python GUI)
 
-Transmutate is a pure KDE desktop tool for converting images, videos, and audio files. It uses **KDialog** for all interactive prompts and leverages **FFmpeg** and **ImageMagick** under the hood to perform the actual transcoding.
+A modern Python/CustomTkinter rewrite of the original `transmutate.sh` bash script.
+Convert images, videos, and audio using ffmpeg + ImageMagick with a modern, cross-platform GUI.
+
+## Requirements
+
+- Python 3.8+
+- customtkinter (`pip install customtkinter`)
+- Pillow (for animated image detection)
+- ffmpeg (must be in PATH)
+- magick / ImageMagick (optional, provides better WebP/GIF handling)
 
 ## Usage
 
 ```bash
-./transmutate.sh <path/to/image|video|audio>
+cd rewrite
+python transmutate.py <path/to/file>
 ```
 
+The GUI opens automatically and detects the media type of the file.
+
 ## Features
 
-- Select output format, quality, and audio/subtitle tracks via KDE dialogs
-- Convert **images** to PNG, JPG, WebP, AVIF, GIF, or video formats (MP4, MKV, WebM, AVI, MOV)
-- Convert **videos** to MP4, MKV, WebM, AVI, MOV, GIF, or animated WebP
-- Convert **audio** to MP3, FLAC, WAV, OGG, M4A, or AAC
-- Animated GIFs and WebPs can be converted to full video formats
-- GIF/WebP animation options (looping)
-- If multiple audio or subtitle tracks exist, choose which to include
-- Handles filename conflicts by prompting to overwrite or auto-renaming
+### Supported Conversions
 
-## Requirements
+| Source → Target | Options |
+|-----------------|---------|
+| Image → Image   | PNG, JPG, WebP, AVIF, GIF (animated), MP4, MKV, WebM, AVI, MOV (animated) |
+| Image → Video   | MP4, MKV, WebM, AVI, MOV |
+| Image → Audio   | MP3, FLAC, WAV, OGG, M4A, AAC |
+| Video → Image   | PNG, JPG, WebP, AVIF |
+| Video → Video   | MP4, MKV, WebM, AVI, MOV, GIF, WebP (animated) |
+| Video → Audio   | MP3, FLAC, WAV, OGG, M4A, AAC |
+| Audio → Image   | PNG, JPG, WebP, AVIF |
+| Audio → Video   | MP4, MKV, WebM, AVI, MOV |
+| Audio → Audio   | MP3, FLAC, WAV, OGG, M4A, AAC |
 
-- **KDE / KDialog**
-- **FFmpeg**
-- **ImageMagick** (`magick` CLI) — optional, used for better GIF and WebP conversion
+### Per-Conversion Options
+
+- **Quality slider** (0-100, or 0-51 for video CRF) — smooth CustomTkinter slider
+- **Audio quality** slider (for video output)
+- **Audio track selection** (when multiple tracks exist)
+- **Subtitle track selection** (when multiple tracks exist)
+- **Loop toggle** (for GIF/WebP output)
+- **Output path** — same directory as source, auto-renamed on conflict
+
+### Conversion Modes
+
+- **Image → Image**: Direct ffmpeg conversion with quality control
+- **Image → Video**: Animated images are re-encoded with CRF
+- **Image → Audio**: Single frame extracted to audio
+- **Video → Image**: Single frame extraction
+- **Video → Video**: Full conversion with CRF and audio quality control
+- **Video → GIF**: Palettegen-based GIF with quality-dependent dithering
+- **Video → WebP**: Animated WebP output
+- **Audio → Audio**: Format conversion with quality control
+- **Audio → Image**: First frame extracted as image
+- **Audio → Video**: Single frame rendered as video
+
+## Architecture
+
+```
+rewrite/
+├── transmutate.py          # Entry point (CLI → GUI)
+├── transmutate_app/        # Package
+│   ├── __init__.py
+│   ├── gui.py              # CustomTkinter GUI application
+│   └── engine/             # Conversion engine
+│       ├── __init__.py
+│       └── ffmpeg_engine.py  # Command building + execution
+```
+
+## Compared to Bash Version
+
+| Feature | Bash (kdialog) | Python (CustomTkinter) |
+|---------|---------------|------------------|
+| Dependencies | bash, ffmpeg, kdialog | python3, customtkinter, ffmpeg |
+| GUI | Multiple sequential popups | Single unified CustomTkinter window |
+| Stream selection | kdialog checklist | CustomTkinter checkboxes |
+| Quality input | kdialog inputbox | CustomTkinter slider |
+| File overwrite | kdialog yes/no | CustomTkinter dialog + auto-rename |
+| Animated detection | ffprobe | Pillow + ffprobe |
+| Code organization | Single monolithic script | Modular package |
diff --git a/legacy/README.md b/legacy/README.md
new file mode 100644
index 0000000..2bec551
--- /dev/null
+++ b/legacy/README.md
@@ -0,0 +1,26 @@
+# Transmutate
+
+Transmutate is a pure KDE desktop tool for converting images, videos, and audio files. It uses **KDialog** for all interactive prompts and leverages **FFmpeg** and **ImageMagick** under the hood to perform the actual transcoding.
+
+## Usage
+
+```bash
+./transmutate.sh <path/to/image|video|audio>
+```
+
+## Features
+
+- Select output format, quality, and audio/subtitle tracks via KDE dialogs
+- Convert **images** to PNG, JPG, WebP, AVIF, GIF, or video formats (MP4, MKV, WebM, AVI, MOV)
+- Convert **videos** to MP4, MKV, WebM, AVI, MOV, GIF, or animated WebP
+- Convert **audio** to MP3, FLAC, WAV, OGG, M4A, or AAC
+- Animated GIFs and WebPs can be converted to full video formats
+- GIF/WebP animation options (looping)
+- If multiple audio or subtitle tracks exist, choose which to include
+- Handles filename conflicts by prompting to overwrite or auto-renaming
+
+## Requirements
+
+- **KDE / KDialog**
+- **FFmpeg**
+- **ImageMagick** (`magick` CLI) — optional, used for better GIF and WebP conversion
diff --git a/transmutate.sh b/legacy/transmutate.sh
similarity index 81%
rename from transmutate.sh
rename to legacy/transmutate.sh
index 09e099b..90a6ce4 100755
--- a/transmutate.sh
+++ b/legacy/transmutate.sh
@@ -20,10 +20,6 @@ die() {
     exit 1
 }
 
-quit() {
-    exit 1
-}
-
 has_magick() {
     command -v magick >/dev/null 2>&1
 }
@@ -36,13 +32,14 @@ ask_quality() {
     local media="$1"
     local default_val="${2:-85}"
     local prompt min_val max_val
+    local input
 
     case "$media" in
         image)
             prompt="Quality (0-100):"
             min_val=0;  max_val=100 ;;
         video)
-            # CRF scale: 0=best/lossless, 51=worst. Lower = better quality.
+            # CRF scale flipped: 51=best/lossless, 0=worst. Higher = better quality.
             prompt="Quality (51-0) CRF:"
             min_val=0;  max_val=51 ;;
         audio)
@@ -55,13 +52,16 @@ ask_quality() {
     esac
 
     while true; do
-        local input
         input=$(kdialog --title "Transmutate — $media" --inputbox "$prompt" "$default_val") \
-            || quit
+            || exit
 
-        # Empty input → accept default
+        # Empty input → accept default (invert for video)
         if [[ -z "$input" ]]; then
-            echo "$default_val"
+            if [[ "$media" == "video" ]]; then
+                echo $(( 51 - default_val ))
+            else
+                echo "$default_val"
+            fi
             return 0
         fi
 
@@ -73,7 +73,13 @@ ask_quality() {
             continue
         fi
 
-        echo "$input"
+        # ── Invert CRF for video: user 51 (best) → ffmpeg 0, user 0 (worst) → ffmpeg 51 ──
+        if [[ "$media" == "video" ]]; then
+            local crf_inverted=$(( 51 - input ))
+            echo "$crf_inverted"
+        else
+            echo "$input"
+        fi
         return 0
     done
 }
@@ -131,12 +137,12 @@ case "$MEDIA_TYPE" in
             MENU_ARGS+=("gif" "GIF" "mp4" "MP4" "mkv" "MKV" "webm" "WebM" "avi" "AVI" "mov" "MOV")
         fi
 
-        SELECTED=$(kdialog --title "Transmutate — Image Conversion" --menu \
+        TARGET=$(kdialog --title "Transmutate — Image Conversion" --menu \
             "Choose output format for: $INPUT_BASENAME" \
-            "${MENU_ARGS[@]}") || quit
+            "${MENU_ARGS[@]}") || exit
         ;;
     video)
-        SELECTED=$(kdialog --title "Transmutate — Video Conversion" --menu \
+        TARGET=$(kdialog --title "Transmutate — Video Conversion" --menu \
             "Choose output format for: $INPUT_BASENAME" \
             "mp4" "MP4" \
             "mkv" "MKV" \
@@ -144,55 +150,46 @@ case "$MEDIA_TYPE" in
             "avi" "AVI" \
             "mov" "MOV" \
             "gif" "GIF" \
-            "webp" "WebP (animated)") || quit
+            "webp" "WebP (animated)") || exit
         ;;
     audio)
-        SELECTED=$(kdialog --title "Transmutate — Audio Conversion" --menu \
+        TARGET=$(kdialog --title "Transmutate — Audio Conversion" --menu \
             "Choose output format for: $INPUT_BASENAME" \
             "mp3" "MP3" \
             "flac" "FLAC" \
             "wav" "WAV" \
             "ogg" "OGG" \
             "m4a" "M4A" \
-            "aac" "AAC") || quit
-            ;;
+            "aac" "AAC") || exit
+        ;;
 esac
 
-TARGET="$SELECTED"
-echo "DEBUG: SELECTED='$SELECTED' TARGET='$TARGET'"
-
 # ─── 4.5. Stream selection (video/audio only) ──────────────────────────────────
-# Probes the file for audio and subtitle tracks.
-# Single track → auto-carried. Multiple tracks → checklist to pick.
-# No selection for audio means "take first audio". No selection for subs means "none".
-
-# Arrays to hold selected stream indices
-map_audio=()   # ffmpeg indices of audio streams to carry over
-map_sub=()     # ffmpeg indices of subtitle streams to carry over
+map_audio=()   
+map_sub=()     
 
 if [[ "$MEDIA_TYPE" == "video" || "$MEDIA_TYPE" == "audio" ]]; then
-    # Probe all streams: index, codec_type, codec_name, language
-    # Use key=value output — ffprobe csv doesn't guarantee field order
+    # Probe all streams
     STREAMS_RAW=$(ffprobe -v error \
         -show_entries stream=index,codec_type,codec_name,language \
         -of default=noprint_wrappers=1 "$INPUT_FILE" 2>/dev/null) || STREAMS_RAW=""
 
-    # Parse key=value pairs per stream block
-    # Each stream has: index=X, codec_type=..., codec_name=..., language=...
-    idx=0
-    audio_entries=()
-    audio_real_idx=()
-    sub_entries=()
-    sub_real_idx=()
+    # We will store the ABSOLUTE ffmpeg indices here
+    audio_absolute_indices=()
+    audio_labels=()
+    sub_absolute_indices=()
+    sub_labels=()
+
+    # Temporary variables for parsing
     current_idx=""
     current_type=""
     current_name=""
     current_lang=""
 
     while IFS='=' read -r key value; do
-        # Strip whitespace from key
         key=$(echo "$key" | tr -d ' ')
         value=$(echo "$value" | tr -d ' ')
+        
         case "$key" in
             index)       current_idx="$value" ;;
             codec_type)  current_type="$value" ;;
@@ -200,28 +197,18 @@ if [[ "$MEDIA_TYPE" == "video" || "$MEDIA_TYPE" == "audio" ]]; then
             language)    current_lang="$value" ;;
         esac
 
-        # When we have a complete stream (after index or type), process it
+        # When we have a complete stream block
         if [[ -n "$current_idx" && -n "$current_type" ]]; then
             if [[ "$current_type" == "audio" ]]; then
-                if [[ -n "$current_lang" && "$current_lang" != "und" ]]; then
-                    label="Audio #$current_idx — $current_name (${current_lang})"
-                else
-                    label="Audio #$current_idx — $current_name"
-                fi
-                audio_entries+=("$((idx + 1))" "$label" "on")
-                audio_real_idx+=("$idx")
-                idx=$(( idx + 1 ))
+                label="Audio #$current_idx — $current_name${current_lang:+ (${current_lang})}"
+                audio_absolute_indices+=("$current_idx")
+                audio_labels+=("$label")
             elif [[ "$current_type" == "subtitle" ]]; then
-                if [[ -n "$current_lang" && "$current_lang" != "und" ]]; then
-                    label="Subtitle #$current_idx — $current_name (${current_lang})"
-                else
-                    label="Subtitle #$current_idx — $current_name"
-                fi
-                sub_entries+=("$((idx + 1))" "$label" "on")
-                sub_real_idx+=("$idx")
-                idx=$(( idx + 1 ))
+                label="Subtitle #$current_idx — $current_name${current_lang:+ (${current_lang})}"
+                sub_absolute_indices+=("$current_idx")
+                sub_labels+=("$label")
             fi
-            # Reset for next stream
+            # Reset
             current_idx=""
             current_type=""
             current_name=""
@@ -230,63 +217,53 @@ if [[ "$MEDIA_TYPE" == "video" || "$MEDIA_TYPE" == "audio" ]]; then
     done <<< "$STREAMS_RAW"
 
     # --- Audio track selection ---
-    # Convert checklist entries to (id label on/off) pairs for kdialog
-    audio_pairs=()
-    for (( a=0; a<${#audio_entries[@]}; a+=3 )); do
-        audio_pairs+=("${audio_entries[$a]}" "${audio_entries[$((a+1))]}" "${audio_entries[$((a+2))]:-on}")
-    done
-
-    num_audio=$(( ${#audio_real_idx[@]} ))
-    if (( num_audio == 0 )); then
-        : # no audio streams, nothing to map
-    elif (( num_audio == 1 )); then
-        # Single audio stream — auto-select it
-        map_audio=("${audio_real_idx[0]}")
+    num_audio=${#audio_absolute_indices[@]}
+    if (( num_audio == 1 )); then
+        map_audio=("${audio_absolute_indices[0]}")
     elif (( num_audio > 1 )); then
-        # Multiple audio streams — let user pick
+        audio_pairs=()
+        for (( i=0; i<num_audio; i++ )); do
+            audio_pairs+=("$((i+1))" "${audio_labels[$i]}" "on")
+        done
+        
         audio_check=$(kdialog --title "Transmutate — Audio Tracks" --checklist \
-            "Select audio tracks to carry over:" \
-            "${audio_pairs[@]}") || quit
+            "Select audio tracks to carry over:" "${audio_pairs[@]}") || exit
+        
         if [[ -n "$audio_check" ]]; then
             while IFS=$'\t' read -r cid cstatus; do
                 [[ -z "$cid" ]] && continue
                 if [[ "$cstatus" == "on" ]]; then
-                    # Map checklist id (1-based) to real index
-                    real=$(( cid - 1 ))
-                    map_audio+=("${audio_real_idx[$real]}")
+                    # Map UI ID (1-based) back to the absolute index
+                    real_idx_pos=$(( cid - 1 ))
+                    map_audio+=("${audio_absolute_indices[$real_idx_pos]}")
                 fi
             done <<< "$audio_check"
         fi
     fi
 
     # --- Subtitle track selection ---
-    sub_pairs=()
-    for (( s=0; s<${#sub_entries[@]}; s+=3 )); do
-        sub_pairs+=("${sub_entries[$s]}" "${sub_entries[$((s+1))]}" "${sub_entries[$((s+2))]:-on}")
-    done
-
-    num_sub=${#sub_real_idx[@]}
-    if (( num_sub == 0 )); then
-        : # no subtitles
-    elif (( num_sub == 1 )); then
-        # Single subtitle — auto-select
-        map_sub=("${sub_real_idx[0]}")
+    num_sub=${#sub_absolute_indices[@]}
+    if (( num_sub == 1 )); then
+        map_sub=("${sub_absolute_indices[0]}")
     elif (( num_sub > 1 )); then
+        sub_pairs=()
+        for (( i=0; i<num_sub; i++ )); do
+            sub_pairs+=("$((i+1))" "${sub_labels[$i]}" "on")
+        done
+
         sub_check=$(kdialog --title "Transmutate — Subtitle Tracks" --checklist \
-            "Select subtitle tracks to carry over:" \
-            "${sub_pairs[@]}") || quit
+            "Select subtitle tracks to carry over:" "${sub_pairs[@]}") || exit
+        
         if [[ -n "$sub_check" ]]; then
             while IFS=$'\t' read -r cid cstatus; do
                 [[ -z "$cid" ]] && continue
                 if [[ "$cstatus" == "on" ]]; then
-                    real=$(( cid - 1 ))
-                    map_sub+=("${sub_real_idx[$real]}")
+                    real_idx_pos=$(( cid - 1 ))
+                    map_sub+=("${sub_absolute_indices[$real_idx_pos]}")
                 fi
             done <<< "$sub_check"
         fi
     fi
-
-    echo "DEBUG: map_audio=[${map_audio[*]}] map_sub=[${map_sub[*]}]"
 fi
 
 # ─── 5. Quality / options dialog ───────────────────────────────────────────────
@@ -301,7 +278,8 @@ LOOPING=false
 # Outputs two lines: first is CRF, second is audio quality.
 ask_video_quality() {
     local target="$1"
-    local crf_default=23
+    local crf_user_default=28
+    local crf_ffmpeg_default=23
     local audio_default=85
     local audio_label audio_range audio_hint
 
@@ -337,17 +315,18 @@ ask_video_quality() {
     while true; do
         local crf_input
         crf_input=$(kdialog --title "Transmutate — Video Quality" --inputbox \
-            "Video CRF (0 = best, 51 = worst):" \
-            "$crf_default") || quit
+            "Video CRF (51 = best, 0 = worst):" \
+            "$crf_user_default") || exit
 
         # Empty → accept default
         if [[ -z "$crf_input" ]]; then
-            crf_input=$crf_default
+            crf_input=$crf_user_default
             break
         fi
 
         if [[ "$crf_input" =~ ^[0-9]+$ ]] && (( crf_input >= 0 && crf_input <= 51 )); then
-            crf_default="$crf_input"
+            # Invert: user 51 (best) → ffmpeg 0, user 0 (worst) → ffmpeg 51
+            crf_ffmpeg_default=$(( 51 - crf_input ))
             break
         fi
 
@@ -360,7 +339,7 @@ ask_video_quality() {
         local audio_input
         audio_input=$(kdialog --title "Transmutate — Video Quality" --inputbox \
             "${audio_label} ${audio_range}:" \
-            "$audio_default") || quit
+            "$audio_default") || exit
 
         # Empty → accept default
         if [[ -z "$audio_input" ]]; then
@@ -377,10 +356,10 @@ ask_video_quality() {
             --msgbox "Please enter a valid integer."
     done
 
-    echo "$crf_default $audio_default"
+    echo "$crf_ffmpeg_default $audio_default"
 }
 
-# Per-media-type quality picker: image=0-100(default 85), video=CRF 0-51(default 23), audio=0-100(default 85)
+# Per-media-type quality picker: image=0-100(default 85), video=CRF 51-0(default 28), audio=0-100(default 85)
 # But video→GIF/WebP uses 0-100 (same as images) since the output encoders use that scale.
 AUDIO_QUALITY=""
 case "$MEDIA_TYPE" in
@@ -399,13 +378,12 @@ case "$MEDIA_TYPE" in
             QUALITY=$(ask_quality image 50)  # 0-100 scale, default 50
         else
             # Video → video: combined dialog for both video CRF and audio quality
-            read -r VIDEO_QUALITY AUDIO_QUALITY <<< $(ask_video_quality "$TARGET")
+            read -r VIDEO_QUALITY AUDIO_QUALITY <<< "$(ask_video_quality "$TARGET")"
             QUALITY="$VIDEO_QUALITY"
         fi
         ;;
 esac
-echo "Quality=$QUALITY"
-echo "AudioQuality=$AUDIO_QUALITY"
+echo "Quality=$QUALITY, AudioQuality=${AUDIO_QUALITY:-unset}"
 
 # GIF & WebP get extra animation options
 if [[ "$TARGET" == "gif" || "$TARGET" == "webp" ]]; then
@@ -420,7 +398,7 @@ if [[ "$TARGET" == "gif" || "$TARGET" == "webp" ]]; then
         CHECKS=$(kdialog --title "Transmutate — Animation Settings" --checklist \
             "Options for: $INPUT_BASENAME" \
             "1" "Animated" "on" \
-            "2" "Looping" "off") || quit
+            "2" "Looping" "off") || exit
         # Kdialog returns space-separated quoted IDs, e.g. "1" "2" or just "2"
         # Check if "2" (Looping) is in the selection
         if [[ -n "$CHECKS" ]] && [[ "$CHECKS" == *"2"* ]]; then
@@ -484,10 +462,11 @@ echo "Output: $OUTPUT_FILE"
 # ─── 8. Build ffmpeg command ────────────────────────────────────────────────────
 
 # For images/audio: user-facing 0-100, ffmpeg q-scale is inverted (lower=better).
-# For video: user enters CRF directly (0=best, 51=worst) — no inversion needed.
+# For video: user-facing CRF is 51-0 (higher=better), inverted to ffmpeg 0-51 (lower=better).
 Q=""
 if [[ "$MEDIA_TYPE" == "video" ]]; then
-    CRF="$QUALITY"  # CRF used directly
+    # Already inverted in ask_video_quality or ask_quality, use directly
+    :
 else
     # Invert: user 100 (best) → ffmpeg q=1 (best), user 0 (worst) → ffmpeg q=100 (worst)
     Q=$(( 100 - QUALITY ))
@@ -525,24 +504,40 @@ if [[ "$MEDIA_TYPE" == "video" && -n "$AUDIO_QUALITY" ]]; then
     esac
 fi
 
-# Build -map arguments for video streams
-# -map 0:v:0    = always map first video stream
-# -map 0:a:idx  = map selected audio streams (stream-copy when no -c:a)
-# -map 0:s:idx  = map selected subtitle streams
-# When AQ_ARG is set → add audio map + -c:a forces re-encode
-# When AQ_ARG is empty → only add audio map for stream-copy
+# ── Pre-compute subtitle codec argument ──────────────────────────
+SQ_ARG=""
+if (( ${#map_sub[@]} > 0 )); then
+    case "$TARGET" in
+        mp4|mov)
+            SQ_ARG="-c:s mov_text"
+            ;;
+        mkv)
+            SQ_ARG="-c:s copy"
+            ;;
+        webm)
+            SQ_ARG="-c:s webvtt"
+            ;;
+        avi)
+            SQ_ARG="-c:s copy"
+            ;;
+    esac
+fi
+
+# Build -map arguments for streams
+# We use absolute indices (e.g., -map 0:1) instead of category indices (e.g., -map 0:a:0)
 MAP_ARGS="-map 0:v:0"
 if (( ${#map_audio[@]} > 0 )); then
     for ai in "${map_audio[@]}"; do
-        MAP_ARGS+=" -map 0:a:$ai"
+        MAP_ARGS+=" -map 0:$ai"
     done
 elif [[ -n "$AQ_ARG" ]]; then
     # Fallback if no audio selected but we need audio
     MAP_ARGS+=" -map 0:a:0"
 fi
+
 if (( ${#map_sub[@]} > 0 )); then
     for si in "${map_sub[@]}"; do
-        MAP_ARGS+=" -map 0:s:$si"
+        MAP_ARGS+=" -map 0:$si"
     done
 fi
 
@@ -581,7 +576,6 @@ case "$MEDIA_TYPE" in
                         # Map quality (0-100) to palette colors (3-256)
                         # Minimum 3 because ffmpeg doesn't support max_colors=2 with reserve_transparent
                         IMG_PALETTE_COLORS=$(( QUALITY * 253 / 100 + 3 ))
-                        echo "DEBUG: image→GIF fallback QUALITY=$QUALITY IMG_PALETTE_COLORS=$IMG_PALETTE_COLORS"
                         if $LOOPING; then
                             CMD="ffmpeg -y -v error -i '${INPUT_FILE}' -vf 'fps=$FPS,scale=$SCALE,split[s0][s1];[s0]palettegen=max_colors=$IMG_PALETTE_COLORS:reserve_transparent=1[p];[s1][p]paletteuse=dither=sierra2_4a' -loop 0 '${OUTPUT_FILE}'"
                         else
@@ -623,23 +617,23 @@ case "$MEDIA_TYPE" in
                 ;;
             mp4)
                 # Animated image → MP4: use libwebp/ffmpeg to extract frames, re-encode as h264
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY -pix_fmt yuv420p -movflags +faststart '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY -pix_fmt yuv420p -movflags +faststart '${OUTPUT_FILE}'"
                 ;;
             mkv)
                 # Animated image → MKV
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
                 ;;
             webm)
                 # Animated image → WebM
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
                 ;;
             avi)
                 # Animated image → AVI
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY '${OUTPUT_FILE}'"
                 ;;
             mov)
                 # Animated image → MOV
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
                 ;;
         esac
         ;;
@@ -647,19 +641,19 @@ case "$MEDIA_TYPE" in
     video)
         case "$TARGET" in
             mp4)
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY -pix_fmt yuv420p -movflags +faststart '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY -pix_fmt yuv420p -movflags +faststart '${OUTPUT_FILE}'"
                 ;;
             mkv)
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
                 ;;
             webm)
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
                 ;;
             avi)
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY '${OUTPUT_FILE}'"
                 ;;
             mov)
-                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
+                CMD="ffmpeg -y -v error -i '${INPUT_FILE}' $MAP_ARGS $AQ_ARG $SQ_ARG -crf $QUALITY -pix_fmt yuv420p '${OUTPUT_FILE}'"
                 ;;
             gif)
                 # Video → GIF: use palettegen with quality-based dithering
@@ -670,12 +664,7 @@ case "$MEDIA_TYPE" in
                 SRC_W=$(ffprobe -v error -select_streams v:0 -show_entries stream=width -of csv=p=0 "${INPUT_FILE}" 2>/dev/null || echo "0")
                 SRC_H=$(ffprobe -v error -select_streams v:0 -show_entries stream=height -of csv=p=0 "${INPUT_FILE}" 2>/dev/null || echo "0")
                 SCALE="${SRC_W}:${SRC_H}:flags=lanczos"
-                # If fps is a fraction like 24000/1001, keep as-is; otherwise use as integer
-                if [[ "$SRC_FPS" == */* ]]; then
-                    FPS="$SRC_FPS"
-                else
-                    FPS="$SRC_FPS"
-                fi
+                FPS="$SRC_FPS"
                 # Map quality (0-100) to palette generation colors (3-256)
                 # Minimum 3 colors because ffmpeg doesn't support max_colors=2 with reserve_transparent
                 PALETTE_COLORS=$(( QUALITY * 253 / 100 + 3 ))
@@ -689,7 +678,6 @@ case "$MEDIA_TYPE" in
                 else
                     DITHER="none"
                 fi
-                echo "DEBUG: video→GIF QUALITY=$QUALITY PALETTE_COLORS=$PALETTE_COLORS DITHER=$DITHER"
                 if $LOOPING; then
                     CMD="ffmpeg -y -v error -i '${INPUT_FILE}' -vf 'fps=$FPS,scale=$SCALE,split[s0][s1];[s0]palettegen=max_colors=$PALETTE_COLORS:reserve_transparent=1[p];[s1][p]paletteuse=dither=$DITHER' -loop 0 '${OUTPUT_FILE}'"
                 else
@@ -714,7 +702,7 @@ case "$MEDIA_TYPE" in
         if (( ${#map_audio[@]} > 0 )); then
             AUDIO_MAP=""
             for ai in "${map_audio[@]}"; do
-                AUDIO_MAP+="-map 0:a:$ai "
+                AUDIO_MAP+="-map 0:$ai "
             done
         fi
 
@@ -755,10 +743,7 @@ esac
 # Skipped for WebP→GIF since ImageMagick handles those better.
 
 USE_MAGICK=false
-if [[ "$MEDIA_TYPE" == "image" && "$TARGET" == "gif" ]] && has_magick; then
-    USE_MAGICK=true
-fi
-if [[ "$MEDIA_TYPE" == "image" && "$TARGET" == "webp" ]] && has_magick; then
+if [[ "$MEDIA_TYPE" == "image" ]] && { [[ "$TARGET" == "gif" ]] || [[ "$TARGET" == "webp" ]]; } && has_magick; then
     USE_MAGICK=true
 fi
 
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..36dab53
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1,3 @@
+customtkinter
+Pillow
+ffmpeg-python
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..caf6f13
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1 @@
+"""Test package for transmutate_app."""
diff --git a/tests/test_engine.py b/tests/test_engine.py
new file mode 100644
index 0000000..d9220e0
--- /dev/null
+++ b/tests/test_engine.py
@@ -0,0 +1,113 @@
+"""Tests for the FFmpeg engine module."""
+
+import os
+import sys
+import unittest
+
+# Ensure the rewrite directory is on sys.path
+rewrite_dir = os.path.join(os.path.dirname(__file__), '..')
+if rewrite_dir not in sys.path:
+    sys.path.insert(0, rewrite_dir)
+
+from transmutate_app.engine.ffmpeg_engine import (
+    StreamInfo,
+    ProbeResult,
+    _parse_time_to_seconds,
+)
+
+
+class TestStreamInfo(unittest.TestCase):
+    """Tests for the StreamInfo dataclass and label method."""
+
+    def test_label_with_language(self):
+        stream = StreamInfo(
+            index=1,
+            codec_type="audio",
+            codec_name="aac",
+            language="eng",
+        )
+        self.assertEqual(stream.label(), "Audio #1 — aac (eng)")
+
+    def test_label_without_language(self):
+        stream = StreamInfo(
+            index=0,
+            codec_type="video",
+            codec_name="h264",
+            language=None,
+        )
+        self.assertEqual(stream.label(), "Video #0 — h264")
+
+    def test_label_with_empty_string_language(self):
+        stream = StreamInfo(
+            index=2,
+            codec_type="subtitle",
+            codec_name="mov_text",
+            language="",
+        )
+        # Empty string language should be treated as None
+        self.assertEqual(stream.label(), "Subtitle #2 — mov_text")
+
+    def test_label_defaults(self):
+        stream = StreamInfo(index=3, codec_type="data", codec_name="unknown")
+        self.assertEqual(stream.label(), "Data #3 — unknown")
+
+
+class TestProbeResult(unittest.TestCase):
+    """Tests for the ProbeResult dataclass defaults."""
+
+    def test_default_values(self):
+        result = ProbeResult()
+        self.assertEqual(result.streams, [])
+        self.assertFalse(result.is_animated)
+        self.assertEqual(result.mime_type, "")
+
+    def test_custom_values(self):
+        stream = StreamInfo(0, "audio", "aac", "eng")
+        result = ProbeResult(
+            streams=[stream],
+            is_animated=True,
+            mime_type="video/mp4",
+        )
+        self.assertEqual(result.streams, [stream])
+        self.assertTrue(result.is_animated)
+        self.assertEqual(result.mime_type, "video/mp4")
+
+
+class TestParseTime(unittest.TestCase):
+    """Tests for the time parser used in progress reporting."""
+
+    def test_hh_mm_ss(self):
+        result = _parse_time_to_seconds("01:23:45.67")
+        self.assertAlmostEqual(result, 5025.67, places=2)
+
+    def test_hh_mm_ss_int(self):
+        result = _parse_time_to_seconds("00:00:01")
+        self.assertAlmostEqual(result, 1.0, places=2)
+
+    def test_mm_ss(self):
+        result = _parse_time_to_seconds("01:23.45")
+        self.assertAlmostEqual(result, 83.45, places=2)
+
+    def test_ss_only(self):
+        result = _parse_time_to_seconds("12.34")
+        self.assertAlmostEqual(result, 12.34, places=2)
+
+    def test_integer_seconds(self):
+        result = _parse_time_to_seconds("42")
+        self.assertAlmostEqual(result, 42.0, places=2)
+
+    def test_zero(self):
+        result = _parse_time_to_seconds("0")
+        self.assertAlmostEqual(result, 0.0, places=2)
+
+    def test_invalid(self):
+        result = _parse_time_to_seconds("invalid")
+        self.assertAlmostEqual(result, 0.0, places=2)
+
+    def test_empty(self):
+        result = _parse_time_to_seconds("")
+        self.assertAlmostEqual(result, 0.0, places=2)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/transmutate.py b/transmutate.py
new file mode 100644
index 0000000..e7893cb
--- /dev/null
+++ b/transmutate.py
@@ -0,0 +1,31 @@
+#!/usr/bin/env python3
+"""Transmutate — Media Conversion GUI.
+
+Usage:
+    python transmutate.py <path/to/image|video|audio>
+
+Opens a CustomTkinter GUI for converting the specified file.
+"""
+
+import sys
+import os
+from transmutate_app.gui import open_file
+
+
+def main():
+    """CLI entry point: validate arguments and open the GUI."""
+    if len(sys.argv) < 2:
+        print("Usage: python transmutate.py <path/to/image|video|audio>")
+        sys.exit(1)
+
+    filepath = os.path.abspath(sys.argv[1])
+
+    if not os.path.isfile(filepath):
+        print(f"Error: File does not exist: {filepath}", file=sys.stderr)
+        sys.exit(1)
+
+    open_file(filepath)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/transmutate_app/__init__.py b/transmutate_app/__init__.py
new file mode 100644
index 0000000..e90d365
--- /dev/null
+++ b/transmutate_app/__init__.py
@@ -0,0 +1 @@
+"""transmutate_app — Media Conversion GUI package."""
diff --git a/transmutate_app/__main__.py b/transmutate_app/__main__.py
new file mode 100644
index 0000000..1686219
--- /dev/null
+++ b/transmutate_app/__main__.py
@@ -0,0 +1,19 @@
+"""Allow running transmutate_app as a module: python -m transmutate_app <file>."""
+
+import sys
+import os
+from .gui import open_file
+
+
+def main():
+    """CLI entry point for ``python -m transmutate_app <file>``."""
+    if len(sys.argv) < 2:
+        print("Usage: python -m transmutate_app <path/to/file>")
+        sys.exit(1)
+
+    filepath = sys.argv[1]
+    if not os.path.isfile(filepath):
+        print(f"Error: File does not exist: {filepath}", file=sys.stderr)
+        sys.exit(1)
+
+    open_file(filepath)
diff --git a/transmutate_app/engine/__init__.py b/transmutate_app/engine/__init__.py
new file mode 100644
index 0000000..7484adb
--- /dev/null
+++ b/transmutate_app/engine/__init__.py
@@ -0,0 +1 @@
+"""Engine module — FFmpeg and ImageMagick command builders."""
diff --git a/transmutate_app/engine/ffmpeg_engine.py b/transmutate_app/engine/ffmpeg_engine.py
new file mode 100644
index 0000000..d41800c
--- /dev/null
+++ b/transmutate_app/engine/ffmpeg_engine.py
@@ -0,0 +1,728 @@
+"""FFmpeg engine — command builders, probing, and execution for Transmutate."""
+
+from __future__ import annotations
+
+import json
+import os
+import subprocess
+from dataclasses import dataclass, field
+from importlib import import_module
+from pathlib import Path
+from typing import Optional
+
+# ---------------------------------------------------------------------------
+# Data classes
+# ---------------------------------------------------------------------------
+
+
+@dataclass
+class StreamInfo:
+    """Metadata for a single media stream."""
+
+    index: int
+    codec_type: str
+    codec_name: str
+    language: Optional[str] = None
+
+    def label(self) -> str:
+        """Return a human-readable label for this stream."""
+        part = f"#{self.index} \u2014 {self.codec_name}"
+        lang = self.language if self.language else None
+        if lang:
+            part += f" ({lang})"
+        return f"{self.codec_type.capitalize()} {part}"
+
+
+@dataclass
+class ProbeResult:
+    """Result of probing a media file with ffprobe."""
+
+    streams: list[StreamInfo] = field(default_factory=list)
+    is_animated: bool = False
+    mime_type: str = ""
+
+
+# ---------------------------------------------------------------------------
+# Helper — _parse_time_to_seconds  (used in progress reporting)
+# ---------------------------------------------------------------------------
+
+
+def _parse_time_to_seconds(t: str) -> float:
+    """Parse an ``HH:MM:SS.frac`` / ``MM:SS.frac`` / ``SS.frac`` / ``SS``
+    timecode to a float of seconds.  Returns 0.0 on failure."""
+    if not t:
+        return 0.0
+    try:
+        parts = t.split(":")
+        if len(parts) == 3:
+            h, m, s = parts
+            return int(h) * 3600 + int(m) * 60 + float(s)
+        elif len(parts) == 2:
+            m, s = parts
+            return int(m) * 60 + float(s)
+        else:
+            return float(parts[0])
+    except (ValueError, IndexError):
+        return 0.0
+
+
+# ---------------------------------------------------------------------------
+# FFmpeg / ffmpeg-python availability
+# ---------------------------------------------------------------------------
+
+_ffmpeg = None
+
+try:
+    _ffmpeg = import_module("ffmpeg")
+except ImportError:
+    pass
+
+
+def has_ffmpeg() -> bool:
+    """Return ``True`` when the ``ffmpeg`` binary is available in PATH."""
+    try:
+        subprocess.run(
+            ["ffmpeg", "-version"],
+            capture_output=True,
+            check=True,
+            timeout=10,
+        )
+        return True
+    except Exception:
+        return False
+
+
+# ---------------------------------------------------------------------------
+# MIME detection
+# ---------------------------------------------------------------------------
+
+def _get_magic_mime(filepath: str) -> str:
+    """Attempt to get the MIME type via ``file --mime-type`` / ``file -b``.
+
+    Falls back to extension-based detection.
+    """
+    # Try Python ``filetype`` / ``mimetypes`` first
+    try:
+        import filetype  # type: ignore[import-not-found]
+        kind = filetype.guess(filepath)
+        if kind is not None and kind.mime:
+            return kind.mime
+    except ImportError:
+        pass
+
+    # Try ``file --mime-type``
+    try:
+        result = subprocess.run(
+            ["file", "--mime-type", "-b", filepath],
+            capture_output=True,
+            text=True,
+            timeout=10,
+        )
+        if result.returncode == 0:
+            return result.stdout.strip()
+    except Exception:
+        pass
+
+    # Fallback: extension-based
+    ext = Path(filepath).suffix.lstrip(".").lower()
+    _EXT_MIME = {
+        "png": "image/png",
+        "jpg": "image/jpeg",
+        "jpeg": "image/jpeg",
+        "gif": "image/gif",
+        "webp": "image/webp",
+        "bmp": "image/bmp",
+        "tiff": "image/tiff",
+        "tif": "image/tiff",
+        "avif": "image/avif",
+        "heic": "image/heic",
+        "mp4": "video/mp4",
+        "mkv": "video/x-matroska",
+        "webm": "video/webm",
+        "avi": "video/x-msvideo",
+        "mov": "video/quicktime",
+        "flv": "video/x-flv",
+        "wmv": "video/x-ms-wmv",
+        "m4v": "video/x-m4v",
+        "mpg": "video/mpeg",
+        "mpeg": "video/mpeg",
+        "3gp": "video/3gpp",
+        "ts": "video/mp2t",
+        "ogv": "video/ogg",
+        "m2ts": "video/MP2T",
+        "mp3": "audio/mpeg",
+        "flac": "audio/flac",
+        "wav": "audio/wav",
+        "ogg": "audio/ogg",
+        "m4a": "audio/mp4",
+        "aac": "audio/aac",
+        "opus": "audio/opus",
+        "wma": "audio/x-ms-wma",
+        "aiff": "audio/aiff",
+        "ape": "audio/ape",
+        "alac": "audio/x-alac",
+    }
+    return _EXT_MIME.get(ext, "")
+
+
+def detect_mime(filepath: str) -> str:
+    """Detect the MIME type of *filepath*."""
+    mime = _get_magic_mime(filepath)
+    if mime:
+        return mime
+    # Also try ffprobe as fallback
+    probe_result = _probe_json(filepath)
+    if probe_result:
+        fmt = probe_result.get("format", {})
+        for k in ("mime_type", "format_name"):
+            v = fmt.get(k)
+            if v:
+                return v
+    return ""
+
+
+def detect_media_type(mime: str) -> str:
+    """Return ``image``, ``video``, or ``audio`` based on MIME, or empty."""
+    if not mime:
+        return ""
+    if mime.startswith("image/"):
+        return "image"
+    if mime.startswith("video/"):
+        return "video"
+    if mime.startswith("audio/"):
+        return "audio"
+    # Map some common format names
+    name_map = {
+        "image/png": "image",
+        "image/jpeg": "image",
+        "image/gif": "image",
+        "image/webp": "image",
+        "image/avif": "image",
+        "image/bmp": "image",
+        "image/tiff": "image",
+        "video/mp4": "video",
+        "video/x-matroska": "video",
+        "video/webm": "video",
+        "video/x-msvideo": "video",
+        "video/quicktime": "video",
+        "video/x-flv": "video",
+        "video/x-ms-wmv": "video",
+        "video/mpeg": "video",
+        "video/3gpp": "video",
+        "video/ogg": "video",
+        "audio/mpeg": "audio",
+        "audio/flac": "audio",
+        "audio/wav": "audio",
+        "audio/ogg": "audio",
+        "audio/mp4": "audio",
+        "audio/aac": "audio",
+        "audio/opus": "audio",
+    }
+    return name_map.get(mime, "")
+
+
+# ---------------------------------------------------------------------------
+# Probing — ffprobe
+# ---------------------------------------------------------------------------
+
+
+def _probe_json(filepath: str, timeout: int = 10) -> Optional[dict]:
+    """Run ffprobe via ffmpeg-python and return parsed JSON, or None on failure.
+
+    Falls back to a raw subprocess call if ffmpeg-python fails.
+    """
+    # Try ffmpeg-python first
+    if _ffmpeg is not None:
+        try:
+            metadata: dict = _ffmpeg.probe(filepath)  # type: ignore[attr-defined]
+            if isinstance(metadata, dict) and metadata.get("streams"):
+                return metadata
+        except Exception:  # noqa: BLE001
+            pass
+
+    # Fallback: use ffprobe directly via subprocess
+    try:
+        result = subprocess.run(
+            ["ffprobe", "-v", "quiet", "-show_format", "-show_streams",
+             "-of", "json", filepath],
+            capture_output=True, text=False, timeout=timeout,
+        )
+        if result.returncode == 0:
+            return json.loads(result.stdout.decode("utf-8"))
+    except Exception:  # noqa: BLE001
+        pass
+
+    return None
+
+
+def _get_animated_image_mime(filepath: str) -> Optional[str]:
+    """Attempt to detect if an image is animated using Pillow, and return its MIME."""
+    try:
+        from PIL import Image  # type: ignore[import-not-found]
+        img = Image.open(filepath)
+        # Animated images have multiple frames (GIF, WebP)
+        if hasattr(img, "n_frames") and int(getattr(img, "n_frames", 0)) > 1:
+            ext = Path(filepath).suffix.lower()
+            return {
+                ".gif": "image/gif",
+                ".webp": "image/webp",
+            }.get(ext)
+        if hasattr(img, "is_animated") and img.is_animated:
+            ext = Path(filepath).suffix.lower()
+            return {
+                ".gif": "image/gif",
+                ".webp": "image/webp",
+            }.get(ext)
+    except Exception:
+        pass
+    return None
+
+
+def probe_file(filepath: str, mime: str = "") -> ProbeResult:
+    """Probe *filepath* with ffprobe and return a :class:`ProbeResult`.
+
+    Handles both media files and animated images (GIF/WebP).
+    """
+    result = ProbeResult()
+    result.mime_type = mime
+
+    # Special case: animated images
+    if mime in ("image/gif", "image/webp"):
+        animated_mime = _get_animated_image_mime(filepath)
+        if animated_mime:
+            result.is_animated = True
+            result.mime_type = animated_mime
+
+    data = _probe_json(filepath)
+    if not data:
+        return result
+
+    # Parse streams
+    for s in data.get("streams", []):
+        stype = s.get("codec_type", "unknown")
+        cname = s.get("codec_name", "unknown")
+        lang = s.get("tags", {}).get("language", None)
+        if lang == "und":
+            lang = None
+        idx = s.get("index", 0)
+        result.streams.append(StreamInfo(
+            index=idx,
+            codec_type=stype,
+            codec_name=cname,
+            language=lang,
+        ))
+
+    return result
+
+
+# ---------------------------------------------------------------------------
+# Preflight — make sure ffmpeg can read the source file
+# ---------------------------------------------------------------------------
+
+
+def preflight_check(filepath: str) -> tuple[bool, str]:
+    """Verify that *filepath* is readable by ffmpeg.
+
+    Returns ``(ok, error_message)``.
+    """
+    if not os.path.isfile(filepath):
+        return False, f"File does not exist: {filepath}"
+
+    try:
+        result = subprocess.run(
+            ["ffprobe", "-v", "error", "-show_format", "-show_streams",
+             filepath],
+            capture_output=True,
+            text=True,
+            timeout=30,
+        )
+        if result.returncode != 0:
+            return False, f"ffprobe failed: {result.stderr.strip()}"
+        return True, ""
+    except subprocess.TimeoutExpired:
+        return False, "ffprobe timed out"
+    except Exception as exc:
+        return False, f"ffprobe error: {exc}"
+
+
+# ---------------------------------------------------------------------------
+# Execution — run_command
+# ---------------------------------------------------------------------------
+
+
+def run_command(cmd: list[str]) -> tuple[bool, str]:
+    """Execute *cmd* (a list of strings) via subprocess.
+
+    Returns ``(success, message)``.  *message* is empty on success.
+    """
+    try:
+        proc = subprocess.run(
+            cmd,
+            capture_output=True,
+            text=True,
+            timeout=3600,  # generous 1-hour timeout
+        )
+        if proc.returncode == 0:
+            return True, ""
+
+        stderr = proc.stderr.strip()
+        # Summarise the last few lines of ffmpeg output for the user
+        lines = stderr.split("\n")
+        summary = "\n".join(lines[-10:]) if len(lines) > 10 else stderr
+        return False, summary
+    except subprocess.TimeoutExpired:
+        return False, "Conversion timed out (exceeded 1 hour)"
+    except Exception as exc:
+        return False, str(exc)
+
+
+# ---------------------------------------------------------------------------
+# Quality helpers
+# ---------------------------------------------------------------------------
+
+
+def _quality_to_crf(quality: int) -> int:
+    """Map user quality (0–100) to a CRF value (0–51).
+
+    Higher quality → lower CRF.
+    """
+    # Invert so quality=100 → CRF=0, quality=0 → CRF=51
+    return max(0, min(51, 51 - int(quality * 51 / 100)))
+
+
+def _audio_quality_to_abr(codec: str, quality: int) -> tuple[str, str]:
+    """Return (param_flag, value) for audio quality.
+
+    Supports AAC, MP3, Opus, FLAC, Vorbis.
+    """
+    quality = max(0, min(100, quality))
+
+    if codec == "aac":
+        # AAC uses -b:a with a rate mapping
+        # Map 0-100 → 32-320 kbps
+        abr = max(32, int(32 + quality * 288 / 100))
+        return ("-b:a", f"{abr}k")
+    elif codec == "libmp3lame":
+        # MP3 uses -q:a (VBR) 0-9 → map quality
+        q = max(0, min(9, int(9 * (100 - quality) / 100)))
+        return ("-q:a", str(q))
+    elif codec == "libvorbis":
+        # Vorbis uses -q:a 0-10 → map quality
+        q = max(0, min(10, round(quality * 10 / 100)))
+        return ("-q:a", str(q))
+    elif codec == "libopus":
+        # Opus uses -b:a
+        abr = max(32, int(32 + quality * 256 / 100))
+        return ("-b:a", f"{abr}k")
+    elif codec == "flac":
+        # FLAC is lossless — quality is irrelevant, just use default
+        return ("-compression_level", "5")
+    elif codec == "pcm_s16le":
+        # WAV is lossless
+        return ("", "")
+    else:
+        # Generic: use bitrate
+        abr = max(32, int(32 + quality * 288 / 100))
+        return ("-b:a", f"{abr}k")
+
+
+# ---------------------------------------------------------------------------
+# Image → Image command builder
+# ---------------------------------------------------------------------------
+
+
+def _build_image_command(
+    src: str,
+    dst: str,
+    fmt: str,
+    quality: int,
+    is_animated: bool,
+    loop: bool,
+    mime: str = "",
+) -> list[str]:
+    """Build an ffmpeg command for image → image conversion.
+
+    Handles both single-frame and animated images.
+    """
+    ext = fmt.lower()
+    cmd = ["ffmpeg", "-y", "-i", src]
+
+    if is_animated:
+        # Animated: preserve all frames
+        cmd.extend(["-c:v", "libx264", "-pix_fmt", "yuv420p"])
+        if ext == "png":
+            # For animated PNG, extract frames then use ImageMagick or ffmpeg
+            # We'll use ffmpeg with a numbered sequence
+            pass
+        elif ext == "webp":
+            cmd.extend(["-c:v", "libwebp_anim", "-loop", "0"])
+        elif ext == "gif":
+            # Palettegen-based GIF
+            return _build_video_gif(src, dst, quality)
+        elif ext in ("mp4", "mkv", "webm", "avi", "mov"):
+            cmd.extend(["-c:v", "libx264", "-pix_fmt", "yuv420p"])
+            if ext == "webm":
+                cmd.extend(["-c:a", "libvorbis"])
+        if loop and ext in ("webp", "gif"):
+            cmd.extend(["-loop", "0"])
+    else:
+        # Single frame — just encode it
+        cmd.extend(["-frames:v", "1"])
+        if ext == "png":
+            cmd.extend(["-c:v", "png"])
+        elif ext in ("jpg", "jpeg"):
+            q = _quality_to_crf(quality)
+            cmd.extend(["-c:v", "libjpeg-turbo", "-q:v", str(q)])
+        elif ext == "webp":
+            lossless = quality >= 100
+            if lossless:
+                cmd.extend(["-c:v", "libwebp", "-lossless", "1"])
+            else:
+                q = _quality_to_crf(quality)
+                cmd.extend(["-c:v", "libwebp", "-lossless", "0", "-q:v", str(q)])
+        elif ext == "avif":
+            q = _quality_to_crf(quality)
+            cmd.extend(["-c:v", "libaom-av1", "-cpu-used", "4", "-q:v", str(q)])
+        elif ext == "bmp":
+            cmd.extend(["-c:v", "bmp"])
+
+    cmd.extend(["-pix_fmt", "yuv420p", dst])
+    return cmd
+
+
+# ---------------------------------------------------------------------------
+# Video → Video command builder
+# ---------------------------------------------------------------------------
+
+
+def _build_video_command(
+    src: str,
+    dst: str,
+    fmt: str,
+    quality: int,
+    audio_quality: int,
+    audio_streams: list[int],
+    sub_streams: list[int],
+    mime: str = "",
+) -> list[str]:
+    """Build an ffmpeg command for video → video conversion.
+
+    Handles audio stream selection and subtitle streams.
+    """
+    cmd = ["ffmpeg", "-y"]
+
+    # Input
+    cmd.extend(["-i", src])
+
+    # Select audio streams if specified
+    # audio_streams contains ffprobe stream indices (global), e.g. [1]
+    # Use the global-index form ``0:<index>`` — not ``0:a:N`` (invalid syntax).
+    if audio_streams:
+        for idx in audio_streams:
+            cmd.extend(["-map", f"0:{idx}"])
+    else:
+        # No specific audio selection — copy first audio stream if present
+        cmd.extend(["-map", "0:a:0"])
+
+    # Subtitles
+    # sub_streams contains ffprobe stream indices (global), e.g. [2]
+    if sub_streams:
+        for idx in sub_streams:
+            cmd.extend(["-map", f"0:{idx}"])
+    else:
+        # Try to copy subtitle streams
+        cmd.extend(["-map", "0:s?"])
+
+    # Video encoding
+    cmd.extend(["-c:v", "libx264", "-pix_fmt", "yuv420p"])
+
+    # CRF mode
+    crf = _quality_to_crf(quality)
+    cmd.extend(["-crf", str(crf)])
+
+    # Audio encoding
+    ext = fmt.lower()
+    audio_codec_map = {
+        "mp4": "aac",
+        "mkv": "aac",
+        "webm": "libvorbis",
+        "avi": "aac",
+        "mov": "aac",
+    }
+    ac = audio_codec_map.get(ext, "aac")
+
+    af, av = _audio_quality_to_abr(ac, audio_quality)
+    if af:
+        cmd.extend([af, av])
+
+    # Container-specific options
+    if ext == "webm":
+        cmd.extend(["-c:a", "libvorbis"])
+    elif ext == "mp4":
+        cmd.extend(["-movflags", "+faststart"])
+
+    cmd.extend(["-c:s", "mov_text"])
+
+    # Output
+    cmd.append(dst)
+    return cmd
+
+
+# ---------------------------------------------------------------------------
+# Video → Audio command builder
+# ---------------------------------------------------------------------------
+
+
+def _build_audio_command(
+    src: str,
+    dst: str,
+    fmt: str,
+    quality: int,
+    audio_streams: list[int],
+) -> list[str]:
+    """Build an ffmpeg command for audio extraction / format conversion.
+
+    Can be called from video→audio or audio→audio conversions.
+    """
+    cmd = ["ffmpeg", "-y", "-i", src]
+
+    # Select audio stream(s)
+    # audio_streams contains ffprobe stream indices (global), e.g. [1]
+    # We must use the global-index form ``0:<index>`` — not ``0:a:N``
+    # which is invalid ffmpeg syntax.
+    if audio_streams:
+        for idx in audio_streams:
+            cmd.extend(["-map", f"0:{idx}"])
+    else:
+        cmd.extend(["-map", "0:a:0"])
+
+    # Discard video — keep audio only
+    cmd.extend(["-vn"])
+
+    # Determine codec from format
+    fmt_lower = fmt.lower()
+    _codec_defaults: dict[str, tuple[str, str, str]] = {
+        "mp3": ("libmp3lame", "", ""),
+        "flac": ("flac", "-compression_level", "5"),
+        "wav": ("pcm_s16le", "", ""),
+        "ogg": ("libvorbis", "", ""),
+        "m4a": ("aac", "", ""),
+        "aac": ("aac", "", ""),
+    }
+    ac, af1, af2 = _codec_defaults.get(fmt_lower, ("aac", "-b:a", "128k"))
+
+    cmd.extend(["-c:a", ac])
+    if af1:
+        cmd.extend([af1])
+    if af2:
+        cmd.extend([af2])
+
+    # Quality override
+    qf, qv = _audio_quality_to_abr(ac, quality)
+    if qf:
+        # Replace the default codec params with quality-based ones
+        cmd = cmd[:-2] if len(cmd) >= 2 and cmd[-2] == af1 else cmd
+        cmd.extend([qf, qv])
+
+    # Output
+    cmd.append(dst)
+    return cmd
+
+
+# ---------------------------------------------------------------------------
+# Video → GIF (palettegen-based)
+# ---------------------------------------------------------------------------
+
+
+def _build_video_gif(src: str, dst: str, quality: int) -> list[str]:
+    """Build a palettegen-based ffmpeg command for video → GIF.
+
+    Uses a two-pass approach: first generate a palette, then use it.
+    """
+    quality = max(0, min(100, quality))
+
+    # Map quality to dithering mode and palette quality
+    # Higher quality = better palette + more dithering
+    if quality > 80:
+        dither = "bayer"
+        bayer_scale = "5"
+    elif quality > 60:
+        dither = "sierra2_4a"
+        bayer_scale = "4"
+    elif quality > 40:
+        dither = "burkes"
+        bayer_scale = "3"
+    else:
+        dither = "none"
+        bayer_scale = "1"
+
+    # Palette generation
+    palette_temp = dst + ".palette"
+
+    # Two-pass command:
+    # Pass 1: generate palette
+    # Pass 2: use palette for GIF
+    cmd = [
+        "ffmpeg", "-y",
+        "-i", src,
+        "-vf",
+        "fps=24,scale=640:-1:flags=lanczos,palettegen=max_colors=256:stats_mode=diff",
+        "-y", palette_temp,
+    ]
+    # Run pass 1 separately, then append pass 2
+    subprocess.run(cmd, capture_output=True, timeout=300)
+
+    # Pass 2: apply palette
+    cmd = [
+        "ffmpeg", "-y",
+        "-i", src,
+        "-i", palette_temp,
+        "-lavfi",
+        f"fps=24,scale=640:-1:flags=lanczos [x]; [x][1:v] paletteuse=dither={dither}:bayer_scale={bayer_scale}:diff_mode=rectangle",
+        dst,
+    ]
+
+    # Cleanup palette temp file
+    try:
+        os.remove(palette_temp)
+    except OSError:
+        pass
+
+    return cmd
+
+
+# ---------------------------------------------------------------------------
+# Video → Animated WebP (libwebp_anim)
+# ---------------------------------------------------------------------------
+
+
+def _build_video_webp(
+    src: str,
+    dst: str,
+    quality: int,
+    loop: bool,
+) -> list[str]:
+    """Build an ffmpeg command for video → animated WebP using libwebp_anim."""
+    quality = max(0, min(100, quality))
+
+    # WebP uses lossless/lossy quality
+    # Map quality: 0-100 → lossless or quality factor 0-100
+    lossiness = max(0, int((100 - quality) * 10))
+    cmd = [
+        "ffmpeg", "-y",
+        "-i", src,
+        "-c:v", "libwebp_anim",
+        "-pix_fmt", "yuv420p",
+        "-lossless", "0",
+        "-lossiness", str(lossiness),
+    ]
+
+    if loop:
+        cmd.extend(["-loop", "0"])
+
+    # Frame rate
+    cmd.extend(["-r", "24"])
+
+    # Output
+    cmd.append(dst)
+    return cmd
diff --git a/transmutate_app/gui.py b/transmutate_app/gui.py
new file mode 100644
index 0000000..121bc9e
--- /dev/null
+++ b/transmutate_app/gui.py
@@ -0,0 +1,969 @@
+"""Main GUI module for Transmutate — CustomTkinter application."""
+
+# flake8: noqa: F401 (imports used by TransmutateApp, not exposed at module level)
+
+import os
+import sys
+import threading
+import customtkinter as ctk
+
+from .engine.ffmpeg_engine import (
+    detect_mime,
+    detect_media_type,
+    probe_file,
+    has_ffmpeg,
+    preflight_check,
+    run_command,
+    _build_image_command,
+    _build_video_command,
+    _build_audio_command,
+    _build_video_gif,
+    _build_video_webp,
+    StreamInfo,
+)
+
+
+# Supported output formats
+IMAGE_FORMATS = {
+    "png": "PNG",
+    "jpg": "JPG",
+    "webp": "WebP",
+    "avif": "AVIF",
+}
+
+ANIMATED_IMAGE_FORMATS = {
+    **IMAGE_FORMATS,
+    "gif": "GIF",
+    "mp4": "MP4",
+    "mkv": "MKV",
+    "webm": "WebM",
+    "avi": "AVI",
+    "mov": "MOV",
+}
+
+VIDEO_FORMATS = {
+    "mp4": "MP4",
+    "mkv": "MKV",
+    "webm": "WebM",
+    "avi": "AVI",
+    "mov": "MOV",
+    "gif": "GIF",
+    "webp": "WebP (animated)",
+}
+
+AUDIO_FORMATS = {
+    "mp3": "MP3",
+    "flac": "FLAC",
+    "wav": "WAV",
+    "ogg": "OGG",
+    "m4a": "M4A",
+    "aac": "AAC",
+}
+
+class ConversionConfig:
+    """Dataclass-like holder for all user-selected conversion parameters."""
+
+    def __init__(self):
+        self.source_file: str = ""
+        self.media_type: str = ""  # image, video, audio
+        self.mime_type: str = ""
+        self.is_animated: bool = False
+        self.target_format: str = ""  # png, mp4, mp3, etc.
+        self.audio_streams: list[int] = []  # ffprobe stream indices
+        self.sub_streams: list[int] = []  # ffprobe stream indices
+        self.quality: int = 85  # user-facing 0-100
+        self.audio_quality: int = 85  # per-output-type scale
+        self.loop: bool = False
+        self.output_file: str = ""
+
+
+# ---------------------------------------------------------------------------
+# CustomTkinter Dialog Helpers
+# ---------------------------------------------------------------------------
+
+def _die(root: ctk.CTk, message: str) -> None:
+    """Show an error dialog, then destroy the root window and exit."""
+    _show_message(root, "Transmutate — Error", message, kind="error")
+    root.destroy()
+    sys.exit(1)
+
+
+def _show_message(
+    parent: ctk.CTk, title: str, message: str,
+    kind: str = "info",
+) -> None:
+    """Show a CustomTkinter modal dialog with a single OK button."""
+    dialog = ctk.CTkToplevel(parent)
+    dialog.title(title)
+    dialog.resizable(False, False)
+
+    # Center the dialog
+    parent.update_idletasks()
+    pw, ph = parent.winfo_width(), parent.winfo_height()
+    px, py = parent.winfo_x(), parent.winfo_y()
+    dw, dh = 420, 240
+    dx = px + (pw - dw) // 2
+    dy = py + (ph - dh) // 2
+    dialog.geometry(f"{dw}x{dh}+{dx}+{dy}")
+
+    # Icon based on kind
+    icons = {"info": "ℹ️", "warning": "⚠️", "error": "❌", "question": "❓"}
+    icon = icons.get(kind, "ℹ️")
+
+    # Theme based on kind
+    themes = {
+        "info":    ("#0ea5e9", "#0284c7", "#0369a1"),
+        "warning": ("#f59e0b", "#d97706", "#b45309"),
+        "error":   ("#ef4444", "#dc2626", "#b91c1c"),
+        "question":("#8b5cf6", "#7c3aed", "#6d28d9"),
+    }
+    bg_color, btn_color, btn_hover = themes.get(kind, themes["info"])
+
+    body = ctk.CTkFrame(dialog, fg_color="transparent")
+    body.pack(fill="both", padx=20, pady=16)
+
+    icon_lbl = ctk.CTkLabel(
+        body, text=icon, font=ctk.CTkFont(size=20),
+        fg_color="transparent", text_color=bg_color,
+    )
+    icon_lbl.pack()
+
+    ctk.CTkLabel(
+        body, text=message, wraplength=360,
+        font=ctk.CTkFont(size=13), justify="center",
+    ).pack(pady=(8, 8))
+
+    btn_frame = ctk.CTkFrame(body, fg_color="transparent")
+    btn_frame.pack(pady=(0, 8))
+
+    def on_close():
+        dialog.destroy()
+
+    close_btn = ctk.CTkButton(
+        btn_frame, text="OK", width=80,
+        fg_color=btn_color,
+        hover_color=btn_hover,
+        text_color="#ffffff",
+        font=ctk.CTkFont(size=13, weight="bold"),
+        corner_radius=8,
+        command=on_close,
+    )
+    close_btn.pack()
+
+    # Must be visible before grab_set so grab doesn't fail
+    dialog.update_idletasks()
+    dialog.grab_set()
+
+
+def _ask_question(
+    parent: ctk.CTk, title: str, message: str,
+) -> bool:
+    """Show a modal dialog asking the user to overwrite or rename.
+
+    Returns ``True`` for overwrite, ``False`` for rename.
+    """
+    dialog = ctk.CTkToplevel(parent)
+    dialog.title(title)
+    dialog.resizable(False, False)
+
+    # Center the dialog
+    parent.update_idletasks()
+    pw, ph = parent.winfo_width(), parent.winfo_height()
+    px, py = parent.winfo_x(), parent.winfo_y()
+    dialog.geometry("360x120" + f"+{px + (pw - 360) // 2}+{py + (ph - 120) // 2}")
+
+    # Message label — directly on the dialog, no wrapper frames
+    msg_lbl = ctk.CTkLabel(
+        dialog, text=message, wraplength=320,
+        font=ctk.CTkFont(size=13), justify="center",
+    )
+    msg_lbl.pack(pady=(20, 8))
+
+    result = {"value": False}
+
+    btn_row = ctk.CTkFrame(dialog, fg_color="transparent")
+    btn_row.pack(pady=(0, 20))
+
+    def on_overwrite():
+        result["value"] = True
+        dialog.destroy()
+
+    def on_rename():
+        result["value"] = False
+        dialog.destroy()
+
+    ctk.CTkButton(
+        btn_row, text="Overwrite", width=100,
+        fg_color="#059669",
+        hover_color="#047857",
+        text_color="#ffffff",
+        font=ctk.CTkFont(size=13, weight="bold"),
+        corner_radius=6,
+        command=on_overwrite,
+    ).pack(side="left", padx=6)
+
+    ctk.CTkButton(
+        btn_row, text="Rename", width=100,
+        fg_color="#6b7280",
+        hover_color="#4b5563",
+        text_color="#ffffff",
+        font=ctk.CTkFont(size=13, weight="bold"),
+        corner_radius=6,
+        command=on_rename,
+    ).pack(side="left", padx=6)
+
+    dialog.update_idletasks()
+    dialog.grab_set()
+    return result["value"]
+
+
+# ---------------------------------------------------------------------------
+# Main Application
+# ---------------------------------------------------------------------------
+
+class TransmutateApp:
+    """Main CustomTkinter application for Transmutate."""
+
+    def __init__(self, root: ctk.CTk, filepath: str):
+        self.root = root
+        self.config = ConversionConfig()
+        self.config.source_file = os.path.abspath(filepath)
+
+        # Basic validation
+        if not os.path.isfile(self.config.source_file):
+            _die(root, f"File does not exist: {filepath}")
+
+        self.config.mime_type = detect_mime(self.config.source_file)
+        self.config.media_type = detect_media_type(self.config.mime_type)
+
+        if not self.config.media_type:
+            _die(root, f"Unsupported file type: {self.config.mime_type}")
+
+        if not has_ffmpeg():
+            _die(root, "ffmpeg is not installed or not in PATH")
+
+        probe = probe_file(self.config.source_file, self.config.mime_type)
+        self.config.is_animated = probe.is_animated
+        self.streams = list(probe.streams)  # copy to avoid mutation
+
+        base = os.path.basename(self.config.source_file)
+        self._input_name, _ = os.path.splitext(base)
+        self._input_dir = os.path.dirname(self.config.source_file)
+
+        # Build UI
+        self.root.title("Transmutate")
+        self._build_ui()
+
+    # ──────────────────────────────────────────────────────────────
+    # UI Construction
+    # ──────────────────────────────────────────────────────────────
+
+    def _build_ui(self):
+        """Build the main application window."""
+        self.root.geometry("720x720")
+        self.root.minsize(480, 400)
+
+        main = ctk.CTkFrame(self.root)
+        main.pack(fill="both", expand=True, padx=16, pady=16)
+
+        self._build_info_header(main)
+        self._build_category_buttons(main)
+
+        self.options_frame = ctk.CTkScrollableFrame(
+            main, fg_color="transparent",
+            label_text="Options", label_font=ctk.CTkFont(size=11, weight="bold"),
+        )
+        self.options_frame.pack(fill="both", expand=True, pady=(8, 4))
+
+        self._build_bottom(main)
+
+    def _build_info_header(self, parent):
+        """Display file info (name, media type, extension) at the top."""
+        info_frame = ctk.CTkFrame(parent, fg_color="transparent")
+        info_frame.pack(fill="x", pady=(0, 8))
+
+        media_label = self.config.media_type.upper()
+        ext = os.path.splitext(self.config.source_file)[1].lstrip(".").upper()
+        self.info_label = ctk.CTkLabel(
+            info_frame,
+            text=f"📁 {os.path.basename(self.config.source_file)}  "
+                 f"[{media_label} · {ext}]",
+            font=ctk.CTkFont(family="", size=10),
+        )
+        self.info_label.pack(anchor="center")
+
+        if self.config.is_animated:
+            ctk.CTkLabel(info_frame, text="  ⚡ Animated", text_color="#b87333").pack(anchor="center")
+
+    def _build_category_buttons(self, parent):
+        """Source-to-target type selector buttons (Image / Video / Audio)."""
+        btn_frame = ctk.CTkFrame(parent)
+        btn_frame.pack(fill="x", pady=(0, 4))
+
+        ctk.CTkLabel(btn_frame, text="Convert to:", font=ctk.CTkFont(family="", size=11)).pack()
+
+        target_types = {"image": "Image", "video": "Video", "audio": "Audio"}
+        current = self.config.media_type
+
+        row_frame = ctk.CTkFrame(btn_frame, fg_color="transparent")
+        row_frame.pack()
+
+        for key, label in target_types.items():
+            if key == current:
+                state = "normal"
+            elif self.config.media_type == "video" and key == "image":
+                state = "disabled"
+            elif self.config.media_type == "image" and key in ("video", "audio"):
+                state = "disabled"
+            elif self.config.media_type == "audio" and key in ("video", "image"):
+                state = "disabled"
+            else:
+                state = "normal"
+            btn = ctk.CTkButton(
+                row_frame, text=label,
+                command=lambda k=key: self._on_target_type(k),
+                state=state,
+            )
+            btn.pack(side="left", padx=(4, 4))
+
+    def _build_bottom(self, parent):
+        """Conversion button anchored at the bottom."""
+        bottom_container = ctk.CTkFrame(parent, fg_color="transparent")
+        bottom_container.pack(fill="x", side="bottom", pady=(4, 12))
+        bottom_container.pack_propagate(False)
+
+        self.transform_btn = ctk.CTkButton(
+            bottom_container, text="Transmutate", command=self._on_transform,
+            font=ctk.CTkFont(family="", size=12, weight="bold"),
+            height=36,
+            width=160,
+        )
+        self.transform_btn.pack()
+
+        # Disable until user sets options
+        self.transform_btn.configure(state="disabled")
+
+    # Dynamic option panels
+
+    def _on_target_type(self, target_type: str):
+        """Switch to a new conversion target panel."""
+        self.config.quality = 85
+        self.config.audio_quality = 85
+        self.config.loop = False
+        self.config.audio_streams = []
+        self.config.sub_streams = []
+        self.transform_btn.configure(state="disabled")
+
+        for w in self.options_frame.winfo_children():
+            w.destroy()
+
+        # Clear stale audio-quality widget references
+        for attr in ("_audio_quality_frame", "_audio_quality_slider",
+                     "_audio_quality_var", "_audio_quality_label",
+                     "_audio_quality_title_label", "_audio_quality_widgets"):
+            if hasattr(self, attr):
+                delattr(self, attr)
+
+        dispatch = {
+            ("image", "image"): self._build_image_image_panel,
+            ("image", "video"): self._build_image_video_panel,
+            ("image", "audio"): self._build_image_audio_panel,
+            ("video", "image"): self._build_video_image_panel,
+            ("video", "video"): self._build_video_video_panel,
+            ("video", "audio"): self._build_video_audio_panel,
+            ("audio", "image"): self._build_audio_image_panel,
+            ("audio", "video"): self._build_audio_video_panel,
+            ("audio", "audio"): self._build_audio_audio_panel,
+        }
+        fn = dispatch.get((self.config.media_type, target_type))
+        if fn:
+            fn()
+            if hasattr(self, "_format_var"):
+                self.config.target_format = self._format_var.get()
+                self.transform_btn.configure(state="normal")
+
+    # ──────────────────────────────────────────────────────────────
+    # Helper: create options panel widgets
+    # ──────────────────────────────────────────────────────────────
+
+    def _add_radio_group(self, parent, formats: dict[str, str], default_key: str):
+        """Add format radio buttons inside *parent*. Returns the ``StringVar``."""
+        frame = ctk.CTkFrame(parent, fg_color="transparent")
+        frame.pack(fill="x", pady=(4, 0))
+
+        ctk.CTkLabel(frame, text="Format:", font=ctk.CTkFont(family="", size=11, weight="bold")).pack(anchor="center")
+
+        radios_frame = ctk.CTkFrame(frame, fg_color="transparent")
+        radios_frame.pack(fill="x", pady=(2, 0))
+
+        selected = ctk.StringVar(value=default_key)
+
+        for key, label in formats.items():
+            rb = ctk.CTkRadioButton(
+                radios_frame, text=label, value=key,
+                variable=selected, command=self._on_format_change,
+            )
+            rb.pack(side="left", padx=(0, 8))
+
+        # Track it on self for later reading
+        self._format_var = selected
+        return selected
+
+    def _add_quality_slider(self, parent, default: int = 85,
+                            label_text: str = "Quality: ",
+                            crf_max: int = 0):
+        """Add a quality slider and label, returning the parent ``Frame``.
+
+        When *crf_max* > 0 (CRF mode) the slider runs *crf_max .. 0* so
+        the right side is best quality (lowest CRF).  *default* is on the
+        user-facing scale (0 – crf_max).
+
+        When *crf_max* == 0 the slider runs 0 – 100 (higher = better).
+        """
+        frame = ctk.CTkFrame(parent, fg_color="transparent")
+        frame.pack(fill="x", pady=(4, 0))
+
+        # Use grid for everything in this frame
+        frame.grid_columnconfigure(0, weight=1)
+
+        qty_label_text = ctk.CTkLabel(frame, text=label_text, font=ctk.CTkFont(family="", size=11, weight="bold"))
+        qty_label_text.grid(row=0, column=0, columnspan=2, pady=(0, 2))
+
+        qty = ctk.IntVar(value=default)
+
+        # CRF: slider right → best quality (lower CRF), i.e. crf_max down to 0
+        # Standard: slider right → better quality, i.e. 0 up to 100
+        if crf_max > 0:
+            slider = ctk.CTkSlider(
+                frame, from_=crf_max, to=0,
+                variable=qty,
+                command=self._on_quality_scale_clamped,
+            )
+        else:
+            slider = ctk.CTkSlider(
+                frame, from_=0, to=100,
+                variable=qty,
+                command=self._on_quality_scale_clamped,
+            )
+        slider.grid(row=1, column=0, sticky="ew", columnspan=2, padx=(0, 8))
+
+        self._quality_display_label = ctk.CTkLabel(frame, text=str(default), width=40, anchor="e")
+        self._quality_display_label.grid(row=1, column=1)
+
+        self._quality_var = qty
+        self._quality_frame = frame
+        self._quality_slider = slider
+        self._quality_mode = "standard"
+        self._crf_max = crf_max
+        return frame
+
+    def _on_quality_scale_clamped(self, val):
+        """Clamp slider value to bounds, then update display."""
+        v = int(float(val))
+        qty_var = self._quality_var
+        crf_max = getattr(self, "_crf_max", 0)
+
+        if crf_max > 0:
+            # CRF mode: val is in [crf_max, 0], clamp to that range
+            if v < 0:
+                v = 0
+            elif v > crf_max:
+                v = crf_max
+            qty_var.set(v)
+            self._quality_display_label.configure(text=str(v))
+        else:
+            # Standard mode: val is in [0, 100], clamp to that range
+            if v < 0:
+                v = 0
+            elif v > 100:
+                v = 100
+            qty_var.set(v)
+            self._quality_display_label.configure(text=str(v))
+
+    def _add_audio_quality_slider(self, parent, default: int = 85):
+        """Add an audio-quality slider for video output, returning the parent ``Frame``."""
+        frame = ctk.CTkFrame(parent, fg_color="transparent")
+        frame.pack(fill="x", pady=(4, 0))
+
+        # Use grid for everything in this frame — no pack/grid mixing.
+        frame.grid_columnconfigure(0, weight=1)
+
+        title_label = ctk.CTkLabel(frame, text="Audio Quality:", font=ctk.CTkFont(family="", size=11, weight="bold"))
+        title_label.grid(row=0, column=0, columnspan=2, pady=(0, 2))
+
+        qty = ctk.IntVar(value=default)
+
+        slider = ctk.CTkSlider(
+            frame, from_=0, to=100,
+            variable=qty,
+            command=lambda val: self._on_audio_quality_scale_clamped(val, qty),
+        )
+        slider.grid(row=1, column=0, sticky="ew", columnspan=2, padx=(0, 8))
+
+        self._audio_quality_label = ctk.CTkLabel(frame, text=str(default), width=40, anchor="e")
+        self._audio_quality_label.grid(row=1, column=1)
+
+        self._audio_quality_var = qty
+        self._audio_quality_frame = frame
+        self._audio_quality_slider = slider
+        self._audio_quality_title_label = title_label
+        self._audio_quality_widgets = (title_label, slider, self._audio_quality_label)
+        return frame
+
+    def _on_audio_quality_scale_clamped(self, val, qty_var):
+        """Clamp audio-quality slider value to 0-100 bounds."""
+        v = int(float(val))
+        if v < 0:
+            v = 0
+        elif v > 100:
+            v = 100
+        qty_var.set(v)
+        self._audio_quality_label.configure(text=str(v))
+
+    def _add_loop_checkbox(self, parent):
+        """Add a loop checkbox, returning the ``BooleanVar``."""
+        cb_frame = ctk.CTkFrame(parent, fg_color="transparent")
+        cb_frame.pack(fill="x", pady=(2, 0))
+
+        var = ctk.BooleanVar(value=False)
+        cb = ctk.CTkCheckBox(cb_frame, text="Loop output", variable=var)
+        cb.pack(anchor="center")
+        self._loop_var = var
+        self._loop_frame = cb_frame
+        return var
+
+    def _add_stream_selection(self, parent, streams: list[StreamInfo], stream_type: str = ""):
+        """Add checkboxes for selecting audio/subtitle streams."""
+        if not streams:
+            return None
+
+        frame = ctk.CTkFrame(parent, fg_color="transparent")
+        frame.pack(fill="x", pady=(4, 0))
+
+        header = f"Select {stream_type}" if stream_type else "Streams"
+        ctk.CTkLabel(frame, text=f"{header}:", font=ctk.CTkFont(family="", size=11, weight="bold")).pack(anchor="center")
+
+        checkboxes_frame = ctk.CTkFrame(frame, fg_color="transparent")
+        checkboxes_frame.pack(fill="x", pady=(2, 0))
+
+        vars_dict = {}
+        for stream in streams:
+            var = ctk.BooleanVar(value=True)  # default: all selected
+            cb = ctk.CTkCheckBox(
+                checkboxes_frame, text=stream.label(), variable=var,
+            )
+            cb.pack(anchor="center")
+            vars_dict[stream.index] = var
+
+        self._stream_vars = vars_dict
+        return vars_dict
+
+    def _maybe_update_transform(self):
+        """Enable/disable the conversion button based on current selections."""
+        has_format = hasattr(self, "_format_var") and self.config.target_format
+        self.transform_btn.configure(state="normal" if has_format else "disabled")
+
+    def _on_format_change(self):
+        """Handle format selection change."""
+        fmt = self._format_var.get()
+        self.config.target_format = fmt
+        self._maybe_update_transform()
+        self._maybe_update_quality_controls()
+
+    def _maybe_update_quality_controls(self):
+        """Show/hide quality controls based on the current format."""
+        fmt = getattr(self, "_format_var", None)
+        if fmt is None:
+            return
+        fmt = fmt.get()
+
+        # Determine which quality mode to show
+        show_crf = fmt in VIDEO_FORMATS and fmt not in ("gif", "webp")
+        show_loop = fmt in ("gif", "webp")
+
+        # Show/hide quality slider label dynamically
+        if hasattr(self, "_quality_var"):
+            # Find the quality slider's label and update text
+            if hasattr(self, "_quality_frame"):
+                for child in self._quality_frame.winfo_children():
+                    if isinstance(child, ctk.CTkLabel):
+                        if show_crf:
+                            child.configure(text="Video CRF (0-51): ")
+                        elif show_loop:
+                            child.configure(text="Quality (0-100): ")
+                        else:
+                            child.configure(text="Quality (0-100): ")
+                        break
+
+        # Toggle quality slider range and label text
+        if hasattr(self, "_quality_var") and hasattr(self, "_quality_slider") and hasattr(self, "_quality_display_label"):
+            # Determine the target quality mode
+            if show_crf:
+                target_mode = "crf"
+            elif show_loop:
+                target_mode = "loop"
+            else:
+                target_mode = "standard"
+
+            # Only reset slider value when actually changing modes
+            mode_changed = target_mode != getattr(self, "_quality_mode", None)
+
+            if mode_changed:
+                if show_crf:
+                    # Slider already reports 0..crf_max directly in CRF mode
+                    self._quality_var.set(23)
+                    self._quality_display_label.configure(text="23")
+                elif show_loop:
+                    self._quality_var.set(50)
+                    self._quality_display_label.configure(text="50")
+                else:
+                    self._quality_var.set(85)
+                    self._quality_display_label.configure(text="85")
+
+            self._quality_mode = target_mode
+
+        # Show/hide audio quality slider
+        if hasattr(self, "_audio_quality_frame"):
+            if show_crf:
+                self._audio_quality_frame.pack(fill="x", pady=(4, 0))
+            else:
+                self._audio_quality_frame.pack_forget()
+
+        # Show/hide loop checkbox
+        if hasattr(self, "_loop_var") and hasattr(self, "_loop_frame"):
+            if show_loop:
+                self._loop_frame.pack()
+            else:
+                self._loop_frame.pack_forget()
+
+    # Unified panel builders
+
+    def _build_image_image_panel(self):
+        formats = ANIMATED_IMAGE_FORMATS if self.config.is_animated else IMAGE_FORMATS
+        self._add_radio_group(self.options_frame, formats, "png")
+        self._add_quality_slider(self.options_frame, default=85,
+                                 label_text="Quality (0-100): ")
+        self._add_loop_checkbox(self.options_frame)
+        self._maybe_update_transform()
+
+    def _build_image_video_panel(self):
+        self._add_radio_group(self.options_frame, {
+            "mp4": "MP4", "mkv": "MKV", "webm": "WebM",
+            "avi": "AVI", "mov": "MOV",
+        }, "mp4")
+        self._add_quality_slider(self.options_frame, default=23,
+                                 label_text="Video CRF (0-51): ",
+                                 crf_max=51)
+        self._add_audio_quality_slider(self.options_frame, default=85)
+        self._maybe_update_transform()
+
+    def _build_image_audio_panel(self):
+        self._add_radio_group(self.options_frame, AUDIO_FORMATS, "mp3")
+        self._add_quality_slider(self.options_frame, default=85,
+                                 label_text="Quality (0-100): ")
+        self._maybe_update_transform()
+
+    def _build_video_image_panel(self):
+        self._add_radio_group(self.options_frame, {
+            "png": "PNG", "jpg": "JPG", "webp": "WebP", "avif": "AVIF",
+        }, "png")
+        self._add_quality_slider(self.options_frame, default=85,
+                                 label_text="Quality (0-100): ")
+        self._maybe_update_transform()
+
+    def _build_video_video_panel(self):
+        # Audio streams
+        audio_streams = [s for s in self.streams if s.codec_type == "audio"]
+        if len(audio_streams) == 0:
+            pass
+        elif len(audio_streams) == 1:
+            self.config.audio_streams = [audio_streams[0].index]
+        else:
+            self._add_stream_selection(
+                self.options_frame, audio_streams, "audio",
+            )
+
+        # Subtitle streams
+        sub_streams = [s for s in self.streams if s.codec_type == "subtitle"]
+        if len(sub_streams) == 1:
+            self.config.sub_streams = [sub_streams[0].index]
+        elif len(sub_streams) > 1:
+            self._add_stream_selection(
+                self.options_frame, sub_streams, "subtitle",
+            )
+
+        # Regular video formats
+        self._add_radio_group(self.options_frame, {
+            "mp4": "MP4", "mkv": "MKV", "webm": "WebM",
+            "avi": "AVI", "mov": "MOV",
+        }, "mp4")
+
+        # GIF / WebP on a separate line below
+        gif_webp_frame = ctk.CTkFrame(self.options_frame, fg_color="transparent")
+        gif_webp_frame.pack(fill="x", pady=(4, 0))
+        ctk.CTkLabel(gif_webp_frame, text="Animated:", font=ctk.CTkFont(family="", size=11, weight="bold")).pack(anchor="center")
+        for fmt_key, fmt_label in [("gif", "GIF"), ("webp", "WebP (animated)")]:
+            rb = ctk.CTkRadioButton(
+                gif_webp_frame, text=fmt_label, value=fmt_key,
+                variable=self._format_var,
+                command=self._on_format_change,
+            )
+            rb.pack(side="left", padx=(0, 8))
+
+        # Quality controls
+        self._add_quality_slider(self.options_frame, default=23,
+                                 label_text="Video CRF (0-51): ",
+                                 crf_max=51)
+        self._add_audio_quality_slider(self.options_frame, default=85)
+        self._add_loop_checkbox(self.options_frame)
+        self._maybe_update_transform()
+        self._maybe_update_quality_controls()
+
+    def _build_video_audio_panel(self):
+        audio_streams = [s for s in self.streams if s.codec_type == "audio"]
+        if len(audio_streams) == 0:
+            ctk.CTkLabel(self.options_frame, text="No audio tracks found in this video.",
+                         text_color="#cc6666").pack(anchor="center", pady=(4, 0))
+            ctk.CTkLabel(self.options_frame, text="Cannot convert video without audio.",
+                         text_color="gray").pack(anchor="center")
+            self.transform_btn.configure(state="disabled")
+            return
+        if len(audio_streams) == 1:
+            self.config.audio_streams = [audio_streams[0].index]
+        else:
+            self._add_stream_selection(
+                self.options_frame, audio_streams, "audio",
+            )
+
+        self._add_radio_group(self.options_frame, AUDIO_FORMATS, "mp3")
+        self._add_quality_slider(self.options_frame, default=85,
+                                 label_text="Quality (0-100): ")
+        self._maybe_update_transform()
+
+    def _build_audio_image_panel(self):
+        self._add_radio_group(self.options_frame, {
+            "png": "PNG", "jpg": "JPG", "webp": "WebP", "avif": "AVIF",
+        }, "png")
+        self._add_quality_slider(self.options_frame, default=85,
+                                 label_text="Quality (0-100): ")
+        self._maybe_update_transform()
+
+    def _build_audio_video_panel(self):
+        self._add_radio_group(self.options_frame, {
+            "mp4": "MP4", "mkv": "MKV", "webm": "WebM",
+            "avi": "AVI", "mov": "MOV",
+        }, "mp4")
+        self._add_quality_slider(self.options_frame, default=23,
+                                 label_text="Video CRF (0-51): ",
+                                 crf_max=51)
+        self._add_audio_quality_slider(self.options_frame, default=85)
+        self._maybe_update_transform()
+
+    def _build_audio_audio_panel(self):
+        audio_streams = [s for s in self.streams if s.codec_type == "audio"]
+        if len(audio_streams) == 0:
+            ctk.CTkLabel(self.options_frame, text="No audio streams found in file.",
+                         text_color="#cc6666").pack(anchor="center", pady=(4, 0))
+            ctk.CTkLabel(self.options_frame, text="Cannot convert without audio.",
+                         text_color="gray").pack(anchor="center")
+            self.transform_btn.configure(state="disabled")
+            return
+        if len(audio_streams) == 1:
+            self.config.audio_streams = [audio_streams[0].index]
+        else:
+            self._add_stream_selection(
+                self.options_frame, audio_streams, "audio",
+            )
+
+        self._add_radio_group(self.options_frame, AUDIO_FORMATS, "mp3")
+        self._add_quality_slider(self.options_frame, default=85,
+                                 label_text="Quality (0-100): ")
+        self._maybe_update_transform()
+
+    # Action handlers
+
+    def _on_transform(self):
+        """Execute the conversion in a background thread."""
+        # Gather values
+        self.config.target_format = self._format_var.get()
+        quality_val = self._quality_var.get()
+        # Slider already reports the actual quality value directly
+        # (CRF mode: 0..crf_max; Standard mode: 0..100)
+        self.config.quality = quality_val
+
+        self.config.audio_quality = getattr(self, "_audio_quality_var", None) and self._audio_quality_var.get() or 85
+        self.config.loop = getattr(self, "_loop_var", None) and self._loop_var.get() or False
+
+        # Collect selected streams
+        if hasattr(self, "_stream_vars"):  # stream selection only used for audio video conversions
+            self.config.audio_streams = [
+                idx for idx, var in self._stream_vars.items()
+                if var.get()
+            ]
+            if not self.config.audio_streams:
+                msg = "No audio streams selected. Conversion may fail."
+                print(f"Transmutate — Warning: {msg}")
+                _show_message(self.root, "Transmutate — Warning", msg, kind="warning")
+                return
+
+        # Determine output path
+        out_ext = self._ext_for_format(self.config.target_format)
+        self.config.output_file = os.path.join(
+            self._input_dir, f"{self._input_name}.{out_ext}"
+        )
+
+        # Check overwrite
+        if os.path.exists(self.config.output_file):
+            if not _ask_question(
+                self.root,
+                "File Exists",
+                f"{os.path.basename(self.config.output_file)} already exists.",
+            ):
+                # Auto-rename
+                counter = 1
+                while True:
+                    renamed = os.path.join(
+                        self._input_dir,
+                        f"{self._input_name}[{counter}].{out_ext}",
+                    )
+                    if not os.path.exists(renamed):
+                        self.config.output_file = renamed
+                        break
+                    counter += 1
+
+        # Preflight check
+        ok, err_msg = preflight_check(self.config.source_file)
+        if not ok:
+            print(f"Transmutate — Error: {err_msg}")
+            _show_message(self.root, "Transmutate — Error", err_msg, kind="error")
+            return
+
+        # Build command
+        try:
+            cmd = self._build_command()
+        except ValueError as e:
+            msg = str(e)
+            print(f"Transmutate — Error: {msg}")
+            _show_message(self.root, "Transmutate — Error", msg, kind="error")
+            return
+
+        # Disable UI during conversion
+        self._set_ui_enabled(False)
+        self.transform_btn.configure(state="disabled")
+
+        def run_conversion():
+            try:
+                self.transform_btn.configure(text="Converting…", state="disabled")
+
+                success, msg = run_command(cmd)
+
+                if success:
+                    self.root.after(0, lambda: _show_message(
+                        self.root, "Transmutation Complete", "Conversion finished.", kind="info"))
+                else:
+                    error_msg = f"Conversion failed.\n\n{msg}" if msg else "Unknown error."
+                    print(f"Transmutate — Error: {error_msg}")
+                    self.root.after(0, lambda m=error_msg: _show_message(
+                        self.root, "Transmutate — Error", m, kind="error"))
+            except Exception as exc:
+                exc_val = str(exc)
+                print(f"Transmutate — Error: {exc_val}")
+                self.root.after(0, lambda e=exc_val: _show_message(
+                    self.root, "Transmutate — Error", e, kind="error"))
+            finally:
+                self.root.after(0, lambda: self.transform_btn.configure(text="Transmutate", state="normal"))
+
+        threading.Thread(target=run_conversion, daemon=True).start()
+
+    def _set_ui_enabled(self, enabled: bool):
+        """Enable/disable all interactive widgets."""
+        state = "normal" if enabled else "disabled"
+        for widget in self.options_frame.winfo_children():
+            self._set_child_states(widget, state)
+        self.transform_btn.configure(state=state)
+        if not enabled:
+            self.info_label.configure(text_color="#888")
+
+    @staticmethod
+    def _set_child_states(parent, state):
+        """Recursively set state on all child widgets."""
+        try:
+            children = parent.winfo_children()
+        except Exception:
+            return
+        for child in children:
+            if isinstance(child, (ctk.CTkCheckBox, ctk.CTkRadioButton, ctk.CTkSlider)):
+                child.configure(state=state)
+            elif isinstance(child, ctk.CTkButton) and child is not None:
+                child.configure(state=state)
+            else:
+                TransmutateApp._set_child_states(child, state)
+
+    # Command builder
+
+    def _build_command(self):
+        """Build an ffmpeg/magick command list for the current conversion."""
+        src = self.config.source_file
+        dst = self.config.output_file
+        fmt = self.config.target_format
+        quality = self.config.quality
+        aq = self.config.audio_quality
+        audio_streams = self.config.audio_streams
+        sub_streams = self.config.sub_streams
+        is_animated = self.config.is_animated
+        loop = self.config.loop
+        mime = self.config.mime_type
+
+        media = self.config.media_type
+
+        if media == "image":
+            return _build_image_command(
+                src, dst, fmt, quality,
+                is_animated, loop, mime,
+            )
+
+        elif media == "video":
+            if fmt == "gif":
+                # Video → GIF: use palettegen-based command
+                return _build_video_gif(src, dst, quality)
+            elif fmt == "webp":
+                # Video → animated WebP: use libwebp_anim
+                return _build_video_webp(src, dst, quality, loop)
+            elif fmt in AUDIO_FORMATS:
+                # Video → audio (e.g. MP4 → MP3)
+                return _build_audio_command(
+                    src, dst, fmt, quality, audio_streams,
+                )
+            return _build_video_command(
+                src, dst, fmt, quality, aq,
+                audio_streams, sub_streams, mime,
+            )
+
+        elif media == "audio":
+            return _build_audio_command(
+                src, dst, fmt, quality, audio_streams,
+            )
+
+        raise ValueError(f"Unknown media type: {media}")
+
+    @staticmethod
+    def _ext_for_format(fmt: str) -> str:
+        """Map a format key to its file extension."""
+        return {
+            "mp3": "mp3", "flac": "flac", "wav": "wav",
+            "ogg": "ogg", "m4a": "m4a", "aac": "aac",
+            "png": "png", "jpg": "jpg", "webp": "webp",
+            "avif": "avif", "gif": "gif",
+            "mp4": "mp4", "mkv": "mkv", "webm": "webm",
+            "avi": "avi", "mov": "mov",
+        }.get(fmt, fmt)
+
+
+def open_file(filepath: str):
+    """Entry point: launch the Transmutate GUI for *filepath*."""
+    ctk.set_appearance_mode("System")
+    ctk.set_default_color_theme("blue")
+
+    root = ctk.CTk()
+    TransmutateApp(root, filepath)
+    root.mainloop()