docs: image sequence export design

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/plans/2026-04-06-image-sequence-design.md

@@ -0,0 +1,38 @@
+# Image Sequence Export Design
+
+## Overview
+
+Add lossless WebP image sequence export as an alternative to MP4. A format dropdown next to the Export button selects between "MP4" and "WebP sequence". All existing filter chain options (resize, portrait crop) apply to both formats.
+
+## UI Changes
+
+A `QComboBox(["MP4", "WebP sequence"])` is inserted immediately left of the Export button (between the Portrait dropdown and the counter label). No new button. Selection is persisted via `QSettings("8cut", "8cut")` key `"export_format"`.
+
+## Output Path
+
+WebP sequence output: `<folder>/<stem>_NNN/` directory, same counter logic as MP4.
+Frames inside: `frame_0000.webp`, `frame_0001.webp`, …
+
+`build_export_path` is MP4-only. A new `build_sequence_dir` function returns the directory path using the same `<stem>_NNN` counter pattern.
+
+## ffmpeg Changes
+
+`build_ffmpeg_command` gains `image_sequence: bool = False`.
+
+When `True`:
+- Output argument: `<dir>/frame_%04d.webp`
+- Appends: `-vcodec libwebp -lossless 1 -compression_level 4`
+- Filter chain (resize, portrait crop) unchanged
+
+When `False`: unchanged behavior.
+
+## ExportWorker
+
+When `image_sequence=True`, `os.makedirs(output_dir, exist_ok=True)` before running ffmpeg. `_on_export_done` sets `_last_export_path` to the sequence directory path.
+
+## Testing
+
+New `test_utils.py` cases:
+- `build_ffmpeg_command` with `image_sequence=True`: verify `-vcodec libwebp`, `-lossless 1`, `-compression_level 4`, output ends with `frame_%04d.webp`
+- With `image_sequence=True` and `short_side=256`: verify `-vf` still present
+- `build_sequence_dir`: verify path pattern matches `<stem>_NNN` directory form