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

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