| name | wb-preview |
| description | Render a labeled before/after preview clip showing a proposed white-balance, color, or exposure correction. Use when the user (or Claude) wants to evaluate a color tweak before committing to a full re-render — e.g. "show me what +500K would look like", "preview this WB fix", "before/after on the color correction". Output is a short side-by-side or sequential MP4 with on-screen labels. |
| disable-model-invocation | false |
| allowed-tools | Bash(ffmpeg *), Bash(ffprobe *), Bash(test *), Bash(mkdir *), Bash(ls *), Bash(basename *), Bash(dirname *), Bash(awk *), Read, Write |
White-Balance / Color Preview
Render a tiny labeled clip — original on the left, corrected on the right (or sequential before → after) — so the user can eyeball a color correction before re-rendering the whole video.
Pairs naturally with video-timeline: timeline → diagnose, wb-preview → propose.
Procedure
1. Inputs
| Field | Example | Default |
|---|
| Source video | clip.mp4 | required |
| Timestamp | 00:01:23 (start of preview window) | midpoint of video |
| Window | seconds around timestamp | 10 |
| Layout | sidebyside / sequential | sidebyside |
| Correction | filter spec (see below) | required |
2. Correction filter spec
Accept any of these forms, build the filter chain:
| User intent | ffmpeg filter |
|---|
| "warmer +500K" | colortemperature=temperature=6000 (or apply colorbalance=rs=0.05:bs=-0.05) |
| "cooler -300K" | colortemperature=temperature=5000 |
| "+0.3 EV" | eq=brightness=0.06 (rough; 0.06 ≈ 0.3 EV) |
| "less green" | colorbalance=gm=-0.1 |
| "tint magenta" | colorbalance=gm=0.1 |
| Custom | accept a raw filter string verbatim |
Always also offer an "auto" option: eq=auto-ish, or a measured neutralize via curves=preset=increase_contrast + a manual nudge — but only if the user asks.
3. Build the preview
Sequential (simpler, fewer ffmpeg gotchas):
ffmpeg -hide_banner -y \
-ss "$START" -t "$WINDOW" -i "$VIDEO" \
-ss "$START" -t "$WINDOW" -i "$VIDEO" \
-filter_complex "
[0:v]drawtext=text='BEFORE':fontcolor=white:box=1:boxcolor=black@0.6:boxborderw=8:x=20:y=20:fontsize=36[a];
[1:v]<CORRECTION>,drawtext=text='AFTER\\: <label>':fontcolor=white:box=1:boxcolor=black@0.6:boxborderw=8:x=20:y=20:fontsize=36[b];
[a][b]concat=n=2:v=1:a=0[v];
[0:a][1:a]concat=n=2:v=0:a=1[a]
" \
-map '[v]' -map '[a]' \
-c:v libx264 -crf 22 -preset veryfast \
-c:a aac -b:a 128k \
"$OUT"
Side-by-side (hstack):
... -filter_complex "
[0:v]drawtext=text='BEFORE'...[a];
[1:v]<CORRECTION>,drawtext=text='AFTER'...[b];
[a][b]hstack=inputs=2[v]
" -map '[v]' -map '0:a' ...
Use libx264 + CRF 22 here — preview clips don't need GPU encode, and software keeps it portable across systems even if profile-system hasn't run.
4. Output
Write to:
<project>/renders/wb_preview_<timestamp>_<label>.mp4 if inside an index project, OR- sibling to the source:
<dir>/wb_preview/<basename>_<label>.mp4
5. Report
Print the output path, total length, file size. Offer:
- Open in default player (
xdg-open).
- Adjust the correction (re-run with new params).
- Apply the corrected filter to the full video via
transcode once the user is happy.
