wb-preview

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.