wjs-segmenting-video

Cut a long video + SRT into multiple stand-alone short clips, each oriented for the target platform. This skill stops after cutting + cropping — it hands off the raw clips to /wjs-overlaying-video for covers, captions, illustrations, CTA, and final render.

When to use

• Long-form video (≥10 min) with an existing SRT transcript.
• Goal is stand-alone short clips (each viewable without context).
• The user will (or you will) drive post-production separately in /wjs-overlaying-video.

When NOT to use

• Single-topic trimming → just use ffmpeg -ss A -to B.
• No transcript yet → run /wjs-transcribing-audio first (then /wjs-translating-subtitles if the segments need a non-source language).
• Multicam editing → use /wjs-editing-multicam.
• Highlight reel with multiple cuts inside a single topic → that's editing, not segmentation.

What this skill IS — and IS NOT

The pipeline

long video + SRT
   ↓     (agent reads SRT, decides topics — judgment, not parsing)
segments.json
   ↓     segment.py --reencode (accurate seek; clip starts exactly at requested t)
clip_NN.mp4 + frame_NN.jpg
   ↓     ASK: target platform orientation match source?
   ↓     /wjs-reframing-video on each clip (if 16:9 → 9:16, etc.)
   ↓     re-extract frames from cropped clips
clip_NN.mp4 (now in target orientation) + clip_NN.zh-CN.burn.srt
   ↓
HAND OFF → /wjs-overlaying-video
   (does covers + captions + illustrations + CTA + final render)

Step 1 — Read SRT, write segments.json

Don't outsource topic identification to a script. For each candidate segment, judge:

• Self-contained? A cold viewer must understand it without prior context.
• Single thread? One central question / insight; if the speaker pivots mid-clip, that's two segments.
• Length fits platform? 60–180s for 视频号 / 30–60s for 抖音&Shorts. \x3C30s feels truncated; >4min loses retention.
• Hook + payoff? Open on a claim / question / vivid image; close on a takeaway. Never end mid-sentence.
• Snap to SRT cue boundaries — never cut mid-word.

3–6 strong segments from a 10-minute source is normal. Drop boring middles. Quality > quantity.

Schema (full spec in references/segments_schema.json, example in references/example_segments.json):

{
  "source_video": "input.mp4",
  "source_srt": "input.zh-CN.srt",
  "platform": "wechat_channels",
  "segments": [{
    "id": 1, "slug": "intent-not-code",
    "title": "AI 时代不是写代码\
而是写意图",
    "summary": "Two-sentence pitch — what's the insight, what's at stake.",
    "start": "00:00:43.460", "end": "00:02:35.220",
    "cover_prompt": "Visual concept for gpt-image-2 (style anchor, not literal scene)"
  }]
}

slug = kebab-case English (used in filenames). title uses \ for line break, 2 lines max, 8–12 Chinese chars per line. cover_prompt is consumed downstream by /wjs-overlaying-video's cover-generation step — keep it written here so the overlay skill can pick it up without re-asking.

Step 2 — Accurate-seek cut

python3 ~/.claude/skills/wjs-segmenting-video/scripts/segment.py \
    --segments segments.json --out output/ --reencode

--reencode is the default recommended mode. It cuts with ffmpeg -ss N -i src -c:v libx264 -c:a aac so the output starts EXACTLY at the requested timestamp. ~30s per clip on CPU. Also extracts a midpoint frame per segment to output/frame_NN_slug.jpg.

Why default to --reencode and not stream-copy:

Stream-copy via ffmpeg -ss N -c copy seeks to the nearest keyframe before N (it can't re-encode). The output's t=0 then maps to source t=keyframe, so the clip plays a fraction of a second of "lead-in" content before the requested speech. Captions sliced from the master SRT at boundary N appear AHEAD of the audio by exactly that GOP fraction — listeners feel "subtitles lead the voice."

In practice on H.264 source with GOP=2s: every clip is off by 0.6–1.5s. Looks like a synchronization bug downstream; it's actually a cut-time bug upstream.

Stream-copy variant (only if you control the source encode)

If the source has been re-encoded with -force_key_frames at every requested cut boundary, stream-copy IS accurate. Workflow:

# Build the comma-separated keyframe list from segments.json
KF=$(python3 -c "import json; s=json.load(open('segments.json'))
ts=[]
for seg in s['segments']:
    ts += [seg['start'], seg['end']]
print(','.join(ts))")

# Re-encode master once, forcing keyframes at all segment boundaries
ffmpeg -i master.mp4 \
  -c:v libx264 -preset medium -crf 18 \
  -force_key_frames "$KF" \
  -c:a copy master_kf.mp4

# Now stream-copy cuts land exactly:
python3 segment.py --segments segments.json --source master_kf.mp4 --out output/

Use this only when iterating on segment boundaries (you'll re-cut the same source many times). For one-shot work, --reencode is simpler and just as correct.

Diagnosing keyframe-snap on already-cut clips

ffprobe -v error -select_streams v:0 -read_intervals "$((N-2))%$((N+5))" \
  -show_entries packet=pts_time,flags -of csv=p=0 master.mp4 | grep "K_"

Output like 360.023,K__ 362.023,K__ → GOP=2s. A -c copy cut at 361.000 actually starts at 360.023, captions are 0.977s ahead of audio. The retroactive fix is a per-clip SRT offset shim (requested_start − nearest_preceding_keyframe) added to every cue's start/end, but the root fix is to re-cut with --reencode.

Step 3 — Orientation check (ask before continuing)

Compare source video aspect ratio to the target platform:

Probe with ffprobe:

ffprobe -v error -select_streams v:0 \
  -show_entries stream=width,height -of csv=p=0 clip_01_*.mp4

If source aspect already matches the platform → skip this step.

If mismatch → ASK THE USER before converting. Sample phrasing:

源视频是横屏 (1920×1080)，平台 视频号 需要竖屏 (9:16)。是否对每段 调用 /wjs-reframing-video 转成竖屏？(crop 会用 MediaPipe 跟踪正在说话 的人的脸，保持说话人始终在画面中)

Never silently skip the check — finding out at upload time that your horizontal clip needs to be vertical is a frustrating failure mode the skill exists to prevent.

Calling /wjs-reframing-video

The crop script needs mediapipe + opencv + numpy in a Python 3.12 venv (mediapipe doesn't ship wheels for 3.14+). One-time setup:

uv venv --python 3.12 /tmp/_crop_venv
/tmp/_crop_venv/bin/python -m pip install mediapipe opencv-python numpy

Per-clip invocation:

for n in 01 02 03 04 05; do
  slug=$(ls clip_${n}_*.mp4 | grep -v -E "_intro|_burned|_vert" | head -1 | sed -E "s/clip_${n}_(.+)\.mp4/\1/")
  /tmp/_crop_venv/bin/python ~/.claude/skills/wjs-reframing-video/scripts/crop.py \
    "clip_${n}_${slug}.mp4" \
    --out "clip_${n}_${slug}_vert.mp4" \
    --target portrait \
    --bitrate 8M    # 视频号 caps at 10Mbps
done

After cropping, swap the cropped versions to canonical names so downstream pipelines find them:

mkdir -p _horizontal_archive
for n in 01 02 03 04 05; do
  base=$(ls clip_${n}_*_vert.mp4 | sed -E "s/_vert\.mp4$//")
  mv "${base}.mp4" "_horizontal_archive/"
  mv "${base}_vert.mp4" "${base}.mp4"
  # Re-extract midpoint frame:
  mid=$(ffprobe -v error -show_entries format=duration -of csv=p=0 "${base}.mp4" | awk '{print $1/2}')
  slug=$(echo "$base" | sed -E "s/^clip_${n}_//")
  ffmpeg -hide_banner -loglevel error -ss "$mid" -i "${base}.mp4" \
    -frames:v 1 -q:v 3 "frame_${n}_${slug}.jpg" -y
done

Sanity check: face-on-screen detection rate in the crop log can read low (e.g. face#0: 9.6s on screen (9%)) when speakers sit further than ~2 m from the camera. That number being low is OK — the active-speaker hysteresis + fallback-to-largest-face still produces well-centered crops. Verify visually by extracting a midpoint frame and confirming the speaker is centered before committing.

Step 4 — Slice per-clip SRTs

python3 ~/.claude/skills/wjs-segmenting-video/scripts/burn_subs.py \
    --segments segments.json --out output/ --no-burn

The --no-burn flag emits per-clip SRTs (clip_NN_slug.zh-CN.burn.srt) with timestamps already shifted to start at 0 — exactly the input /wjs-overlaying-video captions expect (its compositions start the body at t=cover_duration, not the master clock).

Despite the legacy name burn_subs.py, this step does NOT burn pixels in --no-burn mode — it's just an SRT slicer. (The burn-pixels mode exists for the legacy "Path A" workflow but is deprecated in favor of /wjs-overlaying-video's HTML/CSS caption rendering.)

Hand-off package — what to deliver to /wjs-overlaying-video

After Steps 1–4, deliver EXACTLY these per-segment artifacts:

output/
  clip_NN_slug.mp4                  # raw cropped clip (target orientation, no subs, no cover)
  clip_NN_slug.zh-CN.burn.srt       # per-clip SRT, timestamps shifted to start at 0
  frame_NN_slug.jpg                 # midpoint frame (cover reference)
  segments.json                     # for slug/title/summary/cover_prompt metadata

Then invoke /wjs-overlaying-video to add covers, captions, illustrations, CTA, and produce the upload-ready MP4 per clip. The overlay skill generates ONE final composition per clip and renders it in a single encode (no cascade of re-encodes).

Quick reference

Common mistakes

• Cutting mid-sentence — always snap to SRT cue boundaries.
• Trying to use 100% of the video — 3–6 strong clips from 10 min is normal. Boring middle = drop.
• Letting the LLM write the title — the title is judgment, not summary. Review and rewrite before passing to make_cover.
• Stream-copy without --force_key_frames preprocessing — produces clips with audio ahead of captions by up to 1 GOP. Use --reencode (default) unless the source was specifically prepared.
• Skipping the orientation check — getting a horizontal podcast on 视频号 and finding out at upload time is preventable. Probe aspect and ask the user before cropping.
• Burning subs / generating covers in THIS skill — those moved to /wjs-overlaying-video. This skill stops after Step 4.

Integration with other skills

• /wjs-transcribing-audio — produce the source SRT first if missing. The word-level Whisper output (or Volcano/豆包 ASR output) is preferred for accurate cue timing. If the segments need translating, chain into /wjs-translating-subtitles.
• /wjs-reframing-video — call in Step 3 when source orientation doesn't match target platform. Face-tracked active-speaker following keeps the talker in frame.
• /wjs-editing-multicam — if the source is multi-cam, render the synced single MP4 first, then segment.
• /wjs-overlaying-video — the default downstream for everything after Step 4. Covers, captions, illustrations, CTA, and final render all happen there. Don't add post-production in this skill.

Files & references

• scripts/segment.py — accurate-seek + stream-copy cutting
• scripts/burn_subs.py — SRT slicer (--no-burn mode); legacy libass burn-in mode is deprecated in favor of /wjs-overlaying-video
• references/segments_schema.json — JSON Schema for segments.json
• references/example_segments.json — worked example