video-clip-skill

Clips a YouTube video locally using yt-dlp and ffmpeg. Supports auto-highlight detection, translation, and CapCut-style karaoke subtitle burning. Triggers when the user wants local video clipping, highlight extraction, or subtitle generation. Optional GROQ_API_KEY env var enables Whisper transcription fallback when YouTube has no subtitles.

Video Clip (Local)

Requires yt-dlp, ffmpeg, and python3. Check with command -v.

Finding plugin scripts

The ASS karaoke generator is bundled with this plugin. Locate it once at the start (this only searches for the plugin's own bundled file):

bash

ASS_SCRIPT=$(find ~/.claude/plugins -path '*/clip-local/*/scripts/ass-karaoke.py' 2>/dev/null | head -1)

Auto-highlight mode

When the user does NOT specify start/end times (e.g., "幫我剪這個影片的精華" or "clip the best parts"):

1. Download the full transcript (step 1–2 below)
2. Read the entire transcript and identify 3–5 highlight segments. For each, note:
   • Start and end timestamps
   • A short description of why it's interesting (key insight, funny moment, dramatic turn, etc.)
3. Present the highlights to the user as numbered options and ask which ones to clip
4. Clip only the segments the user picks, then continue with the normal pipeline (translate, subtitle, etc.)

Pipeline

1. Get video info and original language

bash

yt-dlp --print title --print duration_string --print language \
  --no-playlist --no-warnings --force-ipv4 "<URL>"

The third line is the original language code (e.g., en, en-US, ja, zh-Hant). Use the base code (before -) for subtitle download.

2. Download original language subtitles

bash

yt-dlp --write-auto-sub --sub-lang "<LANG>*" --sub-format vtt --skip-download \
  --no-playlist --no-warnings --force-ipv4 \
  --extractor-args 'youtube:player-client=default,mweb' \
  -o "subs" "<URL>"

Replace <LANG> with the base language code from step 1 (e.g., en, ja). The * wildcard matches variants like en-orig. Do NOT use YouTube's auto-translated subs — they are low quality. All translation is done by you.

3. Trim VTT to clip range

When clipping a portion (e.g., 10–130s), filter the VTT to only include cues whose timestamps fall within the range. Keep the original absolute timestamps — do NOT adjust them. The --offset flag in ass-karaoke.py handles the time shift.

When filtering, strip any extra metadata from timestamp lines (e.g., align:start position:0%) — keep only HH:MM:SS.mmm --> HH:MM:SS.mmm. The ASS parser regex expects clean timestamp lines.

4. Translate subtitles

Write and execute a Python script that:

1. Parses the trimmed VTT (regex: HH:MM:SS.mmm --> HH:MM:SS.mmm + text lines)
2. Collects all text lines into a list
3. You translate the list (print a Python list of translated strings)
4. Writes a new VTT with identical timestamps and translated text

Example structure:

python

import re

# Parse original VTT
with open("clip.vtt") as f:
    content = f.read()
cues = re.findall(r'(\d{2}:\d{2}:\d{2}\.\d{3} --> \d{2}:\d{2}:\d{2}\.\d{3})\n((?:(?!\d{2}:\d{2}).+\n?)*)', content)

# Translations — fill this list with your translations, one per cue
translations = [
    "translated line 1",
    "translated line 2",
    # ...
]

# Write translated VTT
with open("clip_translated.vtt", "w") as f:
    f.write("WEBVTT\n\n")
    for (timestamp, _), translation in zip(cues, translations):
        f.write(f"{timestamp}\n{translation.strip()}\n\n")

Generate this script with the translations list filled in, then execute it. This ensures timestamps stay exact and VTT format is correct.

5. Generate ASS karaoke subtitles

bash

python3 "$ASS_SCRIPT" <original.vtt> -o subs.ass -t <translated.vtt> --offset <START_SECONDS>

• First arg = original language VTT (karaoke timing on top line)
• -t = translated VTT (shown below karaoke line)
• --offset = clip start time (adjusts timestamps relative to clip start)

Handles YouTube rolling caption dedup, CJK per-char splitting, and bilingual layout.

6. Resolve stream URLs and clip

Get both video + audio URLs in one call:

bash

URLS=$(yt-dlp --get-url -f 'bv[height<=720]+ba/b[height<=720]' \
  --no-playlist --no-warnings --force-ipv4 \
  --extractor-args 'youtube:player-client=default,mweb' "<URL>")
VIDEO_URL=$(echo "$URLS" | head -1)
AUDIO_URL=$(echo "$URLS" | tail -1)

Then clip with ffmpeg:

• With subtitles: -vf "ass=subs.ass" -c:v libx264 -preset fast -crf 23 -c:a aac -b:a 128k -movflags +faststart
• Without subtitles: -c copy -avoid_negative_ts make_zero
• Input seeking: -ss <START> before each -i
• Separate streams: -map 0:v:0 -map 1:a:0

Whisper fallback (no YouTube subs)

If yt-dlp finds no auto-subs and user has GROQ_API_KEY set:

1. Download audio: yt-dlp -f ba -x --audio-format mp3 --postprocessor-args 'ffmpeg:-ac 1 -ar 16000 -b:a 64k'
2. Transcribe: POST https://api.groq.com/openai/v1/audio/transcriptions with model=whisper-large-v3, response_format=verbose_json
3. Convert segments to VTT

If GROQ_API_KEY is not set, inform the user that no subtitles are available and ask how to proceed (clip without subs, or set the key).

Common issues

• YouTube throttling: export cookies to a file and use --cookies cookies.txt
• Missing CJK fonts for ASS: brew install font-noto-sans-cjk-tc (macOS)
• Groq 25MB audio limit: split audio for videos >50min
• Stream URLs expire ~6h: re-resolve if clip fails
• Subtitle burning re-encodes video (~1–3 min for 60s clip)