Beat-Sync Video Editing

Purpose

Provide domain expertise for creating beat-synced video edits: taking a source video and an audio track, selecting clips from the video that align with the music's rhythm, and rendering the final output with FFmpeg.

Core Concept: The EditPlan

Every edit starts as an EditPlan — a JSON structure that describes which video clips to use and where in the audio to place them:

{
  "audio_start": "00:13",
  "audio_duration": 6.5,
  "clips": [
    { "video_start": "00:08", "duration": 2.0, "description": "Opening shot" },
    { "video_start": "00:45", "duration": 1.5, "description": "Action moment" },
    { "video_start": "01:22", "duration": 3.0, "description": "Build-up" }
  ],
  "reasoning": "Matches rising intensity with beat drops"
}

Timestamp format: audio_start and video_start use MM:SS strings (e.g. "01:15" for 1 minute 15 seconds). audio_duration and clip duration use numbers in seconds.

Critical constraints:

audio_start must be valid MM:SS format, non-negative
audio_duration must be positive (seconds)
Every clip must have valid MM:SS video_start and positive duration (seconds)
Sum of all clip durations must equal audio_duration (within 0.5s tolerance)
Clip order is intentional — not necessarily chronological. Non-linear ordering creates dynamic edits.

Workflow: From Files to Final Video

Step 1: Generate EditPlan via Gemini

Run the Gemini script to analyze video + audio and produce a plan:

# Fresh upload (files auto-deleted after):
bash ${CLAUDE_PLUGIN_ROOT}/scripts/gemini-edit-plan.sh \
  --video <video_path> \
  --audio <audio_path> \
  --prompt "<user's edit description>"

# Keep files for reuse (outputs ECLIPTIC_FILES JSON to stderr):
bash ${CLAUDE_PLUGIN_ROOT}/scripts/gemini-edit-plan.sh \
  --video <video_path> \
  --audio <audio_path> \
  --prompt "<description>" --no-cleanup

# Reuse previously uploaded files (skips upload, much faster):
bash ${CLAUDE_PLUGIN_ROOT}/scripts/gemini-edit-plan.sh \
  --video-uri <uri> --video-mime <mime> \
  --audio-uri <uri> --audio-mime <mime> \
  --prompt "<different description>"

Outputs EditPlan JSON to stdout, progress to stderr
Requires GEMINI_API_KEY, curl, and jq
Gemini watches the video and listens to the audio simultaneously
Use --no-cleanup + reuse mode for fast iteration with different prompts

Step 2: Validate the Plan

echo '<plan_json>' | bash ${CLAUDE_PLUGIN_ROOT}/scripts/validate-plan.sh

Outputs {"valid": true, "errors": []} or {"valid": false, "errors": [...]}
Exit code 0 = valid, 1 = invalid

Step 3: Build FFmpeg Filters

echo '<plan_json>' | bash ${CLAUDE_PLUGIN_ROOT}/scripts/build-filter.sh

Outputs {"videoFilter": "...", "audioFilter": "...", "fullFilter": "..."}
The fullFilter field is what goes into FFmpeg's -filter_complex argument

Step 4: Render with FFmpeg

ffmpeg -y -i "<video_path>" -i "<audio_path>" \
  -filter_complex "<fullFilter>" \
  -map "[outv]" -map "[outa]" \
  -c:v libx264 -preset fast -crf 23 \
  -c:a aac -shortest \
  "<output_path>"

FFmpeg Filter Anatomy

For a 3-clip edit, the fullFilter looks like:

[0:v]trim=start=8.000:duration=2.000,setpts=PTS-STARTPTS[v0];
[0:v]trim=start=45.000:duration=1.500,setpts=PTS-STARTPTS[v1];
[0:v]trim=start=22.000:duration=3.000,setpts=PTS-STARTPTS[v2];
[v0][v1][v2]concat=n=3:v=1:a=0[outv];
[1:a]atrim=start=13.000:duration=6.500,asetpts=PTS-STARTPTS[outa]

[0:v] = first input (video), [1:a] = second input (audio)
trim extracts a segment, setpts=PTS-STARTPTS resets timestamps
concat joins all video segments in order
atrim extracts the audio section

For the full FFmpeg filter reference, see references/ffmpeg-filters.md.

Troubleshooting

Duration mismatch error: Clip durations don't sum to audio_duration. Fix by adjusting the last clip's duration to absorb the difference, or re-run Gemini with a stricter prompt.

FFmpeg "Error" in stderr: FFmpeg writes progress and warnings to stderr. Only treat it as a real error if the output file wasn't created. Check for actual error patterns like No such file, Invalid data, or Conversion failed.

Gemini returns poor clips: Add specificity to the prompt. Instead of "make an edit", say "make a fast 30-second action edit, cut every 1-2 seconds on the beat drops, start from the chorus".

Additional Resources

Reference Files

references/ffmpeg-filters.md — Detailed FFmpeg filter_complex syntax, encoding options, common flags
references/edit-plan-schema.md — Full EditPlan JSON schema, validation rules, edge cases

Scripts

${CLAUDE_PLUGIN_ROOT}/scripts/gemini-edit-plan.sh — Upload to Gemini via REST API, get EditPlan (supports --no-cleanup and file reuse)
${CLAUDE_PLUGIN_ROOT}/scripts/validate-plan.sh — Validate EditPlan JSON
${CLAUDE_PLUGIN_ROOT}/scripts/build-filter.sh — Convert EditPlan to FFmpeg filters
${CLAUDE_PLUGIN_ROOT}/scripts/cleanup-gemini-files.sh — Delete uploaded files from Gemini when done iterating

beat-sync-video-editing