Video Compress

Process

1. Obtain input file

2. Probe the source

ffprobe -v quiet -print_format json -show_streams -show_format "$INPUT"

3. Determine mode from user intent

• CRF mode (quality-based): user says "without losing quality", "good quality", "make it smaller", or gives no size target
• 2-pass mode (size-based): user specifies a target ("under 50MB", "around 20MB", "fit on X")

4. Choose codec

• H.265 / libx265: target is ≤50% of original size, or user asks for "maximum compression" / "HEVC"
• H.264 / libx264: otherwise — faster encode, wider device compatibility, safe default

5. Construct command

ffmpeg -i "$INPUT" -c:v libx264 -crf 23 -c:a copy -movflags +faststart "$OUTPUT"
# CRF scale: 18=near-lossless, 23=default quality, 28=aggressive (visible loss)
# -movflags +faststart moves moov atom to front — enables progressive web playback

python3 ${CLAUDE_PLUGIN_ROOT}/skills/compress-video/scripts/calc_bitrate.py "$INPUT" --target-mb "$TARGET_MB"
# Outputs: VIDEO_BITRATE_KBPS (integer)
# Formula: (target_mb * 8192 / duration_s) - audio_bitrate_kbps
# Typical audio budget: 128 kbps
# Exit 1 = target too small (bitrate would go negative); report the error and ask for a larger target before retrying.

# Pass 1 — video analysis only, no output file:
ffmpeg -y -i "$INPUT" -c:v libx264 -b:v ${VIDEO_BITRATE_KBPS}k -pass 1 -an -f null /dev/null

# Pass 2 — final encode with audio:
ffmpeg -i "$INPUT" -c:v libx264 -b:v ${VIDEO_BITRATE_KBPS}k -pass 2 \
  -c:a aac -b:a 128k -movflags +faststart "$OUTPUT"

6. Confirm with user

7. Run and report

Key Decisions

• In CRF mode, use

  -c:a copy

  to preserve audio losslessly. In 2-pass mode, audio must be re-encoded (AAC 128k) because pass 1 is video-only — no audio stream is processed.
• If input is already H.264 and the user only wants to trim or remux, recommend

  convert-video

  with

  -c copy

  instead — instant and lossless.
• For H.265 output, substitute

  libx265

  and add

  -tag:v hvc1

  for Apple device compatibility.
• Clean up

  ffmpeg2pass-0.log

  and

  ffmpeg2pass-0.log.mbtree

  after 2-pass encoding completes.