Media Gpac

Media Gpac

Context: $ARGUMENTS

Quick start

• Inspect a broken MP4: → Step 2 (info / diso)
• Package DASH/CMAF: → Step 3 (dash subcommand)
• Fragment for fMP4: → Step 3 (fragment)
• Extract a raw track (h264/aac ES): → Step 3 (extract-track)
• CENC encrypt for ClearKey testing: → Step 3 (encrypt)
• Hand commercial DRM to Shaka instead: → media-shaka skill

When to use

• Doing ISO-BMFF surgery ffmpeg can't (box-level edits, precise track IDs, edit-list fixes).
• Packaging CMAF/DASH where fragment timing must be accurate (MP4Box beats ffmpeg's dash muxer).
• Dumping the full box tree to XML for diagnostics (-diso).
• Repairing broken moov/moof boxes, rewriting sidx, or setting language/default flags in-place.
• Authoring ClearKey-encrypted CENC content for pipeline testing.

Step 1 — Install

# macOS
brew install gpac

# Debian/Ubuntu
sudo apt install gpac

# Fedora/RHEL
sudo dnf install gpac

# Verify
MP4Box -version
gpac -version

MP4Box is the legacy (stable, widely documented) CLI for ISOBMFF surgery. gpac is the newer filter-graph CLI (similar philosophy to ffmpeg filters). Prefer MP4Box for most tasks — it's documented everywhere.

Step 2 — Pick the operation

Always pass -out when you want a new file — MP4Box edits in-place otherwise. Back up first.

Step 3 — Run via the helper script

The scripts/gpac.py wrapper gives an argparse interface over these operations. Stdlib-only, non-interactive, --dry-run prints the command.

# Inspect
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py info --input in.mp4
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py diso --input in.mp4 --output boxes.xml

# Extract raw track 1 (ES)
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py extract-track --input in.mp4 --track 1 --output track1.h264

# Fragment (2-second fragments, in-place copy with -out)
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py fragment --input in.mp4 --output frag.mp4 --fragment-ms 2000

# DASH/CMAF (live profile)
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py dash --input in.mp4 --outdir dashout --segment-ms 4000 --profile live

# CENC ClearKey encrypt
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py encrypt \
  --input in.mp4 --output enc.mp4 \
  --key-id 0123456789abcdef0123456789abcdef \
  --key fedcba9876543210fedcba9876543210

# Remove track 2 (usually audio #2)
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py remove-track --input in.mp4 --output stripped.mp4 --track 2

# Set language on track 2
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py set-lang --input in.mp4 --track 2 --lang jpn

# Split by time
uv run ${CLAUDE_SKILL_DIR}/scripts/gpac.py split-time --input in.mp4 --output-pattern seg --start 00:00:10 --end 00:00:30

Add --dry-run to print the MP4Box command without executing. Add --verbose to echo the command before running.

Step 4 — Verify

After any authoring step, confirm result with:

MP4Box -info out.mp4

For DASH, validate the manifest:

xmllint --noout manifest.mpd
ls -la dashout/   # should contain init.mp4 + seg_*.m4s (live) OR one onDemand file

For CENC: play with ffplay -decryption_key KEY enc.mp4 or a Shaka-Player ClearKey page. For fragmented MP4: MP4Box -info shows Fragmented: yes and lists moof count.

Available scripts

• scripts/gpac.py — argparse wrapper for MP4Box: check, info, diso, extract-track, fragment, dash, encrypt, decrypt, remove-track, set-lang, split-time. Generates DRM XML for CENC ops in a temp file.

Reference docs

Read references/gpac.md when:

• Choosing a DASH profile (live, onDemand, main, full).
• Looking up an ISO-BMFF box (moov, moof, sidx, senc, saio, saiz, pssh, mvex).
• Writing a CENC XML by hand.
• Translating MP4Box idioms to the new gpac filter graph syntax.

Gotchas

• MP4Box vs gpac: both ship with the GPAC framework. MP4Box is legacy CLI (stable, everywhere). gpac (lowercase) is the newer filter-graph CLI — gpac -i input -o output with a filter pipeline; list plugins via gpac -h filters. Prefer MP4Box unless you need filter-graph semantics.
• In-place edits: MP4Box mutates the input file unless you pass -out. Always back up or pass -out.
• Track IDs start at 1, not 0. MP4Box -info lists them.
• -raw TID extracts the ES (elementary stream: raw H.264 NAL units, raw AAC ADTS, etc.) — not a container. Feed it back in via -add track.h264.
• -diso dump is huge — dumps the entire box tree as XML. Redirect to a file; don't cat it.
• -dash 4000 = 4000 ms segment duration. -frag is fragment duration inside each segment (usually equal to -dash).
• -rap forces each segment to start on a random access point (IDR). Omit and you will get unseekable DASH.
• sidx = Segment Index box. Required for byte-range DASH (onDemand profile). Live profile uses separate .m4s files and doesn't need it.
• CMAF segments are .m4s + init.mp4. Sent as independent files over HTTP.
• -dash-profile live → segmented, fragmented, templated URLs ($Number$). onDemand → single file with indexed byte ranges. main/full = older DASH profiles.
• -dash-ctx <file> saves packaging state — required for continuous live DASH across MP4Box invocations.
• CENC encryption XML follows a GPAC-specific schema (see references). KID and key are hex with 0x prefix. IV_size is 8 (CTR-64) or 16 (CTR-full). first_IV seeds the counter.
• MP4Box does CENC but NOT commercial DRM (Widevine / PlayReady / FairPlay license-server signaling). For those, package with MP4Box first, then hand off to Shaka Packager — see media-shaka skill.
• senc vs saio/saiz: MP4Box writes the correct auxiliary-info boxes; don't mix CENC between tools mid-pipeline.
• ffmpeg philosophy vs MP4Box: ffmpeg is general-purpose (many containers, filters, codecs). MP4Box is ISOBMFF-surgeon — precise box manipulation ffmpeg's demuxer/muxer can't express.
• -tight reorders the moov box for streaming (fast-start). Equivalent to ffmpeg's -movflags +faststart.
• Disk space: MP4Box writes temp files next to the input. Large DASH packaging can briefly 2x disk use.
• -splitx T1:T2 uses timecodes (e.g., 00:01:00:00:02:00). Some versions want -splitx T1/T2 syntax — check MP4Box -h split on your build.

Examples

Example 1: Package a mezzanine MP4 for DASH live

MP4Box -dash 4000 -frag 4000 -rap \
  -segment-name 'seg_$RepresentationID$_$Number$' \
  -dash-profile live \
  -out dashout/manifest.mpd \
  input.mp4#video input.mp4#audio

Result: dashout/manifest.mpd, per-representation init.mp4, and seg_video_1.m4s, seg_audio_1.m4s, ...

Example 2: Repair an MP4 that ffmpeg can't stream

# 1. Inspect: look for missing sidx, broken edit list, or non-fragmented moov
MP4Box -info broken.mp4

# 2. Rewrite with fragments + sidx
MP4Box -frag 2000 -add-sidx 2000 -out fixed.mp4 broken.mp4

# 3. Verify
MP4Box -info fixed.mp4   # "Fragmented: yes", moof count > 0, sidx present

Example 3: CENC ClearKey encrypt for pipeline testing

drm.xml:

<GPACDRM>
  <CrypTrack trackID="1" IsEncrypted="1" IV_size="8"
             first_IV="0x0123456789abcdef" saiSavedBox="senc">
    <key KID="0xABCDEF01234567890ABCDEF012345678"
         value="0x112233445566778899AABBCCDDEEFF00"/>
  </CrypTrack>
</GPACDRM>

MP4Box -crypt drm.xml -out enc.mp4 in.mp4

Decrypt with the same XML: MP4Box -decrypt drm.xml -out dec.mp4 enc.mp4.

Example 4: Extract H.264 ES, remux into new MP4

MP4Box -raw 1 in.mp4          # writes in_track1.h264
MP4Box -raw 2 in.mp4          # writes in_track2.aac
MP4Box -add in_track1.h264 -add in_track2.aac -new combined.mp4

Troubleshooting

Error: Cannot find adaptation set

Cause: DASH packager couldn't correlate streams — usually bad #video/#audio track-selection syntax. Solution: list tracks via MP4Box -info. Use input.mp4#trackID=1 explicit syntax instead of #video.

Error: Segment duration not a multiple of frame rate

Cause: -dash 4000 doesn't align to the GOP. With 24 fps + 2s GOP, pick 4000/8000 ms (multiples of GOP ms). Solution: re-encode source with a fixed GOP matching your segment duration, or use -rap and accept slightly variable segment sizes.

Error: Cannot write to file: permission denied

Cause: MP4Box tries to edit input in-place and the file is read-only. Solution: pass -out newname.mp4 explicitly.

Error: Unknown track reference

Cause: operating on a track ID that doesn't exist. Solution: MP4Box -info file.mp4 — track IDs are 1-based and listed in the summary.

Error: Encryption: wrong KID/key format

Cause: hex values missing 0x prefix or wrong length (KID must be 16 bytes / 32 hex chars; key must be 16 bytes / 32 hex chars). Solution: double-check lengths. openssl rand -hex 16 generates valid values.