av-tools/docs/FORMAT_SUPPORT.md

Format and feature support

FFmpeg capability is determined from the loaded core, not from filename extensions or this document. The app queries muxers, demuxers, decoders, encoders and filters; a preset is disabled with missing components listed. Browser playback is a separate capability checked with MIME/media APIs.

0.3.0 release fixture matrix

All 15 files are generated and re-probed with native FFmpeg 7.1.3 for test preparation and pass the checksum/probe verifier. That proves fixture provenance, not ffmpeg.wasm behavior. A 2026-07-24 Vitest run passed 303 tests with one conditional native subtitle-filter skip and includes broad execution of typed plans against native FFmpeg 7.1.3.

Chromium automation also runs a broad operation matrix against the real pinned ST core. The real MT core is queried and completes an H.264/AAC conversion, but the complete ST operation matrix has not been duplicated under MT. The following table distinguishes exact browser evidence from still-pending coverage; it must not be generalized to Firefox, Safari or mobile.

Input	Fixture	Expected streams	Browser preview expectation	Recorded pinned-core Chromium evidence
MP4 / H.264 + AAC	pattern-av.mp4, compatible clips	H.264 video, AAC audio	commonly playable after canPlayType/media load succeeds	broad ST operations; real MT conversion; exhaustive MT pending
WebM / VP8 + Vorbis	pattern.webm	VP8 video, Vorbis audio	commonly playable in Chromium/Firefox; browser-dependent elsewhere	ST probe, remux and VP8/Vorbis re-encode; MT pending
WAV / PCM s16le	tone.wav	mono PCM	commonly playable	ST WAV output re-probed; exact input fixture and MT pending
MP3	tone.mp3	mono MP3	commonly playable	ST MP3 outputs re-probed; exact input fixture and MT pending
FLAC	tone.flac	mono FLAC	browser/version-dependent	ST input and FLAC output re-probed; MT pending
MP3 + attached PNG	attached-cover.mp3	MP3 audio, attached-picture PNG	audio may play; cover rendering is app-owned	ST probe/stream selection; MT pending
MP4 tags	metadata.mp4	H.264/AAC	same as MP4	ST import/removal and output metadata round-trip; MT pending
MP4 chapters	chapters.mp4	H.264/AAC/data plus two chapters	media may play; browser does not expose all chapter data	ST generated-output chapter round-trip; fixture import pending
malformed MP4	truncated.mp4	no usable streams expected	must not preview	ST bounded failure, cleanup and recovery; MT pending

The runtime query for pinned core 0.12.10 recorded 341 demuxers, 173 muxers, 515 decoders, 204 encoders and 465 filters (1,698 entries total). Runtime discovery only says that FFmpeg exposes an item; the application must still build a valid plan, the input must be compatible, resources must fit and the browser may still be unable to preview the result.

Output presets

Preset availability always depends on detected runtime requirements:

Preset family	Required examples	Browser result preview
MP4 H.264/AAC	mp4, libx264, aac	usually, but verify actual output
WebM VP9/Opus	webm, libvpx-vp9, libopus	enabled when the loaded core advertises all requirements
WebM VP8/Vorbis	webm, libvpx, libvorbis	commonly Chromium/Firefox
Matroska H.264/AAC	matroska, libx264, aac	generally download-only in browsers
animated WebP	webp, libwebp_anim	browser-dependent image preview
GIF	gif, gif, palettegen, paletteuse	broadly image-previewable
MP3	mp3, libmp3lame	commonly playable
M4A/AAC	ipod, aac	commonly playable
Ogg Vorbis	ogg, libvorbis	browser-dependent
Opus	opus, libopus	Chromium ST/MT encode and re-probe recorded
WAV/PCM	wav, pcm_s16le	commonly playable
FLAC	flac, flac	browser-dependent

“Lossless” applies only to a lossless audio codec such as PCM/FLAC relative to its decoded samples. It does not make a lossy source lossless. Stream-copy remux avoids re-encoding compatible streams but may change container metadata.

Structural editing

• The Edit workspace exposes fast/accurate trim for the selected timeline range and fast/normalized concat for clips in visible timeline order. Availability still fails closed against loaded-core capabilities.
• Fast trim uses stream copy where container/stream/timestamp constraints are compatible. It can begin at a nearby keyframe and is not promised frame-accurate.
• Accurate trim decodes/re-encodes selected streams and can still be limited by source timestamps.
• Stream-copy concat requires compatible codec parameters, time bases, dimensions, sample layout and selected streams; the UI requires a completed compatibility check before enabling it. Otherwise normalized re-encoding is required.
• Normalized concat uses an explicit export preset and requires an explicit insert-silence, drop-all-audio or reject policy when clips have mismatched audio presence.
• Normalized concat can overlap every edit with one uniform 0.04–10 second xfade and, when audio is retained, acrossfade. End clips must contain at least one transition duration and internal clips two; the active core must expose each required filter.
• Crop, resize, frame-rate conversion, fades, normalization, burn-in and contact sheets require specific filters discovered at runtime.
• Even-dimension correction is applied when the selected pixel format/encoder requires it and is reported in the plan/result.

Images and scene selection

Thumbnail/contact-sheet scene mode first performs a bounded full-source analysis with select/showinfo, stopping after at most 100 qualifying frames. Extraction then selects those exact source PTS values; no guessed time rounding or remote analysis is used. If no frame meets the 0.01–0.95 threshold, the operation reports that result and asks for a lower threshold or a deterministic selection mode.

Contact-sheet labels require runtime drawtext. The app mounts only its same-origin DejaVu Sans 2.37 asset after verifying 757,076 bytes and SHA-256 7da195a74c55bef988d0d48f9508bd5d849425c1770dba5d7bfc6ce9ed848954. No remote or host-system font fallback is used. SOURCE.md and LICENSES/DEJAVU_FONTS.txt provide provenance and terms.

Subtitles

The text fixtures cover SRT, WebVTT and ASS import with explicit time shift. The Chromium ST matrix converts a generated SRT to WebVTT with a time shift, soft-muxes it into Matroska, burns it into H.264 video and extracts the soft-muxed stream. The exact standalone WebVTT and ASS fixtures and equivalent MT paths are not separately browser-smoked. Soft-subtitle muxing depends on container support (for example converted text in MP4 versus text/ASS in Matroska). ASS burn-in requires subtitles/libass and local font support; no remote font is fetched. Bitmap subtitle streams are identified but are not claimed to support arbitrary conversion or OCR. Unsupported container/codec combinations must remain disabled with reasons.

Metadata and chapters

Format and stream tags can be inspected. Edits/removal are limited to fields the selected muxer writes; “remove metadata” is best effort and the output is re-probed to report remaining structural/encoder/container fields. Stream language, title and dispositions are separate from format tags.

Chapters use validated integer time bases, monotonic non-overlapping ranges and escaped titles. Some output containers do not support chapters. Browser media elements do not reliably expose them, so ffprobe round-trip is authoritative.

Known unsupported or conditional cases

• remote protocols, live capture, DRM/encrypted media and network playlists;
• hardware-accelerated encoding or desktop-FFmpeg performance;
• files above the 2 GiB WASM input ceiling or stricter product policy;
• arbitrary filters/commands, arbitrary compositing and nonlinear overlap;
• frame-accurate stream-copy trimming;
• complete metadata erasure/anonymity;
• browser preview of every FFmpeg-readable result;
• Opus on browsers, inputs or deployments outside the exact reviewed Chromium regression: both ST and MT pass the non-empty encode/re-probe gate, but runtime capability discovery and normal per-job limits still apply;
• any capability omitted by the active core's runtime query.

Update this file only from re-probed generated/public-domain fixtures and record the engine mode, browser and exact core version.