magnus919_agent-skills/ffmpeg/references/ffmpeg-edit-decision-lists.md

FFmpeg Edit Decision Lists

An edit decision list (EDL) is the reviewable source of truth between editorial intent and an FFmpeg render plan. Keep it data-oriented, versioned, and independent of private filesystem paths.

Canonical semantics

Use opaque asset IDs and identify streams explicitly. For each source, retain probe evidence for stream index, time base, start time, duration, cadence, audio layout, and source digest.

Define ranges as half-open intervals, [in, out), on the selected source stream timeline. State the time unit and precision. Decimal seconds are convenient for review; preserve exact timestamps or integer ticks when frame/sample boundaries matter. Never silently treat timecode, container time, wall-clock time, and frame number as interchangeable.

Each event should record:

• stable event ID and action (keep, remove, insert, or treatment);
• source asset and stream references;
• source in and out, plus any transition handles;
• destination order or lane;
• linked audio/video policy;
• rationale, evidence locators, confidence, and reviewer status;
• transformations, transition type/duration, and expected duration effect;
• whether the cut requires decoded precision or permits packet-level copy;
• verification points around the resulting boundary.

Store commands as generated render records, not as the EDL’s only meaning. Raw paths and shell fragments are unsafe substitutes for structured fields.

Pre-render validation

Reject or flag an EDL when:

• a source, stream, time unit, or range endpoint is missing;
• in >= out, a range falls outside declared source bounds, or rounding behavior is undefined;
• events overlap unintentionally or leave an unexplained gap;
• linked streams use incompatible timelines or omit a sync policy;
• transitions lack sufficient handles or their overlap is absent from duration math;
• concat inputs have unresolved format differences;
• frame/sample-accurate intent is paired with an unverified stream-copy strategy;
• required editorial decisions have no evidence or review status.

Calculate expected output duration from kept ranges, inserts, speed changes, and transition overlaps. Mark the result as derived and declare a tolerance for timestamp/time-base rounding.

Mapping to FFmpeg

For decoded segment assembly, use trim/atrim, reset segment timestamps with setpts/asetpts where required, normalize compatible media parameters deliberately, and join with the concat filter. Map output streams explicitly.

For separate compatible files, the concat demuxer consumes an ffconcat list. Its inpoint and outpoint can include packets outside the requested interval because of inter-frame dependencies and packet boundaries; timestamps can also be adjusted globally. Review the decoded joins.

scripts/render-edl validates schema-v1 decimal-second EDLs and emits a plan without executing FFmpeg. Its default concat-filter strategy:

• assigns one input index per declared source and reuses that index across events;
• trims and resets timestamps for every selected video/audio segment;
• applies declared video/audio normalization before concat;
• maps each generated output label exactly once;
• derives duration from event ranges and checks the declared tolerance.

Use --strategy concat-demuxer only when every source carries the same probe-derived compatibility_signature and every event declares boundary_precision: packet with keyframe_status: verified or not_applicable. The plan returns an edit.ffconcat payload; write and review that file before execution. The helper never selects the concat protocol, because a structured EDL needs explicit file/range semantics rather than URL-style concatenation.

Transitions are deliberately rejected until a separately validated design supplies transition duration, handles, stream layout, and output-duration math. Source ranges may be reused or reordered; explicit destination ranges may not overlap.

Fast seek and stream copy may choose seek points or packets that do not correspond to an exact visual/audio edit boundary. Label keyframe/packet status as one of verified, not_verified, or not_applicable; never infer it from a round timestamp.

Record FFmpeg/ffprobe versions, complete generated command, mapping, codec settings, environment-sensitive capabilities, output digest, and acceptance report alongside the rendered artifact.

Evidence and heuristic boundary

• Direct evidence: source probe data, exact EDL fields, reviewed source samples, packet/frame observations, generated command, and output verification records.
• Derived evidence: output order and duration computed from declared EDL semantics and a stated rounding rule.
• Heuristic: transcript-aligned endpoints, scene/silence candidates, guessed keyframes, or assumed concat compatibility. These must be labeled and tested.
• Human decision: rationale, continuity, context, and approval are editorial evidence only when attributed.
• Not established: an internally valid EDL does not prove render precision, sync, semantic correctness, rights, or downstream acceptance.

Official FFmpeg sources

• ffmpeg Documentation — seeking, timestamps, stream selection/mapping, filtering, and codec-copy behavior.
• FFmpeg Filters Documentation — trim, atrim, setpts, asetpts, concat, xfade, and acrossfade semantics.
• FFmpeg Formats Documentation — concat demuxer syntax, duration, inpoint, outpoint, and format behavior.
• FFmpeg Utilities Documentation — duration and time expression syntax.

The sources define FFmpeg’s timeline and assembly mechanisms. The EDL conventions above are workflow rules; they are not an FFmpeg-native interchange standard.