manga-recap-pipeline/decisions/chapter-assembly.md

chapter-assembly

Settled questions about _assemble_batched / _assemble_once / _xfade_chain in worker_render.py.

xfade offsets are computed from min(video, audio), never format=duration

Closed, 2026-08-12.

_xfade_chain accumulates cum += dur[i] - td and hands each boundary offset=cum-td. That offset is an assertion about where input i-1 still has frames. It fed on _audio_dur, which probes format=duration, which is max(video, audio). A rendered clip's audio outlasts its video by about a frame. So every boundary pushed the accumulator further ahead of the picture.

Once the accumulated overshoot exceeds the transition width, the xfade window starts after the last frame of input i-1. ffmpeg emits the transition and then silently discards input i and every clip downstream of it. rc 0, no warning on stderr, output file present and playable. Measured on a group of 8 real clips, chain truncated at each stage:

k=7 out= 52.52   correct
k=8 out= 52.52   the last xfade contributed nothing
    [v6][n7]xfade=duration=0.050:offset=52.500   <- [v6] is only 52.52s long, so 0.02s of margin

This is the whole cause of the shipped chapter being video 436.39s over audio 363.67s. It reproduces with synthetic clips in about four minutes and needs no GPU.

Two changes hold it closed:

• _assemble_once probes min(_stream_dur(p, "v"), _stream_dur(p, "a")). The minimum, because either stream running long breaks a different half of the graph.
• _xfade_chain floors every input to a whole frame count. It applies trim and atrim to both streams, so the accumulator tracks the real timeline instead of estimating it. Transition widths are quantized to frames for the same reason.

Verified over the 49 real clips of chapter 7c944dd4. The round that previously turned 359s of video into 100s now loses 0.85s. The chapter comes out video 358.76s against audio 358.76s, agreeing to the frame.

Forbidden from here: _audio_dur in anything that positions a filter. It is fine for "how long is this clip roughly", nothing else.

Assembly verifies its own output instead of trusting ffmpeg's exit code

Closed, 2026-08-12.

Both assembly branches drop stream time without failing. xfade discards inputs as above. The concat demuxer with -vsync cfr drops video frames to force a constant rate. Neither is an error to ffmpeg.

So _assemble_once calls _check_assembled(out, expect) after every encode. It compares the output's video stream against the predicted timeline and against its own audio stream. It raises when either is off by more than ASSEMBLE_TOL_S, which is 0.5s. That tolerance covers frame boundaries and aac padding. A dropped input is off by whole seconds.

Without this the failure stays invisible until somebody watches the video. That is how a 50MiB chapter with 72.7s of silent picture reached the bucket while every stage counter read success.

The single-item passthrough is not the bug

Void, 2026-08-12. Cited in HANDOFF.md for 2026-08-11 as the prime suspect and must not be cited again.

With 49 clips and ASSEMBLE_BATCH=8, round 0 makes six groups of 8 plus a leftover group of 1, which _assemble_batched carries forward un-encoded. The theory was that mixing that raw clip with six encoded intermediates broke round 1. The instrumented run disproves it: two round-0 groups of 8 fresh clips collapse on their own, before any passthrough exists.

n=8 XFADE  in v=  63.52 a=  63.60 -> out v=  52.52 a=  62.77   lost_v= +11.00
n=8 XFADE  in v=  58.12 a=  58.20 -> out v=  12.04 a=  56.83   lost_v= +46.08

Round 1's 259s loss was the cascade. Its inputs already held 309s of video against 364s of audio, and durs read the audio.

Consequence for the plan: collapsing concat, xfade and the passthrough into one path was the recommended fix in NEXT.md and is not needed. Three paths are fine once each one positions filters on a real timeline. The tree keeps the bounded memory it was built for.