User's idea, as an OPTION (default stays extend_last). The three existing tail modes
all refill the freed end so the clip keeps its length, which means pass 2 has to invent
an ending and blend a joint — the source of the "thin ending" and of most of the reason
to reach for a rewind restart. 'crop' regrows nothing: the scene ends anchor_shift_frames
earlier and every surviving frame was generated as part of one continuous shot.
Checked the model side before assuming this was free, since a bad latent size would
mismatch rather than degrade:
- LTXAV's video patchifier is SymmetricPatchifier(1) -> temporal patch size 1, so there
is no divisibility rule on video latent frames.
- The streams reach the transformer as independent tensors (separate_audio_and_video_
latents just unbinds the list); no video:audio ratio is enforced there.
- comfy re-packs per sample_custom call and re-derives latent_shapes, and the denoise
masks off them (comfy/samplers.py outer_sample), so nothing is baked in from
scene-build time and a shorter latent needs no conditioning rebuild.
- VAE decode is (f-1)*time_scale+1 for any f >= 1.
So no size is rejected. What IS real is proportion: audio timings come from the audio
stream's own index and video's from its own RoPE, so cropping the two by different
amounts of TIME desyncs the clip silently — no error, just drifting sound. Each extra
stream's KEPT length is therefore derived from the new video length directly rather than
by subtracting a separately-rounded drop, so rounding cannot leave them describing clips
of different durations. fresh_audio zeroes after the crop, at the new length.
Downstream needed nothing: _prepend_soft_continuation already clamps to the previous
scene's real length and _derived_overlap clamps to the stream, so a shorter scene flows
through the chain as-is.
The scene report now states the cropped length in latent frames and in real frames,
because a scene coming out shorter than the length that was set must never be something
the user has to infer from the output.
Tests: crop shortens instead of refilling and the surviving frames are the untouched
originals; audio/video stay in proportion (including four rates that do not divide
evenly); fresh_audio resets at the cropped length; the other three modes still keep the
clip length. Suite at baseline: 22 pre-existing failures, 288 passing (+5).
Verified live in the running app: the Engine ▸ Experimental dropdown offers
extend_last/wrap/empty/crop, default unchanged, hint present.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
