Port mechanism 2 of SCAIL-2 (arXiv:2606.10804) to LTX-2: extra per-token
in-context conditioning channels (1 environment switch + K=6 character binding
slots) concatenated onto the latent before the first projection.
Faithful temporal encoding for LTX's VAE (temporal factor 8): each latent frame
stacks its 8 pixel sub-frames along the channel dim, giving 8*(K+1)=56 channels
(vs the paper's 4*(K+1)=28 on Wan 2.1).
Plumbing (backward compatible; cond_channels=None leaves every existing pipeline
unchanged):
- LatentState/Modality gain an optional cond_channels field (patchified [B,T,C]).
- LTXModel(mask_conditioning_channels=0) config-gates a widened patchify_proj;
TransformerArgsPreprocessor concatenates cond_channels (or zero-pads) before it.
- tools.clear_conditioning trims it; token-appending conditioning items
(reference video/audio, driving, keyframe) extend it via extend_cond_channels.
New:
- VideoConditionByMaskChannels: encodes (K+1) pixel masks -> 56 channels, written
onto the trailing driving tokens (noisy target stays all-zero, per the paper).
- widen_patchify_proj_for_mask_channels: zero-init checkpoint surgery so a
converted model reproduces the base output exactly until finetuned.
Verified (CPU, random weights): backward compat, zero-init widened forward is
bit-identical to baseline for any cond_channels, and the driving+mask pipeline
forwards without crashing with correct placement/clipping. Visual quality
requires Phase 3 finetuning; Replacement-mode z_ref height shift still deferred.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Port mechanisms 1+3 of SCAIL-2 (arXiv:2606.10804) to LTX-2: concatenate a
driving video latent directly into the DiT token sequence with a width-axis
RoPE offset (ΔW) so driving coords stay detached from the target video.
- New VideoConditionByDrivingLatent + DrivingMode in ltx-core conditioning,
modeled on VideoConditionByReferenceLatent (patchify -> positions -> append
-> attention mask). Applies ΔW width shift, aligns time to the target, and
guards against RoPE wrap (max_pos) and target/driving shape mismatch.
- Export both from conditioning packages.
- docs/plan.md and docs/tasks.md track the phased port.
Inference-only: no weight changes. Mechanism 2 (in-context mask channels,
patchify_proj widening) and training are deferred to Phase 2+. Validated via
plumbing checks and a real (random-weight) transformer forward smoke run;
visual quality is not validated (requires Phase 2/3 finetuning).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
indigo