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Fix: per-frame timestep allocation for video2world (image2world) mode in CosmosPredict2 #26

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Fix: per-frame timestep allocation for video2world (image2world) mode in CosmosPredict2 #26

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7 changes: 7 additions & 0 deletions fastgen/networks/cosmos_predict2/network.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1352,6 +1352,13 @@ def forward(
conditioning_latents_full = conditioning_latents
# Replace conditioning frames
model_input = conditioning_latents_full * condition_mask_C + x_t * (1 - condition_mask_C)
# Per-frame timesteps: assign timestep 0 to conditioning (clean) frames so the
# model knows they are noise-free and should not be denoised. Without this, the
# model treats the clean first frame as a fully-noised frame, causing incoherent
# video2world (image2world) generation.
t_expanded = t.unsqueeze(1).expand(B, T)
mask_B_T = condition_mask[:, 0, :, 0, 0] # (B, T)
t = t_expanded * (1 - mask_B_T)

model_outputs = self.transformer(
x_B_C_T_H_W=model_input,
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