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fix: convert boundary_ratio from timestep space to index space via inverse sigma_shift #32

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fix: convert boundary_ratio from timestep space to index space via inverse sigma_shift #32

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12 changes: 10 additions & 2 deletions mova/diffusion/pipelines/mova_train.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1386,10 +1386,18 @@ def training_step(
mode_scale=1.0,
independent_timesteps=False,
)
# The sigma_shift formula in flow_match.py L57:
# sigma_shifted = shift * sigma_linear / (1 + (shift - 1) * sigma_linear)
# boundary_ratio is in shifted space; invert to get sigma_linear:
# sigma_linear = boundary_ratio / (shift - (shift - 1) * boundary_ratio)
# index_boundary = 1 - sigma_linear, which simplifies to:
# shift * (1 - boundary_ratio) / (1 + (shift - 1) * (1 - boundary_ratio))
# e.g. shift=3, boundary_ratio=0.9 => sigma_linear=0.75 => index=0.25
boundary = self.scheduler.shift * (1 - self.boundary_ratio) / (1 + (self.scheduler.shift - 1) * (1 - self.boundary_ratio))
if global_step % 2 == 0:
timestep_config.max_timestep_boundary = self.boundary_ratio
timestep_config.max_timestep_boundary = boundary
else:
timestep_config.min_timestep_boundary = self.boundary_ratio
timestep_config.min_timestep_boundary = boundary

timestep, audio_timestep = self.sample_timestep_pair(timestep_config)
timestep = timestep.to(device=device)
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