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'alphas_prev' of the DDIM sampler should start with 1. #345

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'alphas_prev' of the DDIM sampler should start with 1. #345

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4 changes: 3 additions & 1 deletion ldm/modules/diffusionmodules/util.py
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Original file line number Diff line number Diff line change
Expand Up @@ -70,7 +70,9 @@ def make_ddim_timesteps(ddim_discr_method, num_ddim_timesteps, num_ddpm_timestep
def make_ddim_sampling_parameters(alphacums, ddim_timesteps, eta, verbose=True):
# select alphas for computing the variance schedule
alphas = alphacums[ddim_timesteps]
alphas_prev = np.asarray([alphacums[0]] + alphacums[ddim_timesteps[:-1]].tolist())

# alphas_prev should start with 1.
alphas_prev = np.append(1., alphas[:-1])

# according the the formula provided in https://arxiv.org/abs/2010.02502
sigmas = eta * np.sqrt((1 - alphas_prev) / (1 - alphas) * (1 - alphas / alphas_prev))
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