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Common Diffusion Noise Schedules and Sample Steps are Flawed

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arxiv 2305.08891 v4 pith:C5E4ZX6P submitted 2023-05-15 cs.CV

Shanchuan Lin , Bingchen Liu , Jiashi Li , Xiao Yang This is my paper
classification cs.CV
keywords diffusionmodelnoiseflawedinferencelasttimestepcommon
verification ladder T0 review T1 audit T2 compute T3 formal
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We discover that common diffusion noise schedules do not enforce the last timestep to have zero signal-to-noise ratio (SNR), and some implementations of diffusion samplers do not start from the last timestep. Such designs are flawed and do not reflect the fact that the model is given pure Gaussian noise at inference, creating a discrepancy between training and inference. We show that the flawed design causes real problems in existing implementations. In Stable Diffusion, it severely limits the model to only generate images with medium brightness and prevents it from generating very bright and dark samples. We propose a few simple fixes: (1) rescale the noise schedule to enforce zero terminal SNR; (2) train the model with v prediction; (3) change the sampler to always start from the last timestep; (4) rescale classifier-free guidance to prevent over-exposure. These simple changes ensure the diffusion process is congruent between training and inference and allow the model to generate samples more faithful to the original data distribution.

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Cited by 4 Pith papers

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