What's the difference between PNDM and plms

[dingjingzhen]

What's the difference between PNDM and plms，plms for CompVis/stable-diffusion-v1-4-origin

[patrickvonplaten]

Good question!

The short answer:

They are the same!

The long answer:

PNDM was suggested by the PNDM paper (Pseudo Numerical Methods for Diffusion Models on Manifolds) with PNDM standing for Pseudo Numerical Methods for Diffusion Models . The algorithm for PNDMs is defined concisely in Algorithm 2 where you can see that the first loop is called PRK and the second loop is called PLMS. So one can state the PLMS is a submethod/sub-scheduler of the PNDM scheduler. BTW PLMS stands for pseudo-linear multi-step.

Now practically the CompVis PLMS sampler is identical to the diffusers PNDM scheduler if you set config.skip_prk_steps=True which is done in the original diffusers CompVis/stable-diffusion-v1-4 repo: https://huggingface.co/CompVis/stable-diffusion-v1-4/blob/main/scheduler/scheduler_config.json#L8

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