DDIM produces incorrect samples with SDXL (epsilon or v-prediction)

[bghira]

Describe the bug

When generating images with SDXL and DDIM, there is some residual noise in the outputs.

This leads to a "smudgy" look, and in cases where fewer steps are used, DDIM and Euler diverge a lot more than they should because of the cumulative impact of not having the timesteps aligned properly.

In some brief tests, it looks like simply adding an extra timestep with a zero sigma to the end of the schedule resolves the problem.

Reproduction

This script uses a modified Euler scheduler to create fully-denoised images:

import PIL
import requests
import torch
import numpy as np
from diffusers import StableDiffusionXLPipeline, EulerDiscreteScheduler

model_id = "ptx0/terminus-xl-gamma-training"
pipe = StableDiffusionXLPipeline.from_pretrained(model_id, add_watermarker=False, torch_dtype=torch.bfloat16).to("cuda")
generator = torch.Generator("cuda").manual_seed(420420420)

prompt = "the artful dodger, cool dog in sunglasses sitting on a recliner in the dark, with the white noise reflecting on his sunglasses"
num_inference_steps = 30
guidance_scale = 7.5
def rescale_zero_terminal_snr_sigmas(sigmas):
    sigmas = sigmas.flip(0)
    alphas_cumprod = 1 / ((sigmas * sigmas) + 1)
    alphas_bar_sqrt = alphas_cumprod.sqrt()

    # Store old values.
    alphas_bar_sqrt_0 = alphas_bar_sqrt[0].clone()
    alphas_bar_sqrt_T = alphas_bar_sqrt[-1].clone()

    # Shift so the last timestep is zero.
    alphas_bar_sqrt -= (alphas_bar_sqrt_T)

    # Scale so the first timestep is back to the old value.
    alphas_bar_sqrt *= alphas_bar_sqrt_0 / (alphas_bar_sqrt_0 - alphas_bar_sqrt_T)

    # Convert alphas_bar_sqrt to betas
    alphas_bar = alphas_bar_sqrt**2  # Revert sqrt
    alphas_bar[-1] = 4.8973451890853435e-08
    sigmas = ((1 - alphas_bar) / alphas_bar) ** 0.5
    return sigmas.flip(0)


zsnr = getattr(pipe.scheduler.config, 'rescale_betas_zero_snr', False)
pipe.scheduler = EulerDiscreteScheduler.from_config(pipe.scheduler.config)
if zsnr:
    tsbase = pipe.scheduler.set_timesteps
    def tspatch(*args, **kwargs):
        tsbase(*args, **kwargs)
        pipe.scheduler.sigmas = rescale_zero_terminal_snr_sigmas(pipe.scheduler.sigmas)
    pipe.scheduler.set_timesteps = tspatch
    sigmas = pipe.scheduler.betas

edited_image = pipe(
   prompt=prompt, 
   num_inference_steps=num_inference_steps, 
   guidance_scale=guidance_scale,
   generator=generator,
    guidance_rescale=0.7
).images[0]
edited_image.save("edited_image.png")

It uses the Sigmas code ported by @Beinsezii in #6024

However, with vanilla DDIM, the results are far worse:

import PIL
import requests
import torch
import numpy as np
from diffusers import StableDiffusionXLPipeline

model_id = "ptx0/terminus-xl-gamma-training"
pipe = StableDiffusionXLPipeline.from_pretrained(model_id, add_watermarker=False, torch_dtype=torch.bfloat16).to("cuda")
generator = torch.Generator("cuda").manual_seed(420420420)

prompt = "the artful dodger, cool dog in sunglasses sitting on a recliner in the dark, with the white noise reflecting on his sunglasses"
num_inference_steps = 30
guidance_scale = 7.5
edited_image = pipe(
   prompt=prompt, 
   num_inference_steps=num_inference_steps, 
   guidance_scale=guidance_scale,
   generator=generator,
    guidance_rescale=0.7
).images[0]
edited_image.save("edited_image.png")

Logs

No response

System Info

• diffusers version: 0.21.4
• Platform: Linux-5.19.0-45-generic-x86_64-with-glibc2.31
• Python version: 3.9.16
• PyTorch version (GPU?): 2.1.0+cu118 (True)
• Huggingface_hub version: 0.16.4
• Transformers version: 4.30.2
• Accelerate version: 0.18.0
• xFormers version: 0.0.22.post4+cu118
• Using GPU in script?: A100-80G PCIe
• Using distributed or parallel set-up in script?: FALSE

Who can help?

@patrickvonplaten @yiyixuxu

[bghira]

For me, brighter images make it more noticeable.

DDIM

Euler (Patched)

[patrickvonplaten]

@yiyixuxu could you take a look here?

[yiyixuxu]

hi @bghira
is this resolved by #6024?

[bghira]

for euler yes, but that already appends an additional scheduler step

[Beinsezii]

I thought ddim had incorrect samples regardless of ZSNR or not. If the solution is to simply use euler and leave ddim broken then it may as well be deprecated.

The fact that euler needs an extra 0 sigma to avoid the residual noise issue and DPM has such options as euler_at_final leads me to believe there's a bigger problem with how the samplers are called, so either ddim and the rest all need Band-Aids or that off-by-one issue or whatever it is needs to be found.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the contributing guidelines are likely to be ignored.

[bghira]

that stale bot is the worst! @patrickvonplaten it should probably just be removed from the project due to how many good issues just get juked.

[bghira]

also kinda crazy this remains an issue for more than a month?

[patrickvonplaten]

Could it be that this issue is resolved with: #6477 ? cc @yiyixuxu can you check?