FluxPipeline silently rounds the generated image shape

[albertochimentiinbibo]

Describe the bug

When prompting the FluxPipeline class to generate an image with shape (1920, 1080), the output image shape is rounded to (1920, 1072) which to me seems like the nearest multiple of 16 instead of 8.
As the FluxPipeline class accepts input sizes divisible by 8 I would expect them to remain consistent throught the generation process.

By giving a quick look at the code it seems that in the FluxPipeline._unpack_latents method, the height and width are floor divided (//) by the vae_scale_factor which is 16.

I would love to understand why the scale factor is set like the following:
https://github.com/huggingface/diffusers/blob/89e4d6219805975bd7d253a267e1951badc9f1c0/src/diffusers/pipelines/flux/pipeline_flux.py#L197C9-L199C10

Reproduction

Here is the minimal code to reproduce the bug, feel free to change the number of inference steps as it should not influence the scope of the test.

from diffusers.pipelines import FluxPipeline
import torch

bf_repo = "black-forest-labs/FLUX.1-dev"

prompt = "Astronaut drinking coffe on the moon."
shape = (1920, 1080)

pipe = FluxPipeline.from_pretrained(bf_repo, torch_dtype=torch.bfloat16)

pipe.enable_sequential_cpu_offload()

image = pipe(
    prompt,
    height=shape[1],
    width=shape[0],
    num_inference_steps=28,
    generator=torch.Generator('cpu').manual_seed(123)
    ).images[0]

print(f"Prompted shape: {shape}")
print(f"Generated shape: {image.size}")
image.show()

Logs

Prompted shape: (1920, 1080)
Generated shape: (1920, 1072)

System Info

• 🤗 Diffusers version: 0.31.0
• Platform: Windows-10-10.0.22631-SP0
• Running on Google Colab?: No
• Python version: 3.10.6
• PyTorch version (GPU?): 2.4.1+cu124 (True)
• Flax version (CPU?/GPU?/TPU?): not installed (NA)
• Jax version: not installed
• JaxLib version: not installed
• Huggingface_hub version: 0.26.2
• Transformers version: 4.44.2
• Accelerate version: 0.34.2
• PEFT version: not installed
• Bitsandbytes version: not installed
• Safetensors version: 0.4.5
• xFormers version: not installed
• Accelerator: NVIDIA GeForce RTX 4070 Laptop GPU, 8188 MiB
• Using GPU in script?: yes
• Using distributed or parallel set-up in script?: no

Who can help?

@sayakpaul @DN6

[albertochimentiinbibo]

For the record, on the main branch ( aligned with commit SHA dac623b ) the same script provided above errors with the following traceback:

Traceback (most recent call last):
  File "C:\dev\github-issues\diffusers\test.py", line 13, in <module>
    image = pipe(
  File "...\torch\2.4.1+cu124\python\3.10.6\torch\utils\_contextlib.py", line 116, in decorate_context
    return func(*args, **kwargs)
  File "...\diffusers\pipelines\flux\pipeline_flux.py", line 684, in __call__
    latents, latent_image_ids = self.prepare_latents(
  File "...\diffusers\pipelines\flux\pipeline_flux.py", line 520, in prepare_latents
    latents = self._pack_latents(latents, batch_size, num_channels_latents, height, width)
  File "...\diffusers\pipelines\flux\pipeline_flux.py", line 444, in _pack_latents
    latents = latents.view(batch_size, num_channels_latents, height // 2, 2, width // 2, 2)
RuntimeError: shape '[1, 16, 67, 2, 120, 2]' is invalid for input of size 518400

This happens for all sizes which are not multiple of 16.

Checking out the recent PRs I found this one which seems to change what I was pointing out in the comment above.

[sayakpaul]

Cc: @yiyixuxu @DN6

[DN6]

Hi @albertochimentiinbibo thanks for catching this. You're right the FluxPipeline is meant to work with images that are multiples of 16.

The vae_scale_factor is based on how much the VAE spatially compresses an image to a latent. In Flux's case it's 8 (1024x1024 image gets turned into a 128x128 latent). PR: #9711 made the usage of the scale factor clearer,
but I think we missed the fact that in the previous version the division by 16 was intended to factor in the fact that the latent height and width need to be divisible by 2. This is because the packing step breaks up the latent into 2x2 patches.

I'll open a PR to fix. We'll also raise a warning that the image will be resized to a compatible height, width.

[albertochimentiinbibo]

Thank you for the feedback @DN6 will be waiting on the fix!

[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.

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[sayakpaul]

@DN6 can this be closed?

[DN6]

Yes. Rounding is expected on shapes that are not multiples of 16.