diffusers/docs/source/en/using-diffusers/callback.md

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# Pipeline callbacks

A callback runs at the end of a denoising step and can change pipeline state or tensors for later steps. Use it to adjust attributes or tensor variables for new behavior without rewriting the pipeline.

These callbacks apply to classic [`DiffusionPipeline`] loops. In [Modular Diffusers](../modular_diffusers/overview), you can build and add custom pipeline blocks instead of `callback_on_step_end`.

Diffusers provides several callbacks in the pipeline [overview](../api/pipelines/overview#diffusers.callbacks.PipelineCallback).

To enable a callback, configure when the callback is executed after a certain number of denoising steps with one of the following arguments.

- `cutoff_step_ratio` specifies when a callback is activated as a percentage of the total denoising steps. Use when the cutoff should scale with `num_inference_steps` (for example, drop CFG after 40% of run).
- `cutoff_step_index` specifies the exact step number a callback is activated. Use when you care about an absolute step (for example, step `10` on a fixed 25-step schedule).

The example below uses `cutoff_step_ratio=0.4`, which means the callback is activated once denoising reaches 40% of the total inference steps. [`~callbacks.SDXLCFGCutoffCallback`] disables classifier-free guidance (CFG) after a certain number of steps, which can help save compute without significantly affecting performance.

Define a callback with one of the `cutoff` arguments and pass it to the `callback_on_step_end` parameter in the pipeline.

```py
import torch
from diffusers import DPMSolverMultistepScheduler, StableDiffusionXLPipeline
from diffusers.callbacks import SDXLCFGCutoffCallback

callback = SDXLCFGCutoffCallback(cutoff_step_ratio=0.4)
# if using cutoff_step_index
# callback = SDXLCFGCutoffCallback(cutoff_step_ratio=None, cutoff_step_index=10)

pipeline = StableDiffusionXLPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-base-1.0",
    dtype=torch.float16,
    device_map="cuda"  # or "mps", "xpu", "cpu"
)
pipeline.scheduler = DPMSolverMultistepScheduler.from_config(pipeline.scheduler.config, use_karras_sigmas=True)

prompt = "a sports car on the road, best quality, high quality, high detail, 8k resolution"
output = pipeline(
    prompt=prompt,
    negative_prompt="",
    guidance_scale=6.5,
    num_inference_steps=25,
    callback_on_step_end=callback,
)
```

Official callbacks set their own tensor inputs. For a custom function, pass `callback_on_step_end_tensor_inputs` as in [Display intermediate images](#display-intermediate-images).

If you want to add a new official callback, feel free to open a [feature request](https://github.com/huggingface/diffusers/issues/new/choose) or [submit a PR](https://huggingface.co/docs/diffusers/main/en/conceptual/contribution#how-to-open-a-pr). Otherwise, you can also create your own callback as shown below.

## Early stopping

Early stopping is useful if you aren't happy with the intermediate results during generation. This callback sets a hardcoded stop point by setting the `_interrupt` attribute to `True`, which makes the denoising loop skip the remaining steps.

```py
import torch
from diffusers import DiffusionPipeline

def interrupt_callback(pipeline, i, t, callback_kwargs):
    stop_idx = 10
    if i == stop_idx:
        pipeline._interrupt = True
    return callback_kwargs

pipeline = DiffusionPipeline.from_pretrained(
    "Qwen/Qwen-Image",
    dtype=torch.bfloat16,
    device_map="cuda",  # or "mps", "xpu", "cpu"
)
pipeline(
    prompt="A photo of a cat",
    num_inference_steps=50,
    callback_on_step_end=interrupt_callback,
)
```

## Display intermediate images

Visualizing intermediate images is useful for progress monitoring. The preview below is SDXL-only. It maps SDXL latents to RGB with a linear transform for a quick look during denoising. Those weights do not transfer to other models. For Qwen-Image and similar checkpoints, decode with the model VAE instead of this helper.

[Convert](https://huggingface.co/blog/TimothyAlexisVass/explaining-the-sdxl-latent-space) Stable Diffusion XL latents (4 channels) to RGB tensors (3 channels).

```py
import torch
from PIL import Image
from diffusers import AutoPipelineForText2Image

def latents_to_rgb(latents):
    weights = (
        (60, -60, 25, -70),
        (60,  -5, 15, -50),
        (60,  10, -5, -35),
    )

    weights_tensor = torch.t(torch.tensor(weights, dtype=latents.dtype).to(latents.device))
    biases_tensor = torch.tensor((150, 140, 130), dtype=latents.dtype).to(latents.device)
    rgb_tensor = torch.einsum("...lxy,lr -> ...rxy", latents, weights_tensor) + biases_tensor.unsqueeze(-1).unsqueeze(-1)
    image_array = rgb_tensor.clamp(0, 255).byte().cpu().numpy().transpose(1, 2, 0)

    return Image.fromarray(image_array)
```

Extract the latents and convert the first image in the batch to RGB. Save the image as a PNG file with the step number.

```py
def decode_tensors(pipe, step, timestep, callback_kwargs):
    latents = callback_kwargs["latents"]

    image = latents_to_rgb(latents[0])
    image.save(f"{step}.png")

    return callback_kwargs
```

Use `callback_on_step_end_tensor_inputs` to choose which tensors the callback receives, which in this case, are the latents.

```py
pipeline = AutoPipelineForText2Image.from_pretrained(
    "stabilityai/stable-diffusion-xl-base-1.0",
    dtype=torch.float16,
    device_map="cuda"  # or "mps", "xpu", "cpu"
)

image = pipeline(
    prompt="A croissant shaped like a cute bear.",
    negative_prompt="Deformed, ugly, bad anatomy",
    callback_on_step_end=decode_tensors,
    callback_on_step_end_tensor_inputs=["latents"],
).images[0]
```