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# Stable diffusion XL

Stable Diffusion XL was proposed in [SDXL: Improving Latent Diffusion Models for High-Resolution Image Synthesis](https://arxiv.org/abs/2307.01952) by Dustin Podell, Zion English, Kyle Lacey, Andreas Blattmann, Tim Dockhorn, Jonas Müller, Joe Penna, Robin Rombach

The abstract of the paper is the following:

*We present SDXL, a latent diffusion model for text-to-image synthesis. Compared to previous versions of Stable Diffusion, SDXL leverages a three times larger UNet backbone: The increase of model parameters is mainly due to more attention blocks and a larger cross-attention context as SDXL uses a second text encoder. We design multiple novel conditioning schemes and train SDXL on multiple aspect ratios. We also introduce a refinement model which is used to improve the visual fidelity of samples generated by SDXL using a post-hoc image-to-image technique. We demonstrate that SDXL shows drastically improved performance compared the previous versions of Stable Diffusion and achieves results competitive with those of black-box state-of-the-art image generators.*

## Tips

- Stable Diffusion XL works especially well with images between 768 and 1024.
- Stable Diffusion XL output image can be improved by making use of a refiner as shown below.

### Available checkpoints:

- *Text-to-Image (1024x1024 resolution)*: [stabilityai/stable-diffusion-xl-base-0.9](https://huggingface.co/stabilityai/stable-diffusion-xl-base-0.9) with [`StableDiffusionXLPipeline`]
- *Image-to-Image / Refiner (1024x1024 resolution)*: [stabilityai/stable-diffusion-xl-refiner-0.9](https://huggingface.co/stabilityai/stable-diffusion-xl-refiner-0.9) with [`StableDiffusionXLImg2ImgPipeline`]

## Usage Example

Before using SDXL make sure to have `transformers`, `accelerate`, `safetensors` and `invisible_watermark` installed. 
You can install the libraries as follows:

```
pip install transformers
pip install accelerate
pip install safetensors
pip install invisible-watermark>=2.0
```

### Text-to-Image

You can use SDXL as follows for *text-to-image*:

```py
from diffusers import StableDiffusionXLPipeline
import torch

pipe = StableDiffusionXLPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-base-0.9", torch_dtype=torch.float16, variant="fp16", use_safetensors=True
)
pipe.to("cuda")

prompt = "Astronaut in a jungle, cold color palette, muted colors, detailed, 8k"
image = pipe(prompt=prompt).images[0]
```

### Refining the image output

In addition to the [base model checkpoint](https://huggingface.co/stabilityai/stable-diffusion-xl-base-0.9), 
StableDiffusion-XL also includes a [refiner checkpoint](huggingface.co/stabilityai/stable-diffusion-xl-refiner-0.9)
that is specialized in denoising low-noise stage images to generate images of improved high-frequency quality.
This refiner checkpoint can be used as a "second-step" pipeline after having run the base checkpoint to improve
image quality.

When using the refiner, one can easily 
- 1.) employ the base model and refiner as an *Ensemble of Expert Denoisers* as first proposed in [eDiff-I](https://research.nvidia.com/labs/dir/eDiff-I/) or
- 2.) simply run the refiner in [SDEdit](https://arxiv.org/abs/2108.01073) fashion after the base model.

**Note**: The idea of using SD-XL base & refiner as an ensemble of experts was first brought forward by 
a couple community contributors which also helped shape the following `diffusers` implementation, namely:
- [SytanSD](https://github.com/SytanSD)
- [bghira](https://github.com/bghira)
- [Birch-san](https://github.com/Birch-san)

#### 1.) Ensemble of Expert Denoisers

When using the base and refiner model as an ensemble of expert of denoisers, the base model should serve as the 
expert for the high-noise diffusion stage and the refiner serves as the expert for the low-noise diffusion stage.

The advantage of 1.) over 2.) is that it requires less overall denoising steps and therefore should be significantly
faster. The drawback is that one cannot really inspect the output of the base model; it will still be heavily denoised.

To use the base model and refiner as an ensemble of expert denoisers, make sure to define the fraction
of timesteps which should be run through the high-noise denoising stage (*i.e.* the base model) and the low-noise
denoising stage (*i.e.* the refiner model) respectively. This fraction should be set as the [`~StableDiffusionXLPipeline.__call__.denoising_end`] of the base model 
and as the [`~StableDiffusionXLImg2ImgPipeline.__call__.denoising_start`] of the refiner model.

Let's look at an example.
First, we import the two pipelines. Since the text encoders and variational autoencoder are the same
you don't have to load those again for the refiner.

```py
from diffusers import DiffusionPipeline
import torch

base = DiffusionPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-base-0.9", torch_dtype=torch.float16, variant="fp16", use_safetensors=True
)
pipe.to("cuda")

refiner = DiffusionPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-refiner-0.9",
    text_encoder_2=base.text_encoder_2,
    vae=base.vae,
    torch_dtype=torch.float16,
    use_safetensors=True,
    variant="fp16",
)
refiner.to("cuda")
```

Now we define the number of inference steps and the fraction at which the model shall be run through the 
high-noise denoising stage (*i.e.* the base model).

```py
n_steps = 40
high_noise_frac = 0.7
```

A fraction of 0.7 means that 70% of the 40 inference steps (28 steps) are run through the base model
and the remaining 12 steps are run through the refiner. Let's run the two pipelines now.
Make sure to set `denoising_end` and `denoising_start` to the same values and keep `num_inference_steps`
constant. Also remember that the output of the base model should be in latent space:

```py
prompt = "A majestic lion jumping from a big stone at night"

image = base(prompt=prompt, num_inference_steps=n_steps, denoising_end=high_noise_frac, output_type="latent").images
image = refiner(prompt=prompt, num_inference_steps=n_steps, denoising_start=high_noise_frac, image=image).images[0]
```

Let's have a look at the image

![lion_ref](https://huggingface.co/datasets/huggingface/documentation-images/blob/main/diffusers/lion_refined.png)

If we would have just run the base model on the same 40 steps, the image would have been arguably less detailed (e.g. the lion eyes and nose):

![lion_base](https://huggingface.co/datasets/huggingface/documentation-images/blob/main/diffusers/lion_base.png)

<Tip>

The ensemble-of-experts method works well on all available schedulers!

</Tip>

#### Refining the image output from fully denoised base image

In standard [`StableDiffusionImg2ImgPipeline`]-fashion, the fully-denoised image generated of the base model 
can be further improved using the [refiner checkpoint](huggingface.co/stabilityai/stable-diffusion-xl-refiner-0.9).

For this, you simply run the refiner as a normal image-to-image pipeline after the "base" text-to-image 
pipeline. You can leave the outputs of the base model in latent space.

```py
from diffusers import DiffusionPipeline
import torch

pipe = DiffusionPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-base-0.9", torch_dtype=torch.float16, variant="fp16", use_safetensors=True
)
pipe.to("cuda")

refiner = DiffusionPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-refiner-0.9",
    text_encoder_2=pipe.text_encoder_2,
    vae=pipe.vae,
    torch_dtype=torch.float16,
    use_safetensors=True,
    variant="fp16",
)
refiner.to("cuda")

prompt = "Astronaut in a jungle, cold color palette, muted colors, detailed, 8k"

image = pipe(prompt=prompt, output_type="latent" if use_refiner else "pil").images[0]
image = refiner(prompt=prompt, image=image[None, :]).images[0]
```

### Image-to-image 

```py 
import torch
from diffusers import StableDiffusionXLImg2ImgPipeline
from diffusers.utils import load_image

pipe = StableDiffusionXLImg2ImgPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-refiner-0.9", torch_dtype=torch.float16
)
pipe = pipe.to("cuda")
url = "https://huggingface.co/datasets/patrickvonplaten/images/resolve/main/aa_xl/000000009.png"

init_image = load_image(url).convert("RGB")
prompt = "a photo of an astronaut riding a horse on mars"
image = pipe(prompt, image=init_image).images[0]
```

| Original Image | Refined Image |
|---|---|
| ![](https://huggingface.co/datasets/diffusers/docs-images/resolve/main/sd_xl/init_image.png) | ![](https://huggingface.co/datasets/diffusers/docs-images/resolve/main/sd_xl/refined_image.png) |

### Loading single file checkpoints / original file format

By making use of [`~diffusers.loaders.FromSingleFileMixin.from_single_file`] you can also load the 
original file format into `diffusers`:

```py
from diffusers import StableDiffusionXLPipeline, StableDiffusionXLImg2ImgPipeline
import torch

pipe = StableDiffusionXLPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-base-0.9", torch_dtype=torch.float16, variant="fp16", use_safetensors=True
)
pipe.to("cuda")

refiner = StableDiffusionXLImg2ImgPipeline.from_pretrained(
    "stabilityai/stable-diffusion-xl-refiner-0.9", torch_dtype=torch.float16, use_safetensors=True, variant="fp16"
)
refiner.to("cuda")
```

### Memory optimization via model offloading 

If you are seeing out-of-memory errors, we recommend making use of [`StableDiffusionXLPipeline.enable_model_cpu_offload`].

```diff
- pipe.to("cuda")
+ pipe.enable_model_cpu_offload()
```

and 

```diff
- refiner.to("cuda")
+ refiner.enable_model_cpu_offload()
```

### Speed-up inference with `torch.compile`

You can speed up inference by making use of `torch.compile`. This should give you **ca.** 20% speed-up.

```diff
+ pipe.unet = torch.compile(pipe.unet, mode="reduce-overhead", fullgraph=True)
+ refiner.unet = torch.compile(refiner.unet, mode="reduce-overhead", fullgraph=True)
```

### Running with `torch` \< 2.0

**Note** that if you want to run Stable Diffusion XL with `torch` < 2.0, please make sure to enable xformers 
attention:

```
pip install xformers
```

```diff
+pipe.enable_xformers_memory_efficient_attention()
+refiner.enable_xformers_memory_efficient_attention()
```

## StableDiffusionXLPipeline

[[autodoc]] StableDiffusionXLPipeline
	- all
	- __call__

## StableDiffusionXLImg2ImgPipeline

[[autodoc]] StableDiffusionXLImg2ImgPipeline
	- all
	- __call__
