diffusers/pr_12229/en/api/schedulers/heun.md

# HeunDiscreteScheduler

The Heun scheduler (Algorithm 1) is from the [Elucidating the Design Space of Diffusion-Based Generative Models](https://huggingface.co/papers/2206.00364) paper by Karras et al. The scheduler is ported from the [k-diffusion](https://github.com/crowsonkb/k-diffusion) library and created by [Katherine Crowson](https://github.com/crowsonkb/).

## HeunDiscreteScheduler[[diffusers.HeunDiscreteScheduler]]
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<docstring><name>class diffusers.HeunDiscreteScheduler</name><anchor>diffusers.HeunDiscreteScheduler</anchor><source>https://github.com/huggingface/diffusers/blob/vr_12229/src/diffusers/schedulers/scheduling_heun_discrete.py#L95</source><parameters>[{"name": "num_train_timesteps", "val": ": int = 1000"}, {"name": "beta_start", "val": ": float = 0.00085"}, {"name": "beta_end", "val": ": float = 0.012"}, {"name": "beta_schedule", "val": ": str = 'linear'"}, {"name": "trained_betas", "val": ": typing.Union[numpy.ndarray, typing.List[float], NoneType] = None"}, {"name": "prediction_type", "val": ": str = 'epsilon'"}, {"name": "use_karras_sigmas", "val": ": typing.Optional[bool] = False"}, {"name": "use_exponential_sigmas", "val": ": typing.Optional[bool] = False"}, {"name": "use_beta_sigmas", "val": ": typing.Optional[bool] = False"}, {"name": "clip_sample", "val": ": typing.Optional[bool] = False"}, {"name": "clip_sample_range", "val": ": float = 1.0"}, {"name": "timestep_spacing", "val": ": str = 'linspace'"}, {"name": "steps_offset", "val": ": int = 0"}]</parameters><paramsdesc>- **num_train_timesteps** (`int`, defaults to 1000) --
  The number of diffusion steps to train the model.
- **beta_start** (`float`, defaults to 0.0001) --
  The starting `beta` value of inference.
- **beta_end** (`float`, defaults to 0.02) --
  The final `beta` value.
- **beta_schedule** (`str`, defaults to `"linear"`) --
  The beta schedule, a mapping from a beta range to a sequence of betas for stepping the model. Choose from
  `linear` or `scaled_linear`.
- **trained_betas** (`np.ndarray`, *optional*) --
  Pass an array of betas directly to the constructor to bypass `beta_start` and `beta_end`.
- **prediction_type** (`str`, defaults to `epsilon`, *optional*) --
  Prediction type of the scheduler function; can be `epsilon` (predicts the noise of the diffusion process),
  `sample` (directly predicts the noisy sample`) or `v_prediction` (see section 2.4 of [Imagen
  Video](https://imagen.research.google/video/paper.pdf) paper).
- **clip_sample** (`bool`, defaults to `True`) --
  Clip the predicted sample for numerical stability.
- **clip_sample_range** (`float`, defaults to 1.0) --
  The maximum magnitude for sample clipping. Valid only when `clip_sample=True`.
- **use_karras_sigmas** (`bool`, *optional*, defaults to `False`) --
  Whether to use Karras sigmas for step sizes in the noise schedule during the sampling process. If `True`,
  the sigmas are determined according to a sequence of noise levels {σi}.
- **use_exponential_sigmas** (`bool`, *optional*, defaults to `False`) --
  Whether to use exponential sigmas for step sizes in the noise schedule during the sampling process.
- **use_beta_sigmas** (`bool`, *optional*, defaults to `False`) --
  Whether to use beta sigmas for step sizes in the noise schedule during the sampling process. Refer to [Beta
  Sampling is All You Need](https://huggingface.co/papers/2407.12173) for more information.
- **timestep_spacing** (`str`, defaults to `"linspace"`) --
  The way the timesteps should be scaled. Refer to Table 2 of the [Common Diffusion Noise Schedules and
  Sample Steps are Flawed](https://huggingface.co/papers/2305.08891) for more information.
- **steps_offset** (`int`, defaults to 0) --
  An offset added to the inference steps, as required by some model families.</paramsdesc><paramgroups>0</paramgroups></docstring>

Scheduler with Heun steps for discrete beta schedules.

This model inherits from [SchedulerMixin](/docs/diffusers/pr_12229/en/api/schedulers/overview#diffusers.SchedulerMixin) and [ConfigMixin](/docs/diffusers/pr_12229/en/api/configuration#diffusers.ConfigMixin). Check the superclass documentation for the generic
methods the library implements for all schedulers such as loading and saving.





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<docstring><name>scale_model_input</name><anchor>diffusers.HeunDiscreteScheduler.scale_model_input</anchor><source>https://github.com/huggingface/diffusers/blob/vr_12229/src/diffusers/schedulers/scheduling_heun_discrete.py#L237</source><parameters>[{"name": "sample", "val": ": Tensor"}, {"name": "timestep", "val": ": typing.Union[float, torch.Tensor]"}]</parameters><paramsdesc>- **sample** (`torch.Tensor`) --
  The input sample.
- **timestep** (`int`, *optional*) --
  The current timestep in the diffusion chain.</paramsdesc><paramgroups>0</paramgroups><rettype>`torch.Tensor`</rettype><retdesc>A scaled input sample.</retdesc></docstring>

Ensures interchangeability with schedulers that need to scale the denoising model input depending on the
current timestep.








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<docstring><name>set_begin_index</name><anchor>diffusers.HeunDiscreteScheduler.set_begin_index</anchor><source>https://github.com/huggingface/diffusers/blob/vr_12229/src/diffusers/schedulers/scheduling_heun_discrete.py#L227</source><parameters>[{"name": "begin_index", "val": ": int = 0"}]</parameters><paramsdesc>- **begin_index** (`int`) --
  The begin index for the scheduler.</paramsdesc><paramgroups>0</paramgroups></docstring>

Sets the begin index for the scheduler. This function should be run from pipeline before the inference.




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<docstring><name>set_timesteps</name><anchor>diffusers.HeunDiscreteScheduler.set_timesteps</anchor><source>https://github.com/huggingface/diffusers/blob/vr_12229/src/diffusers/schedulers/scheduling_heun_discrete.py#L263</source><parameters>[{"name": "num_inference_steps", "val": ": typing.Optional[int] = None"}, {"name": "device", "val": ": typing.Union[str, torch.device] = None"}, {"name": "num_train_timesteps", "val": ": typing.Optional[int] = None"}, {"name": "timesteps", "val": ": typing.Optional[typing.List[int]] = None"}]</parameters><paramsdesc>- **num_inference_steps** (`int`) --
  The number of diffusion steps used when generating samples with a pre-trained model.
- **device** (`str` or `torch.device`, *optional*) --
  The device to which the timesteps should be moved to. If `None`, the timesteps are not moved.
- **num_train_timesteps** (`int`, *optional*) --
  The number of diffusion steps used when training the model. If `None`, the default
  `num_train_timesteps` attribute is used.
- **timesteps** (`List[int]`, *optional*) --
  Custom timesteps used to support arbitrary spacing between timesteps. If `None`, timesteps will be
  generated based on the `timestep_spacing` attribute. If `timesteps` is passed, `num_inference_steps`
  must be `None`, and `timestep_spacing` attribute will be ignored.</paramsdesc><paramgroups>0</paramgroups></docstring>

Sets the discrete timesteps used for the diffusion chain (to be run before inference).




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<docstring><name>step</name><anchor>diffusers.HeunDiscreteScheduler.step</anchor><source>https://github.com/huggingface/diffusers/blob/vr_12229/src/diffusers/schedulers/scheduling_heun_discrete.py#L472</source><parameters>[{"name": "model_output", "val": ": typing.Union[torch.Tensor, numpy.ndarray]"}, {"name": "timestep", "val": ": typing.Union[float, torch.Tensor]"}, {"name": "sample", "val": ": typing.Union[torch.Tensor, numpy.ndarray]"}, {"name": "return_dict", "val": ": bool = True"}]</parameters><paramsdesc>- **model_output** (`torch.Tensor`) --
  The direct output from learned diffusion model.
- **timestep** (`float`) --
  The current discrete timestep in the diffusion chain.
- **sample** (`torch.Tensor`) --
  A current instance of a sample created by the diffusion process.
- **return_dict** (`bool`) --
  Whether or not to return a `HeunDiscreteSchedulerOutput` or
  tuple.</paramsdesc><paramgroups>0</paramgroups><rettype>`HeunDiscreteSchedulerOutput` or `tuple`</rettype><retdesc>If return_dict is `True`, `HeunDiscreteSchedulerOutput` is
returned, otherwise a tuple is returned where the first element is the sample tensor.</retdesc></docstring>

Predict the sample from the previous timestep by reversing the SDE. This function propagates the diffusion
process from the learned model outputs (most often the predicted noise).








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## SchedulerOutput[[diffusers.schedulers.scheduling_utils.SchedulerOutput]]
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<docstring><name>class diffusers.schedulers.scheduling_utils.SchedulerOutput</name><anchor>diffusers.schedulers.scheduling_utils.SchedulerOutput</anchor><source>https://github.com/huggingface/diffusers/blob/vr_12229/src/diffusers/schedulers/scheduling_utils.py#L62</source><parameters>[{"name": "prev_sample", "val": ": Tensor"}]</parameters><paramsdesc>- **prev_sample** (`torch.Tensor` of shape `(batch_size, num_channels, height, width)` for images) --
  Computed sample `(x_{t-1})` of previous timestep. `prev_sample` should be used as next model input in the
  denoising loop.</paramsdesc><paramgroups>0</paramgroups></docstring>

Base class for the output of a scheduler's `step` function.




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