pythonProject/diffusers-main/src/diffusers/guiders/classifier_free_zero_star_guidance.py

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#     http://www.apache.org/licenses/LICENSE-2.0
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import math
from typing import TYPE_CHECKING, Dict, List, Optional, Tuple, Union

import torch

from ..configuration_utils import register_to_config
from .guider_utils import BaseGuidance, GuiderOutput, rescale_noise_cfg


if TYPE_CHECKING:
    from ..modular_pipelines.modular_pipeline import BlockState


class ClassifierFreeZeroStarGuidance(BaseGuidance):
    """
    Classifier-free Zero* (CFG-Zero*): https://huggingface.co/papers/2503.18886

    This is an implementation of the Classifier-Free Zero* guidance technique, which is a variant of classifier-free
    guidance. It proposes zero initialization of the noise predictions for the first few steps of the diffusion
    process, and also introduces an optimal rescaling factor for the noise predictions, which can help in improving the
    quality of generated images.

    The authors of the paper suggest setting zero initialization in the first 4% of the inference steps.

    Args:
        guidance_scale (`float`, defaults to `7.5`):
            The scale parameter for classifier-free guidance. Higher values result in stronger conditioning on the text
            prompt, while lower values allow for more freedom in generation. Higher values may lead to saturation and
            deterioration of image quality.
        zero_init_steps (`int`, defaults to `1`):
            The number of inference steps for which the noise predictions are zeroed out (see Section 4.2).
        guidance_rescale (`float`, defaults to `0.0`):
            The rescale factor applied to the noise predictions. This is used to improve image quality and fix
            overexposure. Based on Section 3.4 from [Common Diffusion Noise Schedules and Sample Steps are
            Flawed](https://huggingface.co/papers/2305.08891).
        use_original_formulation (`bool`, defaults to `False`):
            Whether to use the original formulation of classifier-free guidance as proposed in the paper. By default,
            we use the diffusers-native implementation that has been in the codebase for a long time. See
            [~guiders.classifier_free_guidance.ClassifierFreeGuidance] for more details.
        start (`float`, defaults to `0.01`):
            The fraction of the total number of denoising steps after which guidance starts.
        stop (`float`, defaults to `0.2`):
            The fraction of the total number of denoising steps after which guidance stops.
    """

    _input_predictions = ["pred_cond", "pred_uncond"]

    @register_to_config
    def __init__(
        self,
        guidance_scale: float = 7.5,
        zero_init_steps: int = 1,
        guidance_rescale: float = 0.0,
        use_original_formulation: bool = False,
        start: float = 0.0,
        stop: float = 1.0,
    ):
        super().__init__(start, stop)

        self.guidance_scale = guidance_scale
        self.zero_init_steps = zero_init_steps
        self.guidance_rescale = guidance_rescale
        self.use_original_formulation = use_original_formulation

    def prepare_inputs(
        self, data: "BlockState", input_fields: Optional[Dict[str, Union[str, Tuple[str, str]]]] = None
    ) -> List["BlockState"]:
        if input_fields is None:
            input_fields = self._input_fields

        tuple_indices = [0] if self.num_conditions == 1 else [0, 1]
        data_batches = []
        for i in range(self.num_conditions):
            data_batch = self._prepare_batch(input_fields, data, tuple_indices[i], self._input_predictions[i])
            data_batches.append(data_batch)
        return data_batches

    def forward(self, pred_cond: torch.Tensor, pred_uncond: Optional[torch.Tensor] = None) -> GuiderOutput:
        pred = None

        if self._step < self.zero_init_steps:
            pred = torch.zeros_like(pred_cond)
        elif not self._is_cfg_enabled():
            pred = pred_cond
        else:
            pred_cond_flat = pred_cond.flatten(1)
            pred_uncond_flat = pred_uncond.flatten(1)
            alpha = cfg_zero_star_scale(pred_cond_flat, pred_uncond_flat)
            alpha = alpha.view(-1, *(1,) * (len(pred_cond.shape) - 1))
            pred_uncond = pred_uncond * alpha
            shift = pred_cond - pred_uncond
            pred = pred_cond if self.use_original_formulation else pred_uncond
            pred = pred + self.guidance_scale * shift

        if self.guidance_rescale > 0.0:
            pred = rescale_noise_cfg(pred, pred_cond, self.guidance_rescale)

        return GuiderOutput(pred=pred, pred_cond=pred_cond, pred_uncond=pred_uncond)

    @property
    def is_conditional(self) -> bool:
        return self._count_prepared == 1

    @property
    def num_conditions(self) -> int:
        num_conditions = 1
        if self._is_cfg_enabled():
            num_conditions += 1
        return num_conditions

    def _is_cfg_enabled(self) -> bool:
        if not self._enabled:
            return False

        is_within_range = True
        if self._num_inference_steps is not None:
            skip_start_step = int(self._start * self._num_inference_steps)
            skip_stop_step = int(self._stop * self._num_inference_steps)
            is_within_range = skip_start_step <= self._step < skip_stop_step

        is_close = False
        if self.use_original_formulation:
            is_close = math.isclose(self.guidance_scale, 0.0)
        else:
            is_close = math.isclose(self.guidance_scale, 1.0)

        return is_within_range and not is_close


def cfg_zero_star_scale(cond: torch.Tensor, uncond: torch.Tensor, eps: float = 1e-8) -> torch.Tensor:
    cond_dtype = cond.dtype
    cond = cond.float()
    uncond = uncond.float()
    dot_product = torch.sum(cond * uncond, dim=1, keepdim=True)
    squared_norm = torch.sum(uncond**2, dim=1, keepdim=True) + eps
    # st_star = v_cond^T * v_uncond / ||v_uncond||^2
    scale = dot_product / squared_norm
    return scale.to(dtype=cond_dtype)