Implement Frequency-Decoupled Guidance (FDG) as a Guider

src/diffusers/__init__.py

@@ -139,6 +139,7 @@
             "AutoGuidance",
             "ClassifierFreeGuidance",
             "ClassifierFreeZeroStarGuidance",
+            "FrequencyDecoupledGuidance",
             "PerturbedAttentionGuidance",
             "SkipLayerGuidance",
             "SmoothedEnergyGuidance",
@@ -804,6 +805,7 @@
             AutoGuidance,
             ClassifierFreeGuidance,
             ClassifierFreeZeroStarGuidance,
+            FrequencyDecoupledGuidance,
             PerturbedAttentionGuidance,
             SkipLayerGuidance,
             SmoothedEnergyGuidance,

src/diffusers/guiders/__init__.py

@@ -22,6 +22,7 @@
     from .auto_guidance import AutoGuidance
     from .classifier_free_guidance import ClassifierFreeGuidance
     from .classifier_free_zero_star_guidance import ClassifierFreeZeroStarGuidance
+    from .frequency_decoupled_guidance import FrequencyDecoupledGuidance
     from .perturbed_attention_guidance import PerturbedAttentionGuidance
     from .skip_layer_guidance import SkipLayerGuidance
     from .smoothed_energy_guidance import SmoothedEnergyGuidance
@@ -32,6 +33,7 @@
         AutoGuidance,
         ClassifierFreeGuidance,
         ClassifierFreeZeroStarGuidance,
+        FrequencyDecoupledGuidance,
         PerturbedAttentionGuidance,
         SkipLayerGuidance,
         SmoothedEnergyGuidance,

src/diffusers/guiders/frequency_decoupled_guidance.py

@@ -0,0 +1,327 @@
+# Copyright 2025 The HuggingFace Team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import math
+from typing import TYPE_CHECKING, Dict, List, Optional, Tuple, Union
+
+import torch
+
+from ..configuration_utils import register_to_config
+from ..utils import is_kornia_available
+from .guider_utils import BaseGuidance, rescale_noise_cfg
+
+
+if TYPE_CHECKING:
+    from ..modular_pipelines.modular_pipeline import BlockState
+
+
+_CAN_USE_KORNIA = is_kornia_available()
+
+
+if _CAN_USE_KORNIA:
+    from kornia.geometry import pyrup as upsample_and_blur_func
+    from kornia.geometry.transform import build_laplacian_pyramid as build_laplacian_pyramid_func
+else:
+    upsample_and_blur_func = None
+    build_laplacian_pyramid_func = None
+
+
+def project(v0: torch.Tensor, v1: torch.Tensor, upcast_to_double: bool = True) -> Tuple[torch.Tensor, torch.Tensor]:
+    """
+    Project vector v0 onto vector v1, returning the parallel and orthogonal components of v0. Implementation from paper
+    (Algorithm 2).
+    """
+    # v0 shape: [B, ...]
+    # v1 shape: [B, ...]
+    # Assume first dim is a batch dim and all other dims are channel or "spatial" dims
+    all_dims_but_first = list(range(1, len(v0.shape)))
+    if upcast_to_double:
+        dtype = v0.dtype
+        v0, v1 = v0.double(), v1.double()
+    v1 = torch.nn.functional.normalize(v1, dim=all_dims_but_first)
+    v0_parallel = (v0 * v1).sum(dim=all_dims_but_first, keepdim=True) * v1
+    v0_orthogonal = v0 - v0_parallel
+    if upcast_to_double:
+        v0_parallel = v0_parallel.to(dtype)
+        v0_orthogonal = v0_orthogonal.to(dtype)
+    return v0_parallel, v0_orthogonal
+
+
+def build_image_from_pyramid(pyramid: List[torch.Tensor]) -> torch.Tensor:
+    """
+    Recovers the data space latents from the Laplacian pyramid frequency space. Implementation from the paper
+    (Algorihtm 2).
+    """
+    # pyramid shapes: [[B, C, H, W], [B, C, H/2, W/2], ...]
+    img = pyramid[-1]
+    for i in range(len(pyramid) - 2, -1, -1):
+        img = upsample_and_blur_func(img) + pyramid[i]
+    return img
+
+
+class FrequencyDecoupledGuidance(BaseGuidance):
+    """
+    Frequency-Decoupled Guidance (FDG): https://huggingface.co/papers/2506.19713
+
+    FDG is a technique similar to (and based on) classifier-free guidance (CFG) which is used to improve generation
+    quality and condition-following in diffusion models. Like CFG, during training we jointly train the model on both
+    conditional and unconditional data, and use a combination of the two during inference. (If you want more details on
+    how CFG works, you can check out the CFG guider.)
+
+    FDG differs from CFG in that the normal CFG prediction is instead decoupled into low- and high-frequency components
+    using a frequency transform (such as a Laplacian pyramid). The CFG update is then performed in frequency space
+    separately for the low- and high-frequency components with different guidance scales. Finally, the inverse
+    frequency transform is used to map the CFG frequency predictions back to data space (e.g. pixel space for images)
+    to form the final FDG prediction.
+
+    For images, the FDG authors found that using low guidance scales for the low-frequency components retains sample
+    diversity and realistic color composition, while using high guidance scales for high-frequency components enhances
+    sample quality (such as better visual details). Therefore, they recommend using low guidance scales (low w_low) for
+    the low-frequency components and high guidance scales (high w_high) for the high-frequency components. As an
+    example, they suggest w_low = 5.0 and w_high = 10.0 for Stable Diffusion XL (see Table 8 in the paper).
+
+    As with CFG, Diffusers implements the scaling and shifting on the unconditional prediction based on the [Imagen
+    paper](https://huggingface.co/papers/2205.11487), which is equivalent to what the original CFG paper proposed in
+    theory. [x_pred = x_uncond + scale * (x_cond - x_uncond)]
+
+    The `use_original_formulation` argument can be set to `True` to use the original CFG formulation mentioned in the
+    paper. By default, we use the diffusers-native implementation that has been in the codebase for a long time.
+
+    Args:
+        guidance_scales (`List[float]`, defaults to `[10.0, 5.0]`):
+            The scale parameter for frequency-decoupled guidance for each frequency component, listed from highest
+            frequency level to lowest. 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. The FDG authors recommend using higher guidance scales for higher frequency components and
+            lower guidance scales for lower frequency components (so `guidance_scales` should typically be sorted in
+            descending order).
+        guidance_rescale (`float` or `List[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). If a list is supplied, it should be the same length as
+            `guidance_scales`.
+        parallel_weights (`float` or `List[float]`, *optional*):
+            Optional weights for the parallel component of each frequency component of the projected CFG shift. If not
+            set, the weights will default to `1.0` for all components, which corresponds to using the normal CFG shift
+            (that is, equal weights for the parallel and orthogonal components). If set, a value in `[0, 1]` is
+            recommended. If a list is supplied, it should be the same length as `guidance_scales`.
+        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` or `List[float]`, defaults to `0.0`):
+            The fraction of the total number of denoising steps after which guidance starts. If a list is supplied, it
+            should be the same length as `guidance_scales`.
+        stop (`float` or `List[float]`, defaults to `1.0`):
+            The fraction of the total number of denoising steps after which guidance stops. If a list is supplied, it
+            should be the same length as `guidance_scales`.
+        guidance_rescale_space (`str`, defaults to `"data"`):
+            Whether to performance guidance rescaling in `"data"` space (after the full FDG update in data space) or in
+            `"freq"` space (right after the CFG update, for each freq level). Note that frequency space rescaling is
+            speculative and may not produce expected results. If `"data"` is set, the first `guidance_rescale` value
+            will be used; otherwise, per-frequency-level guidance rescale values will be used if available.
+        upcast_to_double (`bool`, defaults to `True`):
+            Whether to upcast certain operations, such as the projection operation when using `parallel_weights`, to
+            float64 when performing guidance. This may result in better performance at the cost of increased runtime.
+    """
+
+    _input_predictions = ["pred_cond", "pred_uncond"]
+
+    @register_to_config
+    def __init__(
+        self,
+        guidance_scales: Union[List[float], Tuple[float]] = [10.0, 5.0],
+        guidance_rescale: Union[float, List[float], Tuple[float]] = 0.0,
+        parallel_weights: Optional[Union[float, List[float], Tuple[float]]] = None,
+        use_original_formulation: bool = False,
+        start: Union[float, List[float], Tuple[float]] = 0.0,
+        stop: Union[float, List[float], Tuple[float]] = 1.0,
+        guidance_rescale_space: str = "data",
+        upcast_to_double: bool = True,
+    ):
+        if not _CAN_USE_KORNIA:
+            raise ImportError(
+                "The `FrequencyDecoupledGuidance` guider cannot be instantiated because the `kornia` library on which "
+                "it depends is not available in the current environment. You can install `kornia` with `pip install "
+                "kornia`."
+            )
+
+        # Set start to earliest start for any freq component and stop to latest stop for any freq component
+        min_start = start if isinstance(start, float) else min(start)
+        max_stop = stop if isinstance(stop, float) else max(stop)
+        super().__init__(min_start, max_stop)
+
+        self.guidance_scales = guidance_scales
+        self.levels = len(guidance_scales)
+
+        if isinstance(guidance_rescale, float):
+            self.guidance_rescale = [guidance_rescale] * self.levels
+        elif len(guidance_rescale) == self.levels:
+            self.guidance_rescale = guidance_rescale
+        else:
+            raise ValueError(
+                f"`guidance_rescale` has length {len(guidance_rescale)} but should have the same length as "
+                f"`guidance_scales` ({len(self.guidance_scales)})"
+            )
+        # Whether to perform guidance rescaling in frequency space (right after the CFG update) or data space (after
+        # transforming from frequency space back to data space)
+        if guidance_rescale_space not in ["data", "freq"]:
+            raise ValueError(
+                f"Guidance rescale space is {guidance_rescale_space} but must be one of `data` or `freq`."
+            )
+        self.guidance_rescale_space = guidance_rescale_space
+
+        if parallel_weights is None:
+            # Use normal CFG shift (equal weights for parallel and orthogonal components)
+            self.parallel_weights = [1.0] * self.levels
+        elif isinstance(parallel_weights, float):
+            self.parallel_weights = [parallel_weights] * self.levels
+        elif len(parallel_weights) == self.levels:
+            self.parallel_weights = parallel_weights
+        else:
+            raise ValueError(
+                f"`parallel_weights` has length {len(parallel_weights)} but should have the same length as "
+                f"`guidance_scales` ({len(self.guidance_scales)})"
+            )
+
+        self.use_original_formulation = use_original_formulation
+        self.upcast_to_double = upcast_to_double
+
+        if isinstance(start, float):
+            self.guidance_start = [start] * self.levels
+        elif len(start) == self.levels:
+            self.guidance_start = start
+        else:
+            raise ValueError(
+                f"`start` has length {len(start)} but should have the same length as `guidance_scales` "
+                f"({len(self.guidance_scales)})"
+            )
+        if isinstance(stop, float):
+            self.guidance_stop = [stop] * self.levels
+        elif len(stop) == self.levels:
+            self.guidance_stop = stop
+        else:
+            raise ValueError(
+                f"`stop` has length {len(stop)} but should have the same length as `guidance_scales` "
+                f"({len(self.guidance_scales)})"
+            )
+
+    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) -> torch.Tensor:
+        pred = None
+
+        if not self._is_fdg_enabled():
+            pred = pred_cond
+        else:
+            # Apply the frequency transform (e.g. Laplacian pyramid) to the conditional and unconditional predictions.
+            pred_cond_pyramid = build_laplacian_pyramid_func(pred_cond, self.levels)
+            pred_uncond_pyramid = build_laplacian_pyramid_func(pred_uncond, self.levels)
+
+            # From high frequencies to low frequencies, following the paper implementation
+            pred_guided_pyramid = []
+            parameters = zip(self.guidance_scales, self.parallel_weights, self.guidance_rescale)
+            for level, (guidance_scale, parallel_weight, guidance_rescale) in enumerate(parameters):
+                if self._is_fdg_enabled_for_level(level):
+                    # Get the cond/uncond preds (in freq space) at the current frequency level
+                    pred_cond_freq = pred_cond_pyramid[level]
+                    pred_uncond_freq = pred_uncond_pyramid[level]
+
+                    shift = pred_cond_freq - pred_uncond_freq
+
+                    # Apply parallel weights, if used (1.0 corresponds to using the normal CFG shift)
+                    if not math.isclose(parallel_weight, 1.0):
+                        shift_parallel, shift_orthogonal = project(shift, pred_cond_freq, self.upcast_to_double)
+                        shift = parallel_weight * shift_parallel + shift_orthogonal
+
+                    # Apply CFG update for the current frequency level
+                    pred = pred_cond_freq if self.use_original_formulation else pred_uncond_freq
+                    pred = pred + guidance_scale * shift
+
+                    if self.guidance_rescale_space == "freq" and guidance_rescale > 0.0:
+                        pred = rescale_noise_cfg(pred, pred_cond_freq, guidance_rescale)
+
+                    # Add the current FDG guided level to the FDG prediction pyramid
+                    pred_guided_pyramid.append(pred)
+                else:
+                    # Add the current pred_cond_pyramid level as the "non-FDG" prediction
+                    pred_guided_pyramid.append(pred_cond_freq)
+
+            # Convert from frequency space back to data (e.g. pixel) space by applying inverse freq transform
+            pred = build_image_from_pyramid(pred_guided_pyramid)
+
+            # If rescaling in data space, use the first elem of self.guidance_rescale as the "global" rescale value
+            # across all freq levels
+            if self.guidance_rescale_space == "data" and self.guidance_rescale[0] > 0.0:
+                pred = rescale_noise_cfg(pred, pred_cond, self.guidance_rescale[0])
+
+        return pred, {}
+
+    @property
+    def is_conditional(self) -> bool:
+        return self._count_prepared == 1
+
+    @property
+    def num_conditions(self) -> int:
+        num_conditions = 1
+        if self._is_fdg_enabled():
+            num_conditions += 1
+        return num_conditions
+
+    def _is_fdg_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 = all(math.isclose(guidance_scale, 0.0) for guidance_scale in self.guidance_scales)
+        else:
+            is_close = all(math.isclose(guidance_scale, 1.0) for guidance_scale in self.guidance_scales)
+
+        return is_within_range and not is_close
+
+    def _is_fdg_enabled_for_level(self, level: int) -> bool:
+        if not self._enabled:
+            return False
+
+        is_within_range = True
+        if self._num_inference_steps is not None:
+            skip_start_step = int(self.guidance_start[level] * self._num_inference_steps)
+            skip_stop_step = int(self.guidance_stop[level] * 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_scales[level], 0.0)
+        else:
+            is_close = math.isclose(self.guidance_scales[level], 1.0)
+
+        return is_within_range and not is_close

src/diffusers/utils/__init__.py

@@ -82,6 +82,7 @@
     is_k_diffusion_available,
     is_k_diffusion_version,
     is_kernels_available,
+    is_kornia_available,
     is_librosa_available,
     is_matplotlib_available,
     is_nltk_available,

src/diffusers/utils/dummy_pt_objects.py

@@ -62,6 +62,21 @@ def from_pretrained(cls, *args, **kwargs):
         requires_backends(cls, ["torch"])
 
 
+class FrequencyDecoupledGuidance(metaclass=DummyObject):
+    _backends = ["torch"]
+
+    def __init__(self, *args, **kwargs):
+        requires_backends(self, ["torch"])
+
+    @classmethod
+    def from_config(cls, *args, **kwargs):
+        requires_backends(cls, ["torch"])
+
+    @classmethod
+    def from_pretrained(cls, *args, **kwargs):
+        requires_backends(cls, ["torch"])
+
+
 class PerturbedAttentionGuidance(metaclass=DummyObject):
     _backends = ["torch"]