diffsynth/diffusion/flow_match.py

import torch, math
from typing_extensions import Literal


class FlowMatchScheduler():

    def __init__(self, template: Literal["FLUX.1", "Wan", "Qwen-Image", "FLUX.2", "Z-Image"] = "FLUX.1"):
        self.set_timesteps_fn = {
            "FLUX.1": FlowMatchScheduler.set_timesteps_flux,
            "Wan": FlowMatchScheduler.set_timesteps_wan,
            "Qwen-Image": FlowMatchScheduler.set_timesteps_qwen_image,
            "FLUX.2": FlowMatchScheduler.set_timesteps_flux2,
            "Z-Image": FlowMatchScheduler.set_timesteps_z_image,
        }.get(template, FlowMatchScheduler.set_timesteps_flux)
        self.num_train_timesteps = 1000

    @staticmethod
    def set_timesteps_flux(num_inference_steps=100, denoising_strength=1.0, shift=None):
        sigma_min = 0.003/1.002
        sigma_max = 1.0
        shift = 3 if shift is None else shift
        num_train_timesteps = 1000
        sigma_start = sigma_min + (sigma_max - sigma_min) * denoising_strength
        sigmas = torch.linspace(sigma_start, sigma_min, num_inference_steps)
        sigmas = shift * sigmas / (1 + (shift - 1) * sigmas)
        timesteps = sigmas * num_train_timesteps
        return sigmas, timesteps
    
    @staticmethod
    def set_timesteps_wan(num_inference_steps=100, denoising_strength=1.0, shift=None):
        sigma_min = 0.0
        sigma_max = 1.0
        shift = 5 if shift is None else shift
        num_train_timesteps = 1000
        sigma_start = sigma_min + (sigma_max - sigma_min) * denoising_strength
        sigmas = torch.linspace(sigma_start, sigma_min, num_inference_steps + 1)[:-1]
        sigmas = shift * sigmas / (1 + (shift - 1) * sigmas)
        timesteps = sigmas * num_train_timesteps
        return sigmas, timesteps
    
    @staticmethod
    def _calculate_shift_qwen_image(image_seq_len, base_seq_len=256, max_seq_len=8192, base_shift=0.5, max_shift=0.9):
        m = (max_shift - base_shift) / (max_seq_len - base_seq_len)
        b = base_shift - m * base_seq_len
        mu = image_seq_len * m + b
        return mu
    
    @staticmethod
    def set_timesteps_qwen_image(num_inference_steps=100, denoising_strength=1.0, exponential_shift_mu=None, dynamic_shift_len=None):
        sigma_min = 0.0
        sigma_max = 1.0
        num_train_timesteps = 1000
        shift_terminal = 0.02
        # Sigmas
        sigma_start = sigma_min + (sigma_max - sigma_min) * denoising_strength
        sigmas = torch.linspace(sigma_start, sigma_min, num_inference_steps + 1)[:-1]
        # Mu
        if exponential_shift_mu is not None:
            mu = exponential_shift_mu
        elif dynamic_shift_len is not None:
            mu = FlowMatchScheduler._calculate_shift_qwen_image(dynamic_shift_len)
        else:
            mu = 0.8
        sigmas = math.exp(mu) / (math.exp(mu) + (1 / sigmas - 1))
        # Shift terminal
        one_minus_z = 1 - sigmas
        scale_factor = one_minus_z[-1] / (1 - shift_terminal)
        sigmas = 1 - (one_minus_z / scale_factor)
        # Timesteps
        timesteps = sigmas * num_train_timesteps
        return sigmas, timesteps
    
    @staticmethod
    def compute_empirical_mu(image_seq_len, num_steps):
        a1, b1 = 8.73809524e-05, 1.89833333
        a2, b2 = 0.00016927, 0.45666666

        if image_seq_len > 4300:
            mu = a2 * image_seq_len + b2
            return float(mu)

        m_200 = a2 * image_seq_len + b2
        m_10 = a1 * image_seq_len + b1

        a = (m_200 - m_10) / 190.0
        b = m_200 - 200.0 * a
        mu = a * num_steps + b

        return float(mu)
    
    @staticmethod
    def set_timesteps_flux2(num_inference_steps=100, denoising_strength=1.0, dynamic_shift_len=1024//16*1024//16):
        sigma_min = 1 / num_inference_steps
        sigma_max = 1.0
        num_train_timesteps = 1000
        sigma_start = sigma_min + (sigma_max - sigma_min) * denoising_strength
        sigmas = torch.linspace(sigma_start, sigma_min, num_inference_steps)
        mu = FlowMatchScheduler.compute_empirical_mu(dynamic_shift_len, num_inference_steps)
        sigmas = math.exp(mu) / (math.exp(mu) + (1 / sigmas - 1))
        timesteps = sigmas * num_train_timesteps
        return sigmas, timesteps

    @staticmethod
    def set_timesteps_z_image(num_inference_steps=100, denoising_strength=1.0, shift=None, target_timesteps=None):
        sigma_min = 0.0
        sigma_max = 1.0
        shift = 3 if shift is None else shift
        num_train_timesteps = 1000
        sigma_start = sigma_min + (sigma_max - sigma_min) * denoising_strength
        sigmas = torch.linspace(sigma_start, sigma_min, num_inference_steps + 1)[:-1]
        sigmas = shift * sigmas / (1 + (shift - 1) * sigmas)
        timesteps = sigmas * num_train_timesteps
        if target_timesteps is not None:
            target_timesteps = target_timesteps.to(dtype=timesteps.dtype, device=timesteps.device)
            for timestep in target_timesteps:
                timestep_id = torch.argmin((timesteps - timestep).abs())
                timesteps[timestep_id] = timestep
        return sigmas, timesteps
    
    def set_training_weight(self):
        steps = 1000
        x = self.timesteps
        y = torch.exp(-2 * ((x - steps / 2) / steps) ** 2)
        y_shifted = y - y.min()
        bsmntw_weighing = y_shifted * (steps / y_shifted.sum())
        if len(self.timesteps) != 1000:
            # This is an empirical formula.
            bsmntw_weighing = bsmntw_weighing * (len(self.timesteps) / steps)
            bsmntw_weighing = bsmntw_weighing + bsmntw_weighing[1]
        self.linear_timesteps_weights = bsmntw_weighing
        
    def set_timesteps(self, num_inference_steps=100, denoising_strength=1.0, training=False, **kwargs):
        self.sigmas, self.timesteps = self.set_timesteps_fn(
            num_inference_steps=num_inference_steps,
            denoising_strength=denoising_strength,
            **kwargs,
        )
        if training:
            self.set_training_weight()
            self.training = True
        else:
            self.training = False

    def step(self, model_output, timestep, sample, to_final=False, **kwargs):
        if isinstance(timestep, torch.Tensor):
            timestep = timestep.cpu()
        timestep_id = torch.argmin((self.timesteps - timestep).abs())
        sigma = self.sigmas[timestep_id]
        if to_final or timestep_id + 1 >= len(self.timesteps):
            sigma_ = 0
        else:
            sigma_ = self.sigmas[timestep_id + 1]
        prev_sample = sample + model_output * (sigma_ - sigma)
        return prev_sample
    
    def return_to_timestep(self, timestep, sample, sample_stablized):
        if isinstance(timestep, torch.Tensor):
            timestep = timestep.cpu()
        timestep_id = torch.argmin((self.timesteps - timestep).abs())
        sigma = self.sigmas[timestep_id]
        model_output = (sample - sample_stablized) / sigma
        return model_output
    
    def add_noise(self, original_samples, noise, timestep):
        if isinstance(timestep, torch.Tensor):
            timestep = timestep.cpu()
        timestep_id = torch.argmin((self.timesteps - timestep).abs())
        sigma = self.sigmas[timestep_id]
        sample = (1 - sigma) * original_samples + sigma * noise
        return sample
    
    def training_target(self, sample, noise, timestep):
        target = noise - sample
        return target
    
    def training_weight(self, timestep):
        timestep_id = torch.argmin((self.timesteps - timestep.to(self.timesteps.device)).abs())
        weights = self.linear_timesteps_weights[timestep_id]
        return weights