Fix Zero Terminal SNR option

backend/modules/k_prediction.py

@@ -45,6 +45,26 @@ def flux_time_shift(mu, sigma, t):
     return math.exp(mu) / (math.exp(mu) + (1 / t - 1) ** sigma)
 
 
+def rescale_zero_terminal_snr_sigmas(sigmas):
+    alphas_cumprod = 1 / ((sigmas * sigmas) + 1)
+    alphas_bar_sqrt = alphas_cumprod.sqrt()
+
+    # Store old values.
+    alphas_bar_sqrt_0 = alphas_bar_sqrt[0].clone()
+    alphas_bar_sqrt_T = alphas_bar_sqrt[-1].clone()
+
+    # Shift so the last timestep is zero.
+    alphas_bar_sqrt -= (alphas_bar_sqrt_T)
+
+    # Scale so the first timestep is back to the old value.
+    alphas_bar_sqrt *= alphas_bar_sqrt_0 / (alphas_bar_sqrt_0 - alphas_bar_sqrt_T)
+
+    # Convert alphas_bar_sqrt to betas
+    alphas_bar = alphas_bar_sqrt**2  # Revert sqrt
+    alphas_bar[-1] = 4.8973451890853435e-08
+    return ((1 - alphas_bar) / alphas_bar) ** 0.5
+
+
 class AbstractPrediction(torch.nn.Module):
     def __init__(self, sigma_data=1.0, prediction_type='epsilon'):
         super().__init__()
@@ -114,6 +134,10 @@ class Prediction(AbstractPrediction):
         self.register_buffer('log_sigmas', sigmas.log().float())
         return
 
+    def set_sigmas(self, sigmas):
+        self.register_buffer('sigmas', sigmas.float())
+        self.register_buffer('log_sigmas', sigmas.log().float())
+
     @property
     def sigma_min(self):
         return self.sigmas[0]

modules/processing.py

@@ -35,6 +35,7 @@ from modules.sd_models import apply_token_merging, forge_model_reload
 from modules_forge.utils import apply_circular_forge
 from modules_forge import main_entry
 from backend import memory_management
+from backend.modules.k_prediction import rescale_zero_terminal_snr_sigmas
 
 
 # some of those options should not be changed at all because they would break the model, so I removed them from options.
@@ -969,25 +970,22 @@ def process_images_inner(p: StableDiffusionProcessing) -> Processed:
             if p.n_iter > 1:
                 shared.state.job = f"Batch {n+1} out of {p.n_iter}"
 
+            # TODO: This currently seems broken. It should be fixed or removed.
             sd_models.apply_alpha_schedule_override(p.sd_model, p)
 
-            alphas_cumprod_modifiers = p.sd_model.forge_objects.unet.model_options.get('alphas_cumprod_modifiers', [])
-            alphas_cumprod_backup = None
-
-            if len(alphas_cumprod_modifiers) > 0:
-                alphas_cumprod_backup = p.sd_model.alphas_cumprod
-                for modifier in alphas_cumprod_modifiers:
-                    p.sd_model.alphas_cumprod = modifier(p.sd_model.alphas_cumprod)
-                p.sd_model.forge_objects.unet.model.model_sampling.set_sigmas(((1 - p.sd_model.alphas_cumprod) / p.sd_model.alphas_cumprod) ** 0.5)
+            sigmas_backup = None
+            if opts.sd_noise_schedule == "Zero Terminal SNR" and p is not None:
+                p.extra_generation_params['Noise Schedule'] = opts.sd_noise_schedule
+                sigmas_backup = p.sd_model.forge_objects.unet.model.predictor.sigmas
+                p.sd_model.forge_objects.unet.model.predictor.set_sigmas(rescale_zero_terminal_snr_sigmas(p.sd_model.forge_objects.unet.model.predictor.sigmas))
 
             samples_ddim = p.sample(conditioning=p.c, unconditional_conditioning=p.uc, seeds=p.seeds, subseeds=p.subseeds, subseed_strength=p.subseed_strength, prompts=p.prompts)
 
             for x_sample in samples_ddim:
                 p.latents_after_sampling.append(x_sample)
 
-            if alphas_cumprod_backup is not None:
-                p.sd_model.alphas_cumprod = alphas_cumprod_backup
-                p.sd_model.forge_objects.unet.model.model_sampling.set_sigmas(((1 - p.sd_model.alphas_cumprod) / p.sd_model.alphas_cumprod) ** 0.5)
+            if sigmas_backup is not None:
+                p.sd_model.forge_objects.unet.model.predictor.set_sigmas(sigmas_backup)
 
             if p.scripts is not None:
                 ps = scripts.PostSampleArgs(samples_ddim)