Restore min SNR

1 files changed, 12 insertions, 13 deletions

diff --git a/training/functional.py b/training/functional.py
index 15b95ba..8dc2b9f 100644
--- a/training/functional.py
+++ b/training/functional.py
@@ -261,7 +261,8 @@ def loss_step(
     seed: int,
     step: int,
     batch: dict[str, Any],
-    eval: bool = False
+    eval: bool = False,
+    min_snr_gamma: int = 5
 ):
     # Convert images to latent space
     latents = vae.encode(batch["pixel_values"]).latent_dist.sample()
@@ -307,23 +308,21 @@ def loss_step(
     model_pred = unet(noisy_latents, timesteps, encoder_hidden_states).sample
 
     # Get the target for loss depending on the prediction type
+    alpha_t = noise_scheduler.alphas_cumprod[timesteps].float()
+    snr = alpha_t / (1 - alpha_t)
+    min_snr = snr.clamp(max=min_snr_gamma)
+
     if noise_scheduler.config.prediction_type == "epsilon":
         target = noise
-
-        snr_weights = 1
+        loss_weight = min_snr / snr
     elif noise_scheduler.config.prediction_type == "v_prediction":
         target = noise_scheduler.get_velocity(latents, noise, timesteps)
-
-        p2_gamma = 1
-        p2_k = 1
-
-        alpha_t = noise_scheduler.alphas_cumprod[timesteps].float()
-        snr = 1.0 / (1 - alpha_t) - 1
-        snr_weights = 1 / (p2_k + snr) ** p2_gamma
-        snr_weights = snr_weights[..., None, None, None]
+        loss_weight = min_snr / (snr + 1)
     else:
         raise ValueError(f"Unknown prediction type {noise_scheduler.config.prediction_type}")
 
+    loss_weight = loss_weight[..., None, None, None]
+
     if with_prior_preservation:
         # Chunk the noise and model_pred into two parts and compute the loss on each part separately.
         model_pred, model_pred_prior = torch.chunk(model_pred, 2, dim=0)
@@ -340,7 +339,7 @@ def loss_step(
     else:
         loss = F.mse_loss(model_pred.float(), target.float(), reduction="none")
 
-    loss = (snr_weights * loss).mean([1, 2, 3]).mean()
+    loss = (loss_weight * loss).mean([1, 2, 3]).mean()
     acc = (model_pred == target).float().mean()
 
     return loss, acc, bsz
@@ -413,7 +412,7 @@ def train_loop(
     try:
         for epoch in range(num_epochs):
             if accelerator.is_main_process:
-                if epoch % sample_frequency == 0:
+                if epoch % sample_frequency == 0 and epoch != 0:
                     local_progress_bar.clear()
                     global_progress_bar.clear()