add a stablizing trick for steps < 15

ldm/models/diffusion/dpm_solver/dpm_solver.py

@@ -394,8 +394,8 @@ def data_prediction_fn(self, x, t):
         if self.thresholding:
             p = 0.995   # A hyperparameter in the paper of "Imagen" [1].
             s = torch.quantile(torch.abs(x0).reshape((x0.shape[0], -1)), p, dim=1)
-            s = expand_dims(torch.maximum(s, torch.ones_like(s).to(s.device)), dims)
-            x0 = torch.clamp(x0, -s, s) / (s / self.max_val)
+            s = expand_dims(torch.maximum(s, self.max_val * torch.ones_like(s).to(s.device)), dims)
+            x0 = torch.clamp(x0, -s, s) / s
         return x0
 
     def model_fn(self, x, t):
@@ -436,7 +436,7 @@ def get_time_steps(self, skip_type, t_T, t_0, N, device):
         else:
             raise ValueError("Unsupported skip_type {}, need to be 'logSNR' or 'time_uniform' or 'time_quadratic'".format(skip_type))
 
-    def get_orders_for_singlestep_solver(self, steps, order):
+    def get_orders_and_timesteps_for_singlestep_solver(self, steps, order, skip_type, t_T, t_0, device):
         """
         Get the order of each step for sampling by the singlestep DPM-Solver.
 
@@ -458,6 +458,13 @@ def get_orders_for_singlestep_solver(self, steps, order):
         Args:
             order: A `int`. The max order for the solver (2 or 3).
             steps: A `int`. The total number of function evaluations (NFE).
+            skip_type: A `str`. The type for the spacing of the time steps. We support three types:
+                - 'logSNR': uniform logSNR for the time steps.
+                - 'time_uniform': uniform time for the time steps. (**Recommended for high-resolutional data**.)
+                - 'time_quadratic': quadratic time for the time steps. (Used in DDIM for low-resolutional data.)
+            t_T: A `float`. The starting time of the sampling (default is T).
+            t_0: A `float`. The ending time of the sampling (default is epsilon).
+            device: A torch device.
         Returns:
             orders: A list of the solver order of each step.
         """
@@ -469,20 +476,26 @@ def get_orders_for_singlestep_solver(self, steps, order):
                 orders = [3,] * (K - 1) + [1]
             else:
                 orders = [3,] * (K - 1) + [2]
-            return orders
         elif order == 2:
-            K = steps // 2
             if steps % 2 == 0:
+                K = steps // 2
                 orders = [2,] * K
             else:
-                orders = [2,] * K + [1]
-            return orders
+                K = steps // 2 + 1
+                orders = [2,] * (K - 1) + [1]
         elif order == 1:
-            return [1,] * steps
+            K = 1
+            orders = [1,] * steps
         else:
             raise ValueError("'order' must be '1' or '2' or '3'.")
+        if skip_type == 'logSNR':
+            # To reproduce the results in DPM-Solver paper
+            timesteps_outer = self.get_time_steps(skip_type, t_T, t_0, K, device)
+        else:
+            timesteps_outer = self.get_time_steps(skip_type, t_T, t_0, steps, device)[torch.cumsum(torch.tensor([0,] + orders)).to(device)]
+        return timesteps_outer, orders
 
-    def denoise_fn(self, x, s):
+    def denoise_to_zero_fn(self, x, s):
         """
         Denoise at the final step, which is equivalent to solve the ODE from lambda_s to infty by first-order discretization. 
         """
@@ -950,8 +963,8 @@ def dpm_solver_adaptive(self, x, order, t_T, t_0, h_init=0.05, atol=0.0078, rtol
         return x
 
     def sample(self, x, steps=20, t_start=None, t_end=None, order=3, skip_type='time_uniform',
-        method='singlestep', denoise=False, solver_type='dpm_solver', atol=0.0078,
-        rtol=0.05,
+        method='singlestep', lower_order_final=True, denoise_to_zero=False, solver_type='dpm_solver',
+        atol=0.0078, rtol=0.05,
     ):
         """
         Compute the sample at time `t_end` by DPM-Solver, given the initial `x` at time `t_start`.
@@ -1035,8 +1048,19 @@ def sample(self, x, steps=20, t_start=None, t_end=None, order=3, skip_type='time
             order: A `int`. The order of DPM-Solver.
             skip_type: A `str`. The type for the spacing of the time steps. 'time_uniform' or 'logSNR' or 'time_quadratic'.
             method: A `str`. The method for sampling. 'singlestep' or 'multistep' or 'singlestep_fixed' or 'adaptive'.
-            denoise: A `bool`. Whether to denoise at the final step. Default is False.
-                If `denoise` is True, the total NFE is (`steps` + 1).
+            denoise_to_zero: A `bool`. Whether to denoise to time 0 at the final step.
+                Default is `False`. If `denoise_to_zero` is `True`, the total NFE is (`steps` + 1).
+
+                This trick is firstly proposed by DDPM (https://arxiv.org/abs/2006.11239) and
+                score_sde (https://arxiv.org/abs/2011.13456). Such trick can improve the FID
+                for diffusion models sampling by diffusion SDEs for low-resolutional images
+                (such as CIFAR-10). However, we observed that such trick does not matter for
+                high-resolutional images. As it needs an additional NFE, we do not recommend
+                it for high-resolutional images.
+            lower_order_final: A `bool`. Whether to use lower order solvers at the final steps.
+                Only valid for `method=multistep` and `steps < 15`. We empirically find that
+                this trick is a key to stabilizing the sampling by DPM-Solver with very few steps
+                (especially for steps <= 10). So we recommend to set it to be `True`.
             solver_type: A `str`. The taylor expansion type for the solver. `dpm_solver` or `taylor`. We recommend `dpm_solver`.
             atol: A `float`. The absolute tolerance of the adaptive step size solver. Valid when `method` == 'adaptive'.
             rtol: A `float`. The relative tolerance of the adaptive step size solver. Valid when `method` == 'adaptive'.
@@ -1067,7 +1091,11 @@ def sample(self, x, steps=20, t_start=None, t_end=None, order=3, skip_type='time
                 # Compute the remaining values by `order`-th order multistep DPM-Solver.
                 for step in range(order, steps + 1):
                     vec_t = timesteps[step].expand(x.shape[0])
-                    x = self.multistep_dpm_solver_update(x, model_prev_list, t_prev_list, vec_t, order, solver_type=solver_type)
+                    if lower_order_final and steps < 15:
+                        step_order = min(order, steps + 1 - step)
+                    else:
+                        step_order = order
+                    x = self.multistep_dpm_solver_update(x, model_prev_list, t_prev_list, vec_t, step_order, solver_type=solver_type)
                     for i in range(order - 1):
                         t_prev_list[i] = t_prev_list[i + 1]
                         model_prev_list[i] = model_prev_list[i + 1]
@@ -1077,23 +1105,22 @@ def sample(self, x, steps=20, t_start=None, t_end=None, order=3, skip_type='time
                         model_prev_list[-1] = self.model_fn(x, vec_t)
         elif method in ['singlestep', 'singlestep_fixed']:
             if method == 'singlestep':
-                orders = self.get_orders_for_singlestep_solver(steps=steps, order=order)
-                timesteps = self.get_time_steps(skip_type=skip_type, t_T=t_T, t_0=t_0, N=steps, device=device)
+                timesteps_outer, orders = self.get_orders_and_timesteps_for_singlestep_solver(steps=steps, order=order, skip_type=skip_type, t_T=t_T, t_0=t_0, device=device)
             elif method == 'singlestep_fixed':
                 K = steps // order
                 orders = [order,] * K
-                timesteps = self.get_time_steps(skip_type=skip_type, t_T=t_T, t_0=t_0, N=(K * order), device=device)
-            with torch.no_grad():
-                i = 0
-                for order in orders:
-                    vec_s, vec_t = timesteps[i].expand(x.shape[0]), timesteps[i + order].expand(x.shape[0])
-                    h = self.noise_schedule.marginal_lambda(timesteps[i + order]) - self.noise_schedule.marginal_lambda(timesteps[i])
-                    r1 = None if order <= 1 else (self.noise_schedule.marginal_lambda(timesteps[i + 1]) - self.noise_schedule.marginal_lambda(timesteps[i])) / h
-                    r2 = None if order <= 2 else (self.noise_schedule.marginal_lambda(timesteps[i + 2]) - self.noise_schedule.marginal_lambda(timesteps[i])) / h
-                    x = self.singlestep_dpm_solver_update(x, vec_s, vec_t, order, solver_type=solver_type, r1=r1, r2=r2)
-                    i += order
-        if denoise:
-            x = self.denoise_fn(x, torch.ones((x.shape[0],)).to(device) * t_0)
+                timesteps_outer = self.get_time_steps(skip_type=skip_type, t_T=t_T, t_0=t_0, N=K, device=device)
+            for i, order in enumerate(orders):
+                t_T_inner, t_0_inner = timesteps_outer[i], timesteps_outer[i + 1]
+                timesteps_inner = self.get_time_steps(skip_type=skip_type, t_T=t_T_inner.item(), t_0=t_0_inner.item(), N=order, device=device)
+                lambda_inner = self.noise_schedule.marginal_lambda(timesteps_inner)
+                vec_s, vec_t = t_T_inner.tile(x.shape[0]), t_0_inner.tile(x.shape[0])
+                h = lambda_inner[-1] - lambda_inner[0]
+                r1 = None if order <= 1 else (lambda_inner[1] - lambda_inner[0]) / h
+                r2 = None if order <= 2 else (lambda_inner[2] - lambda_inner[0]) / h
+                x = self.singlestep_dpm_solver_update(x, vec_s, vec_t, order, solver_type=solver_type, r1=r1, r2=r2)
+        if denoise_to_zero:
+            x = self.denoise_to_zero_fn(x, torch.ones((x.shape[0],)).to(device) * t_0)
         return x
 
 

ldm/models/diffusion/dpm_solver/sampler.py

@@ -77,6 +77,6 @@ def sample(self,
         )
 
         dpm_solver = DPM_Solver(model_fn, ns, predict_x0=True, thresholding=False)
-        x = dpm_solver.sample(img, steps=S, skip_type="time_uniform", method="multistep", order=2)
+        x = dpm_solver.sample(img, steps=S, skip_type="time_uniform", method="multistep", order=2, lower_order_final=True)
 
         return x.to(device), None