diff --git a/train.py b/train.py
index 3ae004eb..90a16c3b 100644
--- a/train.py
+++ b/train.py
@@ -171,6 +171,13 @@ def training(dataset, opt, pipe, testing_iterations, saving_iterations, checkpoi
         Ll1 = l1_loss_appearance(image, gt_image, appearances, viewpoint_idx) # use L1 loss for the transformed image if using decoupled appearance
         loss = (1.0 - opt.lambda_dssim) * Ll1 + opt.lambda_dssim * (1.0 - ssim(image, gt_image))
         
+        # alpha loss
+        if opt.lambda_mask > 0:
+            opacity = 1 - render_pkg["rend_alpha"].clamp(1e-6, 1-1e-6)
+            bg = 1 - viewpoint_cam.gt_alpha_mask
+            mask_error = (- bg * torch.log(opacity)).mean()
+            loss += opt.lambda_mask * mask_error
+
         # regularization
         lambda_normal = opt.lambda_normal if iteration > 7000 else 0.0
         lambda_depth = opt.propagation_begin if iteration > opt.propagation_begin else 0.0