diff --git a/comfy_extras/nodes_compositing.py b/comfy_extras/nodes_compositing.py
index 181b36ed..48fe5e3d 100644
--- a/comfy_extras/nodes_compositing.py
+++ b/comfy_extras/nodes_compositing.py
@@ -28,6 +28,14 @@ class PorterDuffMode(Enum):
 
 
 def porter_duff_composite(src_image: torch.Tensor, src_alpha: torch.Tensor, dst_image: torch.Tensor, dst_alpha: torch.Tensor, mode: PorterDuffMode):
+    # convert mask to alpha
+    src_alpha = 1 - src_alpha
+    dst_alpha = 1 - dst_alpha
+    # premultiply alpha
+    src_image = src_image * src_alpha
+    dst_image = dst_image * dst_alpha
+
+    # composite ops below assume alpha-premultiplied images
     if mode == PorterDuffMode.ADD:
         out_alpha = torch.clamp(src_alpha + dst_alpha, 0, 1)
         out_image = torch.clamp(src_image + dst_image, 0, 1)
@@ -35,7 +43,7 @@ def porter_duff_composite(src_image: torch.Tensor, src_alpha: torch.Tensor, dst_
         out_alpha = torch.zeros_like(dst_alpha)
         out_image = torch.zeros_like(dst_image)
     elif mode == PorterDuffMode.DARKEN:
-        out_alpha = src_alpha + dst_alpha  - src_alpha * dst_alpha
+        out_alpha = src_alpha + dst_alpha - src_alpha * dst_alpha
         out_image = (1 - dst_alpha) * src_image + (1 - src_alpha) * dst_image + torch.min(src_image, dst_image)
     elif mode == PorterDuffMode.DST:
         out_alpha = dst_alpha
@@ -84,8 +92,13 @@ def porter_duff_composite(src_image: torch.Tensor, src_alpha: torch.Tensor, dst_
         out_alpha = (1 - dst_alpha) * src_alpha + (1 - src_alpha) * dst_alpha
         out_image = (1 - dst_alpha) * src_image + (1 - src_alpha) * dst_image
     else:
-        out_alpha = None
-        out_image = None
+        return None, None
+
+    # back to non-premultiplied alpha
+    out_image = torch.where(out_alpha > 1e-5, out_image / out_alpha, torch.zeros_like(out_image))
+    out_image = torch.clamp(out_image, 0, 1)
+    # convert alpha to mask
+    out_alpha = 1 - out_alpha
     return out_image, out_alpha