io_scene_fbx/import_fbx.py

@@ -1962,19 +1962,28 @@ def blen_read_shapes(fbx_tmpl, fbx_data, objects, me, scene):
         # Special case for Blender exported Shape Keys with a Vertex Group assigned. The Vertex Group weights are stored
         # in the FullWeights array.
         # XXX - It's possible, though very rare, to get a false positive here and create a Vertex Group when we
-        #       shouldn't. This should only happen if there are extraneous FullWeights or there is a single FullWeight
-        #       and its value is not 100.0.
-        is_vertex_weights = len(full_weights) == len(indices)
-        # Skip creating a Vertex Group if all the weights are 100.0 because such a Vertex Group has no effect and this
-        # avoids creating a Vertex Group for imported Shapes that only move a single vertex because their singular
-        # FullWeight is expected to always be 100.0.
-        if is_vertex_weights and np.any(full_weights != 100.0):
-            # Not doing the division in-place because it's technically possible for FBX Shape Keys to be used by more
-            # than one mesh, though this shouldn't be the case for Blender exported Shape Keys.
+        #       shouldn't. This should only be possible when there are extraneous FullWeights or when there is a single
+        #       FullWeight and its value is not 100.0.
+        if (
+                # Blender exported Shape Keys only ever export as 1 Shape per BlendShapeChannel.
+                num_shapes_assigned_to_channel == 1
+                # There should be one vertex weight for each vertex moved by the Shape.
+                and len(full_weights) == len(indices)
+                # Skip creating a Vertex Group when all the weights are 100.0 because such a Vertex Group has no effect.
+                # This also avoids creating a Vertex Group for imported Shapes that only move a single vertex because
+                # their BlendShapeChannel's singular FullWeight is expected to always be 100.0.
+                and not np.all(full_weights == 100.0)
+                # Blender vertex weights are always within the [0.0, 1.0] range (scaled to [0.0, 100.0] when saving to
+                # FBX). This can eliminate imported BlendShapeChannels from Unreal that have extraneous FullWeights
+                # because the extraneous values are usually negative.
+                and np.all((full_weights >= 0.0) & (full_weights <= 100.0))
+        ):
+            # Not doing the division in-place because it's technically possible for FBX BlendShapeChannels to be used by
+            # more than one FBX BlendShape, though this shouldn't be the case for Blender exported Shape Keys.
             vgweights = full_weights / 100.0
         else:
             vgweights = None
-            # There must be a FullWeight for each Shape. Any excess is ignored.
+            # There must be a FullWeight for each Shape. Any extra FullWeights are ignored.
             assert(len(full_weights) >= num_shapes_assigned_to_channel)
 
         # To add shape keys to the mesh, an Object using the mesh is needed.