Fix T89249: incorrect mesh validate error with zeroed vertex normals

It's not an error for centered vertices to have a zero normal.

1 files changed, 25 insertions, 4 deletions

diff --git a/source/blender/blenkernel/intern/mesh_validate.c b/source/blender/blenkernel/intern/mesh_validate.c
index 3570b1dd933..df84cf6607f 100644
--- a/source/blender/blenkernel/intern/mesh_validate.c
+++ b/source/blender/blenkernel/intern/mesh_validate.c
@@ -217,6 +217,16 @@ static int search_polyloop_cmp(const void *v1, const void *v2)
  * Validate the mesh, \a do_fixes requires \a mesh to be non-null.
  *
  * \return false if no changes needed to be made.
+ *
+ * Vertex Normals
+ * ==============
+ *
+ * While zeroed normals are checked, these checks aren't comprehensive.
+ * Technically, to detect errors here a normal recalculation and comparison is necessary.
+ * However this function is mainly to prevent severe errors in geometry
+ * (invalid data that will crash Blender, or cause some features to behave incorrectly),
+ * not to detect subtle differences in the resulting normals which could be caused
+ * by importers that load normals (for example).
  */
 /* NOLINTNEXTLINE: readability-function-size */
 bool BKE_mesh_validate_arrays(Mesh *mesh,
@@ -328,10 +338,21 @@ bool BKE_mesh_validate_arrays(Mesh *mesh,
     }
 
     if (fix_normal) {
-      PRINT_ERR("\tVertex %u: has zero normal, assuming Z-up normal", i);
-      if (do_fixes) {
-        mv->no[2] = SHRT_MAX;
-        fix_flag.verts = true;
+      /* If the vertex normal accumulates to zero or isn't part of a face, the location is used.
+       * When the location is also zero, a zero normal warning should not be raised.
+       * since this is the expected behavior of normal calculation.
+       *
+       * This avoids false positives but isn't foolproof as it's possible the vertex
+       * is part of a polygon that has a normal which this vertex should be using,
+       * although it's also possible degenerate/opposite faces accumulate to a zero vector.
+       * To detect this a full normal recalculation would be needed, which is out of scope
+       * for a basic validity check (see "Vertex Normal" in the doc-string). */
+      if (!is_zero_v3(mv->co)) {
+        PRINT_ERR("\tVertex %u: has zero normal, assuming Z-up normal", i);
+        if (do_fixes) {
+          mv->no[2] = SHRT_MAX;
+          fix_flag.verts = true;
+        }
       }
     }
   }