[#23673] Modifier construction gives correct result in viewport but incorrect in render.

When there are 2+ consecutive deform modifiers, the second modifier was getting incorrect normals, this only showed up for the displace modifier since its the only deform modifier that uses vertex normals.

It would have been easy to fix this by always calculating normals on deform modifiers, but slow.

To fix this I added a function to check if a deform modifier needs normals, so the normal calculation function only runs if there are 2 modifiers in a row and the second uses normals.

1 files changed, 13 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/DerivedMesh.c b/source/blender/blenkernel/intern/DerivedMesh.c
index 0b3bd106544..1f49d519e7d 100644
--- a/source/blender/blenkernel/intern/DerivedMesh.c
+++ b/source/blender/blenkernel/intern/DerivedMesh.c
@@ -1670,6 +1670,7 @@ static void mesh_calc_modifiers(Scene *scene, Object *ob, float (*inputVertexCos
 	DerivedMesh *dm, *orcodm, *clothorcodm, *finaldm;
 	int numVerts = me->totvert;
 	int required_mode;
+	int isPrevDeform= FALSE;
 
 	md = firstmd = (useDeform<0) ? ob->modifiers.first : modifiers_getVirtualModifierList(ob);
 
@@ -1787,6 +1788,16 @@ static void mesh_calc_modifiers(Scene *scene, Object *ob, float (*inputVertexCos
 				}
 			}
 
+			/* if this is not the last modifier in the stack then recalculate the normals
+			 * to avoid giving bogus normals to the next modifier see: [#23673] */
+			if(isPrevDeform && mti->dependsOnNormals(md)) {
+				/* XXX, this covers bug #23673, but we may need normal calc for other types */
+				if(dm->type == DM_TYPE_CDDM) {
+					CDDM_apply_vert_coords(dm, deformedVerts);
+					CDDM_calc_normals(dm);
+				}
+			}
+
 			mti->deformVerts(md, ob, dm, deformedVerts, numVerts, useRenderParams, useDeform);
 		} else {
 			DerivedMesh *ndm;
@@ -1897,6 +1908,8 @@ static void mesh_calc_modifiers(Scene *scene, Object *ob, float (*inputVertexCos
 			}
 		}
 
+		isPrevDeform= (mti->type == eModifierTypeType_OnlyDeform);
+
 		/* grab modifiers until index i */
 		if((index >= 0) && (modifiers_indexInObject(ob, md) >= index))
 			break;