Fix T59915: Skin modifier produces inverted normals on end-cap faces when vertically aligned edge is assigned root

Sometimes when the end caps pointed strait up (z axis) their faces would
become inverted.

It seems like the code tried to rely on a certain vertex order to always
happen. However this edge case did manage to produce an order that would
produce inverted cap faces.  So now I introduce a normal check instead
so we can be certain that the face will have the normal direction we
want.

Reviewed By: Jacques Lucke

Differential Revision: http://developer.blender.org/D4138

1 files changed, 14 insertions, 7 deletions

diff --git a/source/blender/modifiers/intern/MOD_skin.c b/source/blender/modifiers/intern/MOD_skin.c
index a344141bf4a..b6ab3f5b4fe 100644
--- a/source/blender/modifiers/intern/MOD_skin.c
+++ b/source/blender/modifiers/intern/MOD_skin.c
@@ -58,6 +58,7 @@
 #include "BLI_bitmap.h"
 #include "BLI_heap_simple.h"
 #include "BLI_math.h"
+#include "BLI_math_geom.h"
 #include "BLI_stack.h"
 
 #include "DNA_mesh_types.h"
@@ -86,7 +87,7 @@ typedef enum {
   CAP_START = 1,
   CAP_END = 2,
   SEAM_FRAME = 4,
-  ROOT = 8,
+  FLIP_NORMAL = 8,
 } SkinNodeFlag;
 
 typedef struct Frame {
@@ -513,12 +514,18 @@ static void end_node_frames(int v,
       negate_v3(mat[0]);
     }
 
-    /* End frame */
-    create_frame(&skin_nodes[v].frames[0], mvert[v].co, rad, mat, 0);
-  }
+    Frame *frame = &skin_nodes[v].frames[0];
 
-  if (nodes[v].flag & MVERT_SKIN_ROOT) {
-    skin_nodes[v].flag |= ROOT;
+    /* End frame */
+    create_frame(frame, mvert[v].co, rad, mat, 0);
+
+    /* The caps might need to have their normals inverted. So check if they
+     * need to be flipped when creating faces. */
+    float normal[3];
+    normal_quad_v3(normal, frame->co[0], frame->co[1], frame->co[2], frame->co[3]);
+    if (dot_v3v3(mat[0], normal) < 0.0f) {
+      skin_nodes[v].flag |= FLIP_NORMAL;
+    }
   }
 }
 
@@ -1577,7 +1584,7 @@ static void skin_output_end_nodes(SkinOutput *so, SkinNode *skin_nodes, int totv
     }
 
     if (sn->flag & CAP_START) {
-      if (sn->flag & ROOT) {
+      if (sn->flag & FLIP_NORMAL) {
         add_poly(so,
                  sn->frames[0].verts[0],
                  sn->frames[0].verts[1],