Fix more polygon winding, culling confusion.

If the determinant of the triangle is positive, its winding is CCW (right-handed coord system).

2 files changed, 15 insertions, 9 deletions

diff --git a/src/mesa/pipe/draw/draw_cull.c b/src/mesa/pipe/draw/draw_cull.c
index 863686f150..e563f9f45f 100644
--- a/src/mesa/pipe/draw/draw_cull.c
+++ b/src/mesa/pipe/draw/draw_cull.c
@@ -40,7 +40,7 @@
 
 struct cull_stage {
    struct draw_stage stage;
-   GLuint mode;  /**< one of PIPE_WINDING_x */
+   GLuint winding;  /**< which winding(s) to cull (one of PIPE_WINDING_x) */
 };
 
 
@@ -54,7 +54,7 @@ static void cull_begin( struct draw_stage *stage )
 {
    struct cull_stage *cull = cull_stage(stage);
 
-   cull->mode = stage->draw->setup.cull_mode;
+   cull->winding = stage->draw->setup.cull_mode;
 
    stage->next->begin( stage->next );
 }
@@ -78,10 +78,12 @@ static void cull_tri( struct draw_stage *stage,
    header->det = ex * fy - ey * fx;
 
    if (header->det != 0) {
-      /* non-zero area */
-      GLuint mode = (header->det > 0) ? PIPE_WINDING_CW : PIPE_WINDING_CCW;
+      /* if (det > 0 then Z points toward camera and triangle is 
+       * counter-clockwise winding.
+       */
+      GLuint winding = (header->det > 0) ? PIPE_WINDING_CCW : PIPE_WINDING_CW;
 
-      if ((mode & cull_stage(stage)->mode) == 0) {
+      if ((winding & cull_stage(stage)->winding) == 0) {
          /* triangle is not culled, pass to next stage */
 	 stage->next->tri( stage->next, header );
       }
diff --git a/src/mesa/pipe/draw/draw_twoside.c b/src/mesa/pipe/draw/draw_twoside.c
index e86123e84d..9f26335fb6 100644
--- a/src/mesa/pipe/draw/draw_twoside.c
+++ b/src/mesa/pipe/draw/draw_twoside.c
@@ -35,8 +35,7 @@
 
 struct twoside_stage {
    struct draw_stage stage;
-   
-   GLfloat facing;
+   GLfloat sign;         /**< +1 or -1 */
    const GLuint *lookup;
 };
 
@@ -51,7 +50,12 @@ static void twoside_begin( struct draw_stage *stage )
 {
    struct twoside_stage *twoside = twoside_stage(stage);
 
-   twoside->facing = (stage->draw->setup.front_winding == PIPE_WINDING_CW) ? 1 : -1;
+   /*
+    * We'll multiply the primitive's determinant by this sign to determine
+    * if the triangle is back-facing (negative).
+    * sign = 1 for CCW, -1 for CW
+    */
+   twoside->sign = (stage->draw->setup.front_winding == PIPE_WINDING_CCW) ? 1 : -1;
 
    stage->next->begin( stage->next );
 }
@@ -94,7 +98,7 @@ static void twoside_tri( struct draw_stage *stage,
 {
    struct twoside_stage *twoside = twoside_stage(stage);
 
-   if (header->det * twoside->facing < 0) {
+   if (header->det * twoside->sign < 0.0) {
       /* this is a back-facing triangle */
       struct prim_header tmp;