gallium: split long prims into chunks with an even number of vertices

This fixes culling "parity" errors when splitting long tri strips.  Splitting
strips into chunks with an odd number of vertices causes front/back-face
orientation to get reversed and upsets culling.

diff --git a/src/gallium/auxiliary/draw/draw_pt_fetch_shade_emit.c b/src/gallium/auxiliary/draw/draw_pt_fetch_shade_emit.c
index 5ce3aba2a2de2acfe995e3e47cafa40f61ea0a51..fdf9b6fe6a8462190224554b399739552204ea92 100644 (file)
--- a/src/gallium/auxiliary/draw/draw_pt_fetch_shade_emit.c
+++ b/src/gallium/auxiliary/draw/draw_pt_fetch_shade_emit.c
@@ -193,6 +193,15 @@ static void fse_prepare( struct draw_pt_middle_end *middle,
    *max_vertices = (draw->render->max_vertex_buffer_bytes / 
                     (vinfo->size * 4));
 
+   /* Return an even number of verts.
+    * This prevents "parity" errors when splitting long triangle strips which
+    * can lead to front/back culling mix-ups.
+    * Every other triangle in a strip has an alternate front/back orientation
+    * so splitting at an odd position can cause the orientation of subsequent
+    * triangles to get reversed.
+    */
+   *max_vertices = *max_vertices & ~1;
+
    /* Probably need to do this somewhere (or fix exec shader not to
     * need it):
     */

diff --git a/src/gallium/auxiliary/draw/draw_pt_varray.c b/src/gallium/auxiliary/draw/draw_pt_varray.c
index 4479963db177198162ca11ed5c2e8c0df344ad2f..2cc08a9e9393c2fcb17bb7fd5f5eaad964af2490 100644 (file)
--- a/src/gallium/auxiliary/draw/draw_pt_varray.c
+++ b/src/gallium/auxiliary/draw/draw_pt_varray.c
@@ -147,6 +147,10 @@ static void varray_prepare(struct draw_pt_front_end *frontend,
 
    varray->middle = middle;
    middle->prepare(middle, varray->output_prim, opt, &varray->driver_fetch_max );
+
+   /* check that the max is even */
+   assert((varray->driver_fetch_max & 1) == 0);
+
    varray->fetch_max = MIN2(FETCH_MAX, varray->driver_fetch_max);
 }