Mesa (master): draw: don't cull tris with zero area

[Roland Scheidegger]

Module: Mesa
Branch: master
Commit: 3e4fd2d4b185dac55a481384f8ce3a8c93d78f87
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=3e4fd2d4b185dac55a481384f8ce3a8c93d78f87

Author: Roland Scheidegger <sroland at vmware.com>
Date:   Thu Oct 26 21:23:27 2017 +0200

draw: don't cull tris with zero area

Culling tris with zero area seems like a great idea, but apparently with
fill mode line (and point) we're supposed to draw them, at least some tests
for some other state tracker complained otherwise.
Such tris also always seem to be back facing (not sure if this can be
inferred from anything, since in a mathematical sense it cannot really be
determined), so make sure to account for this when filling in the face
information.
(For solid tris, this is of course unnecessary, drivers will throw the tris
away later in any case.)

Reviewed-by: Brian Paul <brianp at vmware.com>

---

 src/gallium/auxiliary/draw/draw_pipe_cull.c     | 10 ++++++++++
 src/gallium/auxiliary/draw/draw_pipe_unfilled.c |  5 ++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/gallium/auxiliary/draw/draw_pipe_cull.c b/src/gallium/auxiliary/draw/draw_pipe_cull.c
index 3e8e458959..318d743dbb 100644
--- a/src/gallium/auxiliary/draw/draw_pipe_cull.c
+++ b/src/gallium/auxiliary/draw/draw_pipe_cull.c
@@ -181,6 +181,16 @@ static void cull_tri( struct draw_stage *stage,
             /* triangle is not culled, pass to next stage */
             stage->next->tri( stage->next, header );
          }
+      } else {
+         /*
+          * With zero area, this is back facing (because the spec says
+          * it's front facing if sign is positive?).
+          * Some apis apparently do not allow us to cull zero area tris
+          * here, in case of fill mode line (which is rather lame).
+          */
+         if ((PIPE_FACE_BACK & cull_stage(stage)->cull_face) == 0) {
+            stage->next->tri( stage->next, header );
+         }
       }
    }
 }
diff --git a/src/gallium/auxiliary/draw/draw_pipe_unfilled.c b/src/gallium/auxiliary/draw/draw_pipe_unfilled.c
index c465c7526f..f39db0e6a0 100644
--- a/src/gallium/auxiliary/draw/draw_pipe_unfilled.c
+++ b/src/gallium/auxiliary/draw/draw_pipe_unfilled.c
@@ -63,10 +63,9 @@ inject_front_face_info(struct draw_stage *stage,
                        struct prim_header *header)
 {
    struct unfilled_stage *unfilled = unfilled_stage(stage);
-   unsigned ccw = header->det < 0.0;
    boolean is_front_face = (
-      (stage->draw->rasterizer->front_ccw && ccw) ||
-      (!stage->draw->rasterizer->front_ccw && !ccw));
+      (stage->draw->rasterizer->front_ccw && header->det < 0.0f) ||
+      (!stage->draw->rasterizer->front_ccw && header->det > 0.0f));
    int slot = unfilled->face_slot;
    unsigned i;