[Mesa-dev] [PATCH] draw: fix clipping with linear interpolated values and gl_ClipVertex

[sroland at vmware.com]

From: Roland Scheidegger <sroland at vmware.com>

Discovered this when working on other clip code, apparently didn't work
correctly - the combination of linear interpolated values and using
gl_ClipVertex produced wrong values (failing all such combinations
in piglits glsl-1.30 interpolation tests).
Use the pre-clip-pos values when determining the interpolation factor to
fix this.
Unfortunately I have no idea what I'm doing here really, but it fixes all
these failures in piglit (all interpolation-noperspective-XXX-vertex, 10
tests in total). Albeit piglit coverage of clipping isn't great, so hopefully
someone can confirm this actually makes sense, and wouldn't cause failures
elsewhere...
---
 src/gallium/auxiliary/draw/draw_pipe_clip.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/gallium/auxiliary/draw/draw_pipe_clip.c b/src/gallium/auxiliary/draw/draw_pipe_clip.c
index f2b56b0..7f22eef 100644
--- a/src/gallium/auxiliary/draw/draw_pipe_clip.c
+++ b/src/gallium/auxiliary/draw/draw_pipe_clip.c
@@ -192,11 +192,11 @@ static void interp(const struct clip_stage *clip,
       t_nopersp = t;
       /* find either in.x != out.x or in.y != out.y */
       for (k = 0; k < 2; k++) {
-         if (in->clip[k] != out->clip[k]) {
+         if (in->pre_clip_pos[k] != out->pre_clip_pos[k]) {
             /* do divide by W, then compute linear interpolation factor */
-            float in_coord = in->clip[k] / in->clip[3];
-            float out_coord = out->clip[k] / out->clip[3];
-            float dst_coord = dst->clip[k] / dst->clip[3];
+            float in_coord = in->pre_clip_pos[k] / in->pre_clip_pos[3];
+            float out_coord = out->pre_clip_pos[k] / out->pre_clip_pos[3];
+            float dst_coord = dst->pre_clip_pos[k] / dst->pre_clip_pos[3];
             t_nopersp = (dst_coord - out_coord) / (in_coord - out_coord);
             break;
          }
-- 
2.1.4