i965: Fix scissoring when width or height is 0.

We would run into trouble due to the hardware using inclusive numbers
and the subtraction to handle that producing negative (meaning large
positive) coordinates.

Bug #27643.

1 files changed, 14 insertions, 1 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_sf_state.c b/src/mesa/drivers/dri/i965/brw_sf_state.c
index 9712c31afea..1a6c8218fd1 100644
--- a/src/mesa/drivers/dri/i965/brw_sf_state.c
+++ b/src/mesa/drivers/dri/i965/brw_sf_state.c
@@ -76,7 +76,20 @@ static void upload_sf_vp(struct brw_context *brw)
     * Note that the hardware's coordinates are inclusive, while Mesa's min is
     * inclusive but max is exclusive.
     */
-   if (render_to_fbo) {
+
+   if (ctx->DrawBuffer->_Xmin == ctx->DrawBuffer->_Xmax ||
+       ctx->DrawBuffer->_Ymin == ctx->DrawBuffer->_Ymax) {
+      /* If the scissor was out of bounds and got clamped to 0
+       * width/height at the bounds, the subtraction of 1 from
+       * maximums could produce a negative number and thus not clip
+       * anything.  Instead, just provide a min > max scissor inside
+       * the bounds, which produces the expected no rendering.
+       */
+      sfv.scissor.xmin = 1;
+      sfv.scissor.xmax = 0;
+      sfv.scissor.ymin = 1;
+      sfv.scissor.ymax = 0;
+   } else if (render_to_fbo) {
       /* texmemory: Y=0=bottom */
       sfv.scissor.xmin = ctx->DrawBuffer->_Xmin;
       sfv.scissor.xmax = ctx->DrawBuffer->_Xmax - 1;