[Mesa-dev] [PATCH 4/4] Revert "i965: Disable guardband clipping in the smaller-than-viewport case."

[Jason Ekstrand]

Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>

On Sun, Jan 22, 2017 at 10:42 PM, Kenneth Graunke <kenneth at whitecape.org>
wrote:

> This reverts commit 0bac2551e40410e2251daf4fd9faf69310ab34ce.
>
> Now that we position the guardband correctly (applying translations
> in addition to scaling) and made it as large (or larger) than the
> render target, this shouldn't be necessary.
>
> Now we leave guardband clipping enabled 100% of the time, like the
> Windows driver does.
>
> Fixes GL45-CTS.gtf21.GL2FixedTests.clip.clip.  It tries to draw a
> 16384x64 rectangle, and it appears that some kind of numerical
> imprecisions in the clipper result in some edge pixels going missing.
> The Windows driver passes this test because of guardband clipping.
> ---
>  src/mesa/drivers/dri/i965/gen6_clip_state.c | 31
> -----------------------------
>  1 file changed, 31 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/gen6_clip_state.c
> b/src/mesa/drivers/dri/i965/gen6_clip_state.c
> index 8e893f5668f..1c8c23730be 100644
> --- a/src/mesa/drivers/dri/i965/gen6_clip_state.c
> +++ b/src/mesa/drivers/dri/i965/gen6_clip_state.c
> @@ -172,37 +172,6 @@ upload_clip_state(struct brw_context *brw)
>     /* BRW_NEW_VIEWPORT_COUNT */
>     const unsigned viewport_count = brw->clip.viewport_count;
>
> -   /* We need to disable guardband clipping if the guardband (which we
> always
> -    * program to the maximum screen-space bounding box of 8K x 8K) will be
> -    * smaller than the viewport.
> -    *
> -    * Closely examining the clip determination formulas in the
> documentation
> -    * reveals that objects will be discarded entirely if they're outside
> the
> -    * (small) guardband, even if they're within the (large) viewport:
> -    *
> -    *     TR = TR_GB || TR_VPXY || TR_VPZ || TR_UC || TR_NEGW
> -    *     TA   = !TR && TA_GB && TA_VPZ && TA_NEGW
> -    *     MC = !(TA || TR)
> -    *
> -    * (TA is "Trivial Accept", TR is "Trivial Reject", MC is "Must Clip".)
> -    *
> -    * Disabling guardband clipping removes the TR_GB condition, which
> means
> -    * they'll be considered MC ("Must Clip") unless they're rejected for
> -    * some other reason.
> -    *
> -    * Note that there is no TA_VPXY condition.  If there were, objects
> entirely
> -    * inside a 16384x16384 viewport would be trivially accepted, breaking
> the
> -    * "objects must have a screenspace bounding box not exceeding 8K in
> the X
> -    * or Y direction" restriction.  Instead, they're clipped.
> -    */
> -   for (unsigned i = 0; i < viewport_count; i++) {
> -      if (ctx->ViewportArray[i].Width > 8192 ||
> -          ctx->ViewportArray[i].Height > 8192) {
> -         dw2 &= ~GEN6_CLIP_GB_TEST;
> -         break;
> -      }
> -   }
> -
>     /* BRW_NEW_RASTERIZER_DISCARD */
>     if (ctx->RasterDiscard) {
>        dw2 |= GEN6_CLIP_MODE_REJECT_ALL;
> --
> 2.11.0
>
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