On Tue, Aug 26, 2014 at 04:39:55PM -0700, Kenneth Graunke wrote:
> Apparently guardband clipping doesn't work like we thought: objects
> entirely outside fthe guardband are trivially rejected, regardless of
> their relation to the viewport. Normally, the guardband is larger than
> the viewport, so this is not a problem. However, when the viewport is
> larger than the guardband, this means that we would discard primitives
> which were wholly outside of the guardband, but still visible.
>
> We always program the guardband to 8K x 8K to enforce the restriction
> that the screenspace bounding box of a single triangle must be no more
> than 8K x 8K. So, if the viewport is larger than that, we need to
> disable guardband clipping.
>
> Fixes ES3 conformance tests:
> - framebuffer_blit_functionality_negative_height_blit
> - framebuffer_blit_functionality_negative_width_blit
> - framebuffer_blit_functionality_negative_dimensions_blit
> - framebuffer_blit_functionality_magnifying_blit
> - framebuffer_blit_functionality_multisampled_to_singlesampled_blit
>
> Signed-off-by: Kenneth Graunke <kenn...@whitecape.org>
> Cc: Topi Pohjolainen <topi.pohjolai...@intel.com>
> Cc: Ian Romanick <i...@freedesktop.org>
> ---
> src/mesa/drivers/dri/i965/gen6_clip_state.c | 29
> +++++++++++++++++++++++++++++
> 1 file changed, 29 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/gen6_clip_state.c
> b/src/mesa/drivers/dri/i965/gen6_clip_state.c
> index e8c1b91..524219f 100644
> --- a/src/mesa/drivers/dri/i965/gen6_clip_state.c
> +++ b/src/mesa/drivers/dri/i965/gen6_clip_state.c
> @@ -98,6 +98,35 @@ upload_clip_state(struct brw_context *brw)
>
> dw2 |= GEN6_CLIP_GB_TEST;
>
> + /* 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)
> + *
> + * Disabling guardband clipping removes the TR_GB condition, which means
> + * they'll be considered MC ("Must Clip") unless they're rejected for
Maybe add TA (Trivial Accept) and TR (Trivial Reject) somewhere in the
documentation. I didn't know these before and it would have been easier to
decode the formulas if I had.
Otherwise makes sense to me and many thanks for the explanation:
Reviewed-by: Topi Pohjolainen <topi.pohjolai...@intel.com>
> + * 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 < ctx->Const.MaxViewports; i++) {
> + if (ctx->ViewportArray[i].Width > 8192 ||
> + ctx->ViewportArray[i].Height > 8192) {
> + dw2 &= ~GEN6_CLIP_GB_TEST;
> + break;
> + }
> + }
> +
> /* If the viewport dimensions are smaller than the drawable dimensions,
> * we have to disable guardband clipping prior to Gen8. We always program
> * the guardband to a fixed size, which is almost always larger than the
> --
> 2.0.0
>
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