[Mesa-dev] [PATCH] gallivm: add a horrible hack for stencil texturing with border

[Marek Olšák]

This is not well defined in gallium, but r600g and radeonsi use these
default swizzles for depth and stencil surfaces and it's probably the
only reasonable thing to do:

Format = Default swizzle
Z24X8 = XXXX (Z)
Z24S8 = XXXX (Z)
X24S8 = YYYY (S)
X8Z24 = YYYY (Z)
S8Z24 = YYYY (Z)
S8X24 = XXXX (S)
Z32_S8X24 = XXXX (Z)
X32_S8X24 = YYYY (S)

Marek

On Wed, Dec 16, 2015 at 5:37 AM,  <sroland at vmware.com> wrote:
> From: Roland Scheidegger <sroland at vmware.com>
>
> mesa/st doesn't give us a useful swizzle when stencil texturing. Moreover,
> it's not even obvious what the swizzle actually should be - the channel which
> is used for the fetch (Y) is not the same as the one which must be used for
> the border component (X), which is due to a mismatch between GL and gallium
> interface. (On top of that, I have no idea what GL expects in YZW channels in
> the end.)
> So add some special case for stencil texturing with border, to fetch the right
> border component. Though it seems there has to be some better solution...
> This fixes piglit texwrap GL_ARB_texture_stencil8 bordercolor (only the fixed
> version).
> ---
>  src/gallium/auxiliary/gallivm/lp_bld_sample_soa.c | 28 +++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/src/gallium/auxiliary/gallivm/lp_bld_sample_soa.c b/src/gallium/auxiliary/gallivm/lp_bld_sample_soa.c
> index e21933f..efba5a8 100644
> --- a/src/gallium/auxiliary/gallivm/lp_bld_sample_soa.c
> +++ b/src/gallium/auxiliary/gallivm/lp_bld_sample_soa.c
> @@ -187,9 +187,33 @@ lp_build_sample_texel_soa(struct lp_build_sample_context *bld,
>        border_type.length = 4;
>        /*
>         * Only replace channels which are actually present. The others should
> -       * get optimized away eventually by sampler_view swizzle anyway but it's
> -       * easier too.
> +       * get optimized away eventually by sampler_view swizzle in most cases...
> +       * If not, for "ordinary" color textures, fetch will have placed the
> +       * correct default values there, since missing channels must use default
> +       * values regardless of border.
> +       * We do, however, some horrendous hack for stencil textures. We won't
> +       * get a useful swizzle, and furthermore the channel to fetch (Y) doesn't
> +       * match the channel for the border color (X).
>         */
> +      if (util_format_has_stencil(format_desc) &&
> +            !util_format_has_depth(format_desc)) {
> +         LLVMValueRef zero = lp_build_const_int32(bld->gallivm, 0);
> +         LLVMValueRef border_col;
> +         border_col = lp_build_extract_broadcast(bld->gallivm,
> +                                                 border_type,
> +                                                 bld->texel_type,
> +                                                 bld->border_color_clamped,
> +                                                 zero);
> +         /*
> +          * Replace first 3 chans (match what fetch did).
> +          */
> +         for (chan = 0; chan < 3; chan++) {
> +            texel_out[chan] = lp_build_select(&bld->texel_bld, use_border,
> +                                              border_col, texel_out[chan]);
> +         }
> +         return;
> +      }
> +
>        for (chan = 0; chan < 4; chan++) {
>           unsigned chan_s;
>           /* reverse-map channel... */
> --
> 2.1.4
>
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