[Mesa-dev] [Intel-gfx] [PATCH 4/9] intel gen4-5: Fix backface/frontface selection when one one color is written to.

[Eric Anholt]

Olivier Galibert <galibert at pobox.com> writes:

> Shaders, piglit test ones in particular, may write only to one of
> gl_FrontColor/gl_BackColor.  The standard is unclear on whether the
> behaviour is defined in that case, but it seems reasonable to support
> it.
>
> The choice done there to pick up whichever color was actually written
> to.  That makes most of the generated piglit tests useless to test the
> backface selection, but it's simple and it works.
>
> Signed-off-by: Olivier Galibert <galibert at pobox.com>
> ---
>  src/mesa/drivers/dri/i965/brw_fs.cpp     |    9 +++++++++
>  src/mesa/drivers/dri/i965/brw_wm_pass2.c |    9 +++++++++
>  2 files changed, 18 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index 3f98137..3b62952 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -972,6 +972,15 @@ fs_visitor::calculate_urb_setup()
>  	 if (c->key.vp_outputs_written & BITFIELD64_BIT(i)) {
>  	    int fp_index = _mesa_vert_result_to_frag_attrib((gl_vert_result) i);
>  
> +            /* Special case: two-sided vertex option, vertex program
> +             * only writes to the back color.  Map it to the
> +             * associated front color location.
> +             */
> +            if (i >= VERT_RESULT_BFC0 && i <= VERT_RESULT_BFC1 &&
> +                ctx->VertexProgram._TwoSideEnabled &&
> +                urb_setup[i - VERT_RESULT_BFC0 + FRAG_ATTRIB_COL0] == -1)
> +               fp_index = i - VERT_RESULT_BFC0 + FRAG_ATTRIB_COL0;

In the fs_visitor (and brw_wm_pass*), you don't get to look at ctx->
state like that -- you're getting called once with some set of ctx
state, but the program will get reused even if the ctx state changes.
You'd have to get that state into the wm prog key, and use that, which
would guarantee that you have the appropriate program code.
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