[Mesa-dev] [PATCH V2 2/2] meta: Use gl_FragColor to output color values to all the draw buffers

[Anuj Phogat]

_mesa_meta_setup_blit_shader() currently generates a fragment shader
which, irrespective of the number of draw buffers, writes the color
to only one 'out' variable. Current shader rely on an undefined
behavior and possibly works by chance.

>From OpenGL 4.0  spec, page 256:
  "If a fragment shader writes to gl_FragColor, DrawBuffers specifies a
   set of draw buffers into which the single fragment color defined by
   gl_FragColor is written. If a fragment shader writes to gl_FragData,
   or a user-defined varying out variable, DrawBuffers specifies a set
   of draw buffers into which each of the multiple output colors defined
   by these variables are separately written. If a fragment shader writes
   to none of gl_FragColor, gl_FragData, nor any user defined varying out
   variables, the values of the fragment colors following shader execution
   are undefined, and may differ for each fragment color."

OpenGL 4.4 spec, page 463, added an additional line in this section:
  "If some, but not all user-defined output variables are written, the
   values of fragment colors corresponding to unwritten variables are
   similarly undefined."

V2: Write color output to gl_FragColor instead of writing to multiple
    'out' variables. This'll avoid recompiling the shader every time
    draw buffers count is updated.

Cc: <mesa-stable at lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat at gmail.com>
Reviewed-by: Matt Turner <mattst88 at gmail.com> (V1)
---
 src/mesa/drivers/common/meta.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/src/mesa/drivers/common/meta.c b/src/mesa/drivers/common/meta.c
index fab9106..337be1b 100644
--- a/src/mesa/drivers/common/meta.c
+++ b/src/mesa/drivers/common/meta.c
@@ -246,7 +246,6 @@ _mesa_meta_setup_blit_shader(struct gl_context *ctx,
    void *const mem_ctx = ralloc_context(NULL);
    struct blit_shader *shader = choose_blit_shader(target, table);
    const char *vs_input, *vs_output, *fs_input, *vs_preprocess, *fs_preprocess;
-   const char *fs_output_var, *fs_output_var_decl;
 
    if (ctx->Const.GLSLVersion < 130) {
       vs_preprocess = "";
@@ -254,16 +253,12 @@ _mesa_meta_setup_blit_shader(struct gl_context *ctx,
       vs_output = "varying";
       fs_preprocess = "#extension GL_EXT_texture_array : enable";
       fs_input = "varying";
-      fs_output_var_decl = "";
-      fs_output_var = "gl_FragColor";
    } else {
       vs_preprocess = "#version 130";
       vs_input = "in";
       vs_output = "out";
       fs_preprocess = "#version 130";
       fs_input = "in";
-      fs_output_var_decl = "out vec4 out_color;";
-      fs_output_var = "out_color";
       shader->func = "texture";
    }
 
@@ -291,15 +286,13 @@ _mesa_meta_setup_blit_shader(struct gl_context *ctx,
                 "#extension GL_ARB_texture_cube_map_array: enable\n"
                 "uniform %s texSampler;\n"
                 "%s vec4 texCoords;\n"
-                "%s\n"
                 "void main()\n"
                 "{\n"
-                "   vec4 color = %s(texSampler, %s);\n"
-                "   %s = color;\n"
-                "   gl_FragDepth = color.x;\n"
+                "   gl_FragColor = %s(texSampler, %s);\n"
+                "   gl_FragDepth = gl_FragColor.x;\n"
                 "}\n",
-                fs_preprocess, shader->type, fs_input, fs_output_decl,
-                shader->func, shader->texcoords, fs_output);
+                fs_preprocess, shader->type, fs_input,
+                shader->func, shader->texcoords);
 
    _mesa_meta_compile_and_link_program(ctx, vs_source, fs_source,
                                        ralloc_asprintf(mem_ctx, "%s blit",
-- 
1.8.3.1