[Mesa-dev] [RFC PATCH 3/4] mesa: Prefer non-swizzled formats when prefer_no_swizzle set

[Chris Forbes]

Hi Michel, and thanks for the quick feedback.

This is why it's tagged RFC :)

After thinking about it a bit more, I'm not convinced it's the right
thing either.

You're right, the spec is very careful not to say anything about the
memory layout. The 4.2 core profile spec, section 3.9.20 defines
reinterpretation of texels in a roundabout way to avoid having to
specify the memory layout:

"
    The re-interpretation for image
    loads and the read portion of image atomics is performed as though data were
    copied from the texel of the bound texture to a similar texel
represented in the
    format of the image unit. Similarly, the re-interpretation for
image stores and the
    write portion of image atomics is performed as though data were
copied from a
    texel represented in the format of the image unit to the texel in
the bound texture.
    In both cases, this copy operation would be performed by:

     - reading the texel from the source format to scratch memory according to
       the process described for GetTexImage (see section 6.1.4), using default
       pixel storage modes and <format> and <type> parameters corresponding
       to the source format in table 3.22; and

     - writing the texel from scratch memory to the destination format according
       to the process described for TexSubImage3D (see section 3.9.4),
using default
       pixel storage modes and <format> and <type> parameters corresponding
       to the destination format in table 3.22
"

My underspecified "exact matching format" and "correct order" are then
those which make the above operations just a memcpy, so we don't
actually have to do anything beyond aliasing the memory.

I suppose there might be implementations which want to do other things.

CC'ing Francisco on this as he was working on
ARB_shader_image_load_store, which first introduces this -- I care
mostly about the ARB_texture_view bits, which defines reinterpretation
of a texture's data store by piggybacking on the load_store
definition.

-- Chris

On Tue, Mar 11, 2014 at 3:42 PM, Michel Dänzer <michel at daenzer.net> wrote:
> On Mon, 2014-03-10 at 22:20 +1300, Chris Forbes wrote:
>> If prefer_no_swizzle is set, try:
>> - The exact matching format
>> - Formats with the required components in the correct order, plus a junk
>>   component
>
> How are 'exact matching' and 'correct order' defined? My understanding
> of GL internal formats is that they just specify which components are
> there and possibly their sizes, but nothing about their memory layout.
> If my understanding is correct, I think it's a bad idea to hardcode
> driver / platform specific preferences here like this.
>
> I think you would either need to take the 'format' parameter into
> account as well, or just use a driver specific ChooseTextureFormat hook.
>
>
> --
> Earthling Michel Dänzer            |                  http://www.amd.com
> Libre software enthusiast          |                Mesa and X developer
>