[Mesa-dev] [PATCH v3] i965: Fix ETC2/EAC GetCompressed* functions on Gen7 GPUs

[Eleni Maria Stea]

Hi Eero,

On Fri, 4 May 2018 18:29:55 +0300
Eero Tamminen <eero.t.tamminen at intel.com> wrote:

> You mean returning CAVEAT_SUPPORT in params for compressed formats
> which are transparently converted to uncompressed data?

Well, that would be the best solution I think, if it's possible to
modify an existing query in the extension, although I am not certain
which is the best query to modify: TEXTURE_COMPRESSED, or
INTERNALFORMAT_SUPPORTED (or maybe both?). 

There's also another solution that we already have, but we are not sure
if it is correct:

I noticed that both mesa and nvidia drivers return GL_FALSE when the
pname is GL_TEXTURE_COMPRESSED and the format is emulated and GL_TRUE
for the natively supported formats. (Specifically on mesa the code that
performs the check is in src/mesa/main/formatquery.c and tests only
for native support). 

But if you take a look at this part of the extension specification:

TEXTURE_COMPRESSED: If <internalformat> is a compressed format
      that is supported for this type of resource, TRUE is returned in 
      <params>. If the internal format is not compressed, or the type of
      resource is not supported, FALSE is returned.

it is not very clear if we should return true or false for an
emulated format. Maybe returning false when we provide emulation is a
bug in both drivers, just a convenient one in this case. :-)

Is there any way to clarify what should be the correct behavior?

Do you think that even if the current behavior of the
TEXTURE_COMPRESSED query is correct, in which case it should keep
returning GL_FALSE for the emulated formats, we should nevertheless
modify something else, e.g. the INTERNALFORMAT_SUPPORTED query, to
return CAVEAT_SUPPORT? 

> 
> That API's not available for GLES v2, where I think ETC is most widely
> used, so it would be more of a solution for GLES v3.x applications
> only. Sounds OK to me.
> 
> Hardest part will be propagating use of this query to engines &
> toolkits that would benefit from using it. :-)

+1 on that :)

Thanks a lot for the suggestions and the feedback,
Eleni

PS: here is some code to clarify the current situation:

[1]: https://github.com/hikiko/test-compression is a test program to
quickly check the compressed formats supported (see
the function print_compressed_formats at the end of main.c)

[2]: https://pastebin.com/Qif74fFn is the output of [1] on HSW using
the ETC patch and on nvidia where you can see that the natively
supported compression formats return GL_TRUE in both cards whereas the
emulated ones return GL_FALSE in both cards