[Piglit] [Mesa-dev] [PATCH] ext_framebuffer_multisample: check for num_samples<=1

[Brian Paul]

On Mon, Jan 28, 2013 at 9:17 AM, Marek Olšák <maraeo at gmail.com> wrote:
> On Mon, Jan 28, 2013 at 4:54 PM, Brian Paul <brianp at vmware.com> wrote:
>> [cross-posting this reply to the mesa-dev list]
>>
>>
>> On 01/27/2013 09:07 AM, Brian Paul wrote:
>>>
>>> On Sun, Jan 27, 2013 at 2:51 AM, Eric Anholt<eric at anholt.net>  wrote:
>>>>
>>>> Brian Paul<brianp at vmware.com>  writes:
>>>>
>>>>> Even if GL_ARB_multisample is advertised, it's OK for the max number of
>>>>> samples to be zero.
>>>>
>>>>
>>>> Huh?  The EXT_framebuffer_multisample spec says minmax of MAX_SAMPLES is
>>>> 1.
>>>
>>>
>>> And GL_ARB_framebuffer_object says it's zero.
>>>
>>> It looks like the piglit test is checking the wrong extension.  At
>>> line 364 we're calling
>>> piglit_require_extension("GL_ARB_framebuffer_object").  I don't see
>>> any checks for GL_EXT_framebuffer_multisample.  I was looking at
>>> GL_MAX_SAMPLES in the former, not the later.
>>>
>>> I guess the proper fix is to just check for the proper extension.
>>
>>
>>
>> There's several things going on here.
>>
>> First, the piglit test needs a
>> piglit_require_extension("GL_EXT_framebuffer_multisample") check.
>>
>> Second, I think we need to disable advertising this extension for swrast,
>> softpipe and llvmpipe.
>>
>> Third, I think we need to decide what GL_MAX_SAMPLES=1 really means.
>>
>> I've been reading the various multisample-related extensions and none of
>> them really says anything special about GL_MAX_SAMPLES=1.  But in some
>> places (like the GLSL spec) you'll see the phrase "single-sampling" but it
>> seems to mean the non-multisample / conventional rasterization case.
>>
>> My thinking is that it would be theoretically possible to have single-sample
>> multisampling (perhaps with a different sample position per pixel).
>>
>> In any case, regardless of the number of samples per pixel, we still must
>> obey centroid sampling mode (must interpolate at a point that lies in both
>> the pixel and in the primitive being
>> rendered).  swrast, softpipe and llvmpipe don't support that so we really
>> shouldn't advertise GL_MAX_SAMPLES=1 (it should be zero) so
>> GL_EXT_framebuffer_multisample shouldn't be advertised for those drivers.
>>
>> Most likely, the user's going to be disappointed if they want multisampling
>> in their app and the driver only gives him GL_MAX_SAMPLES=1.
>>
>> Does this sound reasonable?
>
> I think that not advertising ARB_framebufer_object (which adds
> GL_MAX_SAMPLES) is a much worse solution than returning GL_MAX_SAMPLES
> <= 1.

Huh?  I wasn't suggesting disabling GL_ARB_framebuffer_object.  I'm
saying advertise GL_MAX_SAMPLES=0 and disable
GL_EXT_framebuffer_multisample in the software drivers.

-Brian