[Mesa-dev] [PATCH 1/2] mesa : MESA_framebuffer_flip_y extension

[Jason Ekstrand]

On June 7, 2018 16:47:17 Ilia Mirkin <imirkin at alum.mit.edu> wrote:

> On Thu, Jun 7, 2018 at 7:31 PM, Fritz Koenig <frkoenig at google.com> wrote:
>> On Thu, Jun 7, 2018 at 4:10 PM Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>>>
>>> It's slightly different than what you've specified, but can't you
>>> achieve this with GL_ARB_clip_control / GL_EXT_clip_control ?
>>
>> It can be, but there is a lot of extra work that needs to be done to
>> get ReadPixels to work correctly.  It would be preferable to have the
>> driver handle all of that work.
>
> A bit more prior art research:
>
> GL_MESA_pack_invert - specifies this precisely for glReadPixels.
>
> Your version of the spec needs more information. What does "flipped"
> mean? Does it affect gl_FragCoord / gl_SamplePosition /
> interpolateAtOffset? Does it affect the winding?
>
> Might help to say that this is designed to undo the winsys flipping
> relative to fbo rendering.

More designed to emulate winsys flipping on a texture that isn't flipped.  
Basically, everything is "like normal" except that the result of your 
rendering is flipped vertically in the texture.  It's a bit wired but 
that's the idea.

--Jason