[Mesa-dev] Adding support for sRGB framebuffers to EGL

[John Kåre Alsaker]

For DRI adding a __DRIdrawable sRGB gamma attribute appears to be the
way to go. There's __DRI_ATTRIB_FRAMEBUFFER_SRGB_CAPABLE already for
__DRIconfig which appears to be unused. That could be used for the
capability flag. Implementing it in the gallium EGL backend seems more
straightforward, once that actually works.

The sorting of EGL_GAMMA_SRGB_BIT also has the desirable function that
it prevents simple EGL applications using eglChooseConfig from
grabbing the deepest buffer format available (which could easily be a
slower 32 bpc floating point format), assuming sRGB support is
available and limited to 8 bpc.

On Thu, Sep 27, 2012 at 8:57 PM, John Kåre Alsaker
<john.kare.alsaker at gmail.com> wrote:
> I failed to consider that premultiplied sRGB color channels can be
> stored two ways. The friendly way, to_srgb_gamma(color * alpha) and
> what applications actually use, to_srgb_gamma(color) * alpha. That
> might affect this proposal.
I don't think this has to involve EGL.

>
> On Thu, Sep 27, 2012 at 3:49 PM, Brian Paul <brianp at vmware.com> wrote:
>> Just some quick comments.
>>
>> I think the term "GAMMA" should be omitted.  I'm no expert but I'm not sure
>> that sRGB is strictly a gamma function.  The GL_EXT_texture_sRGB spec says
>> "The sRGB color space roughly corresponds to 2.2 gamma correction."  Also,
>> the term GAMMA doesn't appear in any of the existing GLenum values related
>> to sRGB.
> Only sRGB's gamma function is used and it is explicitly named so to
> avoid confusion with the whole sRGB color space.
>
>>
>> Therefore, I think EGL_GAMMA_SRGB_BIT should be just EGL_SRGB_BIT,
>> EGL_GAMMA_LINEAR -> EGL_LINEAR,
>> EGL_GAMMA_SRGB -> EGL_SRGB.
>>
>> For the query, you might use "COLOR_ENCODING" like
>> GL_ARB_framebuffer_object.
> You could use glGetFramebufferAttachmentParameter with
> GL_FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING to check if you got a sRGB
> framebuffer, but you can't set it.
>
>>
>> You could also take a look at
>> http://www.opengl.org/registry/specs/EXT/framebuffer_sRGB.txt to see how
>> this was handled for GLX and WGL.
> This is done similarly with a capability flag in GLX/WGL, but with the
> surface attribute moved into OpenGL (and it affects other
> rendertargets).
>
>>
>> -Brian
>>
>>
>>
>> On 09/26/2012 04:49 AM, John Kåre Alsaker wrote:
>>>
>>> EGL_GAMMA_SRGB_BIT should be changed to a boolean EGL config attribute
>>> so it can be optionally sorted by eglChooseConfig. It should be sorted
>>> after EGL_COLOR_BUFFER_TYPE and true values would be preferred. This
>>> sorting should be optional and is done so compositing window managers
>>> can sample the framebuffer as an sRGB texture. EGL_GAMMA_SRGB_BIT
>>> could also optionally be disabled when the format is not supported as
>>> an sRGB texture by the hardware.
>>>
>>> On Wed, Sep 26, 2012 at 10:19 AM, John Kåre Alsaker
>>> <john.kare.alsaker at gmail.com>  wrote:
>>>>
>>>> The way this would interact with ARB_framebuffer_sRGB is that the
>>>> EGL's surface EGL_GAMMA attribute would be ignored. If a EGL config
>>>> has the EGL_GAMMA_SRGB_BIT, it is required to support sRGB for the
>>>> default OpenGL framebuffer.
>>>>
>>>> On Tue, Sep 25, 2012 at 5:58 PM, John Kåre Alsaker
>>>> <john.kare.alsaker at gmail.com>  wrote:
>>>>>
>>>>> Hello,
>>>>>
>>>>> I would like add support for sRGB framebuffers to EGL. Here are my
>>>>> proposed changes to EGL:
>>>>>
>>>>> A new EGL_SURFACE_TYPE bit:
>>>>> EGL_GAMMA_SRGB_BIT - This format supports sRGB framebuffers. This also
>>>>> means that ARB_framebuffer_sRGB is supported for this format.
>>>>>
>>>>> New values:
>>>>> EGL_GAMMA_LINEAR - The gamma is linear.
>>>>> EGL_GAMMA_SRGB - The gamma is as defined by the sRGB standard.
>>>>>
>>>>> New EGL surface attribute:
>>>>> EGL_GAMMA - The gamma of the surface's framebuffer. The default value
>>>>> is EGL_GAMMA_LINEAR.
>>>>>
>>>>> When the surface's EGL_GAMMA attribute's value is EGL_GAMMA_SRGB and
>>>>> this is supported by
>>>>> the format, reads from this framebuffer will be converted from sRGB
>>>>> gamma and writes will
>>>>> be converted to sRGB gamma. This may not apply to all functions in the
>>>>> client API.
>>>>>
>>>>> There may be something related to the ARB_framebuffer_sRGB extension
>>>>> in EGL already, in
>>>>> which case someone should link me to it.
>>>>>
>>>>> There's two assumptions behind this proposal. One is that you won't
>>>>> support sRGB only
>>>>> formats. The other is that linear gamma and sRGB gamma format support
>>>>> is the same.
>>>>> This means that window systems can't add a new linear gamma format
>>>>> without adding an sRGB
>>>>> format and the other way around.
>>>>>
>>>>> I'd like some comments on this proposal and tips on how it could be
>>>>> implemented in the
>>>>> mess that is DRI.
>>>>>
>>>>> Thanks,
>>>>> John Kåre
>>>
>>> _______________________________________________
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>>> mesa-dev at lists.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>
>>