[Mesa-dev] Looking for advice on how MSAA should behave under sRGB

[Roland Scheidegger]

Am 07.06.2012 01:27, schrieb Paul Berry:
> In my current implementation of MSAA for i965, I'm facing a choice in
> how to perform MSAA resolves when sRGB is in use.  Should I:
> 
> (a) always combine samples using a linear average.  For example, if
> multisampling by a factor of 4, and the 4 samples corresponding to a
> pixel have color values of a, b, c, and d, resolve them to a pixel color
> of (a+b+c+d)/4.
> 
> (b) first convert the samples from sRGB to linear, then combine them
> using a linear average, then convert back to sRGB.
> 
> (c) try to make an educated guess between (a) and (b) based on what the
> client program is doing.
> 
> 
> The GL 3.0 spec seems to pull towards (a), but it leaves a great deal of
> latitude to the implementation.  From 4.1.12 (Additional Multisample
> Fragment Operations):
> 
> "After all operations have been completed on the multisample buffer, the
> sample values for each color in the multisample buffer are combined to
> produce a single color value, and that value is written into the
> corresponding color buffers selected by DrawBuffer or DrawBuffers. An
> implementation may defer the writing of the color buffers until a later
> time, but the state of the framebuffer must behave as if the color
> buffers were updated as each fragment was processed. The method of
> combination is not specified, though a simple average computed
> independently for each color component is recommended."

I believe this is an oversight in the 3.0 specification, all later ones
have this instead (yes there's a typo in there not my fault...): "...
was processed. The method of combination is not specified. If the
framebuffer contains sRGB values, then it is recommended that the an
average of sample values is computed in a linearized space, as for
blending (see section 4.1.7). Otherwise, a simple average computed
independently for each color component is recommended."

And really that behavior makes sense to me. All operations are defined
to happen in linear space, like blending, why should resolve be different?

Roland