Multiple render targets and gamma correctness in Direct3D9
- by Mario
Let's say in a deferred renderer when building your G-Buffer you're going to render texture color, normals, depth and whatever else to your multiple render targets at once.
Now if you want to have a gamma-correct rendering pipeline and you use regular sRGB textures as well as rendertargets, you'll need to apply some conversions along the way, because your filtering, sampling and calculations should happen in linear space, not sRGB space. Of course, you could store linear color in your textures and rendertargets, but this might very well introduce bad precision and banding issues.
Reading from sRGB textures is easy: just set SRGBTexture = true; in your texture sampler in your HLSL effect code and the hardware does the conversion sRGB-linear for you.
Writing to an sRGB rendertarget is theoretically easy, too: just set SRGBWriteEnable = true; in your effect pass in HLSL and your linear colors will be converted to sRGB space automatically.
But how does this work with multiple rendertargets?
I only want to do these corrections to the color textures and rendertarget, not to the normals, depth, specularity or whatever else I'll be rendering to my G-Buffer. Ok, so I just don't apply SRGBTexture = true; to my non-color textures, but when using SRGBWriteEnable = true; I'll do a gamma correction to all the values I write out to my rendertargets, no matter what I actually store there.
I found some info on gamma over at Microsoft:
http://msdn.microsoft.com/en-us/library/windows/desktop/bb173460%28v=vs.85%29.aspx
For hardware that supports Multiple Render Targets (Direct3D 9) or Multiple-element Textures (Direct3D 9), only the first render target or element is written.
If I understand correctly, SRGBWriteEnable should only be applied to the first rendertarget, but according to my tests it doesn't and is used for all rendertargets instead.
Now the only alternative seems to be to handle these corrections manually in my shader and only correct the actual color output, but I'm not totally sure, that this'll not have any negative impact on color correctness. E.g. if the GPU does any blending or filtering or multisampling after the Linear-sRGB conversion...
Do I even need gamma correction in this case, if I'm just writing texture color without lighting to my rendertarget? As far as I know, I DO need it because of the texture filtering and mip sampling happening in sRGB space instead, if I don't correct for it.
Anyway, it'd be interesting to hear other people's solutions or thoughts about this.
