questions about alpha to coverage

[steve3d]

according to directx's document, alpha to coverage works with or without multisample, but I've checked the engine source, both directx10 and 11 needs multisample to enable coverage:

 

codes in D3D10RenderState.cpp

   	int alpha = (alpha_func != ALPHA_NONE && render->getAlphaTestLevel101() == 0 && engine.app->getMultisample());

 

same code also exists in D3D11RenderState.cpp, is that unigine must use multisample to enable alpha to coverage? or is there some other restrictions in unigine which makes alpha to coverage must enable multisample?

 

I've googled some article it seems in opengl, alpha to converage need multisample to work, but in directx, it does work without multisample.

[steve3d]

check directx document: http://msdn.microsoft.com/en-us/library/windows/desktop/bb205072%28v=vs.85%29.aspx

[steve3d]

also there is alpha to coverage support in directx9:

 

AMD's way to alpha-to-coverage

http://developer.amd.com/gpu_assets/Advanced%20DX9%20Capabilities%20for%20ATI%20Radeon%20Cards_v2.pdf

 

nvidia's way to alpha-to-coverage

http://download.nvidia.com/developer/SDK/Individual_Samples/DEMOS/Direct3D9/src/AntiAliasingWithTransparency/docs/AntiAliasingWithTransparency.pdf

[frustum]

Alpha to coverage without multisampling looks ugly (huge transition area with dithering). Because of that they are disabled for non-multisampled renders. AMD and NVidia ATOC is used in the Direct3D9 render.

[steve3d]

yes, I just tried with that, alpha to coverage without multisampling, it is ugly, banding alpha effects :(