# What is the AlphaToCoverage blend state useful for?

Alright, just finished most of my early UI stuff and I wanted the windows to have some transparency. So I expanded my application to initialize and bind blend states so that my UI shader could implement alpha blending. I was initially having no luck but I got the blend state configured to achieve my purpose, however the way it is configured doesn't make sense to me so I must be missing something.

Here is how I initialize my blend state.

bool BlendState::StartUp() {
D11DeviceManager* _pDeviceManager = D11DeviceManager::GetSingleton();

D3D11_BLEND_DESC _desc;
_desc.AlphaToCoverageEnable = false;
_desc.IndependentBlendEnable = false;

_desc.RenderTarget[0].BlendEnable            = true;
_desc.RenderTarget[0].SrcBlend               = D3D11_BLEND_SRC_COLOR;
_desc.RenderTarget[0].DestBlend              = D3D11_BLEND_DEST_COLOR;
_desc.RenderTarget[0].SrcBlendAlpha          = D3D11_BLEND_SRC_ALPHA;
_desc.RenderTarget[0].DestBlendAlpha         = D3D11_BLEND_DEST_ALPHA;

return !FAILED(_pDeviceManager->GetDevice()->CreateBlendState(&_desc, &pBlendState));
}


Right now my alpha blending only works when the AlphaToCoverage bool is set to false... I thought this was just a bool enabling the pixel fragment blending (which upon retrospection would be redundant considering the BlendEnable flags...) but looking at the MSDN documentation is appears to do this...

You can use the AlphaToCoverageEnable member of D3D11_BLEND_DESC1 or D3D11_BLEND_DESC to toggle whether the runtime converts the .a component (alpha) of output register SV_Target0 from the pixel shader to an n-step coverage mask (given an n-sample RenderTarget). The runtime performs an AND operation of this mask with the typical sample coverage for the pixel in the primitive (in addition to the sample mask) to determine which samples to update in all the active RenderTargets.

Could someone explain this to me and how it differs from when the AlphaToCoverageEnable bool is false? I see that it adds a new mask to the fragment but I don't quite follow what the mask exactly is.

• The boolean turns on alpha-to-coverage (which is not alpha blending). Are you asking what alpha-to-coverage is or is useful for? – user1430 Jan 16 '12 at 19:42
• Yeah, I had a misconception with it and now I realize I don't understand the concept behind it at all. – KlashnikovKid Jan 16 '12 at 20:15