Alpha to coverage
Alpha to coverage is a multisampling computer graphics technique useful for situations where dense foliage or grass must be rendered in a video game.
This technique use the alpha channel of textures as a coverage mask for anti-aliasing.
Alpha to coverage multisampling is based on regular multisampling, except that the alpha coverage mask is ANDed with the multisample mask. Alpha-to-coverage converts the alpha component output from the pixel shader to a coverage mask. When the multisampling is applied each output pixel gets a transparency of 0 or 1 depending on alpha coverage and the multisampling result.
As the result of the Alpha to coverage is a transparency of 0 or 1, the results often have hard-edges. In order to preserve a smoothness quality for the edges, further z-pass techniques are often applied in modern game engines such as the CryEngine 2[1] or the Source engine[2].
This method do not work if the alpha value of the texture is used, for example when simulating looking through a window.
See also
References
- ^ "Vegetation Procedural Animation and Shading in Crysis". developer.nvidia.com & Crytek. Retrieved 2009-07-05.
- ^ "Improved Alpha-Tested Magnification for Vector Textures and Special Effects" (PDF). Valve Corporation. 2007. Retrieved 2009-07-05.