Alpha to coverage

Alpha to coverage¹²³ is a multisampling computer graphics technique, that replaces alpha blending with a coverage mask. This achieves order-independent transparency for when anti-aliasing or semi-transparent textures are used. This particular technique is useful for situations where dense foliage or grass must be rendered in a video game.⁴

Alpha to coverage is a sub-pixel version of screen-door transparency, a technique which uses a dithering pattern to simulate transparency with only fully opaque and fully transparent pixels.⁵

Alpha-to-coverage converts the alpha component output from the pixel shader into a temporary coverage mask, where the number of bits set is a function of the alpha value.⁶ This temporary mask is then bitwise ANDed with the fragment's coverage mask.⁶ To ensure consistency as alpha values change, the generated mask is required to be monotonic: the mask for a smaller alpha value must be a subset of the mask for a larger value.⁷

Because the mask is applied at the sub-pixel resolution of a multisample render target, the number of available transparency levels is limited by the sample count.⁷ For example, 4x MSAA allows for five distinct coverage levels (0 to 4 samples), which can lead to Quantization (image processing) artifacts.