Alpha to coverage

Abstract: One embodiment of the present invention sets forth a technique for converting alpha values into pixel coverage masks. Geometric coverage is sampled at a number of “real” sample positions within each pixel. Color and depth values are computed for each of these real samples. Fragment alpha values are used to determine an alpha coverage mask for the real samples and additional “virtual” samples, in which the number of bits set in the mask bits is proportional to the alpha value. An alpha-to-coverage mode uses the virtual samples to increase the number of transparency levels for each pixel compared with using only real samples. The alpha-to-coverage mode may be used in conjunction with virtual coverage anti-aliasing to provide higher-quality transparency for rendering anti-aliased images.

Abstract: One embodiment of the present invention sets forth a technique for converting alpha values into pixel coverage masks Geometric coverage is sampled at a number of “real” sample positions within each pixel Color and depth values are computed for each of these real samples Fragment alpha values are used to determine an alpha coverage mask for the real samples and additional “virtual” samples, in which the number of bits set in the mask bits is proportional to the alpha value An alpha-to-coverage mode uses the virtual samples to increase the number of transparency levels for each pixel compared with using only real samples The alpha-to-coverage mode may be used in conjunction with virtual coverage anti-aliasing to provide higher-quality transparency for rendering anti-aliased images

Abstract: An alpha-to-coverage transformation is performed by a pixel shader. The pixel shader compares data of a transparency column of a pixel and thresholds of sub-pixels of the pixel to generate a plurality of coverage masks, and stores the plurality of coverage masks in the LSBs of the transparency column of the pixel, and finally update the data of the sub-pixels according to the coverage masks stored in the transparency column of the pixel. A new instruction “a 2 c” is invented to speed up such thresholds comparison and coverage mask generation.

Abstract: The use of antialiasing techniques is crucial when producing high quality graphics. Up to now, multisampling antialiasing (MSAA) has remained the most advanced solution, offering superior results in real time. However, there are important drawbacks to the use of MSAA in certain scenarios. First, the increase in processing time it consumes is not negligible at all. Further, limitations of MSAA include the impossibility, in a wide range of platforms, of activating multisampling when using multiple render targets (MRT), on which fundamental techniques such as deferred shading [Shishkovtsov 05, Koonce 07] rely. Even on platforms where MRT and MSAA can be simultaneously activated (i.e., DirectX 10), implementation of MSAA is neither trivial nor cost free [Thibieroz 09]. Additionally, MSAA poses a problem for the current generation of consoles. In the case of the Xbox 360, memory constraints force the use of CPU-based tiling techniques in case high-resolution frame buffers need to be used in conjunction with MSAA; whereas on the PS3 multisampling is usually not even applied. Another drawback of MSAA is its inability to smooth nongeometric edges, such as those resulting from the use of alpha testing, frequent when rendering vegetation. As a result, when using MSAA, vegetation can be antialiased only if alpha to coverage is used. Finally, multisampling requires extra memory, which is always a valuable resource, especially on consoles.