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Geometry Buffer Anti-Aliasing (GBAA)
Monday, July 18, 2011 | Permalink

Source code
GBAA.zip (2.3 MB)

Direct3D 10
GBAA is another anti-aliasing technique that works in the same spirit as GPAA, with the same quality, but with a substantially different method to accomplish the results. Whereas GPAA operates entirely as a post-process step (plus optionally pre-processing edges), GBAA uses a mixed method. During main scene rendering the geometry information is stored to a separate buffer, alternatively it uses available channels in an existing buffer. Anti-Aliasing is then performed in the end, in a resolve-pass if you will, as a fullscreen pass using the stored geometric information.

The geometry is represented as a two-channel edge distance. The closest direction (horizontal/vertical) distance is stored and the other is set to zero. It is possible to pack it into one channel and only store a bit indicating the major direction, but this demo uses two channels for simplicity.

A geometry shader is used during main rendering to compute the edge distances within a triangle. These are passed down in three components of an interpolator. The pixel shader simply selects the shortest distance to output to the buffer.

The advantage of GBAA over GPAA is that it gets rid of the second geometry pass. It also does not require any line drawing, something that consumer level GPUs generally suck at. Additionally, having geometry information in a buffer allows us to anti-alias other edges than just geometric ones, most notably alpha-tested edges. The shader just needs to output the distance to the alpha-edge, something that's easily approximated using gradient instructions. This is also demonstrated in the demo. On the down side it requires additional memory for the geometry buffer, but does not balloon up in the same way as MSAA does.

This demo should run on any DX10 capable GPU.