ATi Radeon 4000 Series Anti-Aliasing Investigation

ATi’s MSAA

ATi’s parts first attained MSAA with the 9700 Pro, which offered 2x and 4x rotated grids, along with 6x sparse grid MSAA. 6xAA was noteworthy as being the first in consumer space to debut six-sample MSAA, along with a pseudo random sampling grid.

Starting with the 2000 series, 2xAA and 4xAA remained unchanged, but 6xAA was dropped in favor of 8xAA, also utilizing a sparse grid. Since the 2000 series, ATi refers to these modes as box modes because all samples come from within a single pixel’s area, just like traditional MSAA.

The box modes were performed on the shaders on the 2000 and 3000 series, which is why they had poor MSAA performance. The 4000 series corrects this deficiency by performing the box modes on the ROPs, and also by packing ROPs that are twice as fast in most situations. ROPs are the units responsible for converting the output from the shaders into pixels.

Here are the sample patterns from a 4850 (these are identical across the entire 2000, 3000 and 4000 product lines).

The green dots are shader/texture samples, and in each case there’s only one sampled at the pixel’s center, as is typical with MSAA. The blue dots are the geometry samples and you can see two, four and eight arranged in rotated and sparse grids. This placement of samples ensures optimal coverage (maximum effective edge resolution).