8x OGAA - what's the point?

Discussion in 'General 3D Technology' started by BoddoZerg, Nov 19, 2002.

  1. Bigus Dickus

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    I agree, that 16x grid is anything but random.
     
  2. Althornin

    Althornin Senior Lurker
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    Its not supposed to be, its supposed to be "sparse"
     
  3. SA

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    Once you have enough samples and a good sample pattern, the next source of AA improvement is a better filter.

    Using a simple Bartlett filter instead of box filter will offer a good amount of improvement. You get two benefits, more gradations for the same number of samples, and a more correct filter shape. Of course as soon as you go to a wider weighted filter (involving surrounding pixels) you might as well make the weights programmable.

    One mistake that many make when they create weighted filter shapes such as Gaussian filters is that they make the weights circularly symmetric around the pixel. This causes the sum of the weights for any particular sample point not to sum to 1/n. This creates uneven sampling which results in such artifacts as wavey edges. To solve the problem, Gaussian filters, windowed sync filters, etc. should not use circularly symmetric weights but should use weights that sum to 1/n for all samples. This results in a 4 sided tent shaped filter with horizontal and vertical silhouettes that are the desired Gaussian or windowed sync shape. Note that box filters and Bartlett filters do not have this problem, all sample weights sum to 1/n since they are not circularly symmetric but are 4 sided.

    Ordered grids and staggered grids are easy to use weighted filters with. Programmable sparsely sampled grids are also fairly easy to weight using programmable weights. In this case you have 9 weights per sample in the grid lookup table (1 weight for the center pixel and 8 weights for the adjacent pixels).

    When using a broad area filter, its best to apply the filter after the frame buffer is complete so that all the final colors are in the frame buffer. Some of the filter-as-you-go techniques are harder to implement.
     
  4. SA

    SA
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    Assuming the use of fragment AA with coverage masks to achieve compression, it is important to use a good fall back when there are more fragments than a pixel can handle.

    For current scenes two fragments per pixel is usually good enough since most AAed pixels consist of a foreground edge and a background surface. This is because the triangles are still large and the complexity is low for most scenes.

    For current scenes therefore edge quality (jaggies) is the major AA concern. For future scenes, however, subpixel triangle quality becomes more of a concern.

    One fall back mechanism for fragment AA is to merge the fragments. This works well as long as there are only 2 surfaces in the pixel and merging is based on both the z and the color. Using only the z value for merging as Z3 does can produce artifacts if the polygons on the surface are different colors (such as a beach ball).

    As scene complexity increases a larger portion of the pixels will have more than 2 surfaces, especially for outdoor scenes. In the long run, therefore, a fall back to uncompressed pixels with one color and z per sample works best.
     
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