How Multisampling Affects Different Primitives

How Multisampling Affects Different Primitives

This section briefly discusses multisampled points, lines, polygons, pixels, and bitmaps.

Multisampled Points

If you're using multisampling, the value of the smoothing hint (GL_POINT_SMOOTH_HINT or GL_LINE_SMOOTH_HINT) is ignored. Since the quality of multisampled points may not be as good as that of anti-aliased points, remember that you can turn multisampling on and off as needed to achieve multisampled polygons and anti-aliased points.

Note: On RealityEngine systems, you achieve higher-quality multisampled points by setting point smooth hint GL_NICEST (though this mode is slower and should be used with care).

glHint(GL_POINT_SMOOTH_HINT, GL_NICEST)

The result is round points. Points may disappear or flicker if you use them without this hint. See the Caution: in the next section for caveats on using multisampling with smooth points.

Multisampled Lines

Lines are sampled into the multisample buffer as rectangles centered on the exact zero-area segment. Rectangle width is equal to the current linewidth. Rectangle length is exactly equal to the length of the segment. The rectangles of colinear, abutting line segments abut exactly, so no subsamples are missed or drawn twice near the shared vertex.

Just like points, lines on RealityEngine systems look better when drawn "smooth" than they do with multisampling.

Caution: If you want to draw smooth lines and points by enabling GL_LINE_SMOOTH_HINT or GL_POINT_SMOOTH_HINT, you need to disable multisampling and then draw the lines and points. The trick is that you need to do this after you have finished doing all of the multisampled drawing. If you try to re-enable multisampling and draw more polygons, those polygons are not necessarily anti-aliased correctly.

Multisampled Polygons

Polygons are sampled into the multisample buffer much as they are into the standard single-sample buffer. A single color value is computed for the entire pixel, regardless of the number of subsamples at that pixel. Each sample is then written with this color if and only if it is geometrically within the exact polygon boundary.

If the depth-buffer is enabled, the correct depth value at each multisample location is computed and used to determine whether that sample should be written or not. If stencil is enabled, the test is performed for each sample.

Polygon bit patterns apply equally to all sample locations at a pixel. All sample locations are considered for modification if the pattern bit is 1. None is considered if the pattern bit is 0.

Multisample Rasterization of Pixels and Bitmaps

In IRIS GL, pixels are not multisampled. In OpenGL, if multisampling is on, pixels are considered small rectangles and are subject to multisampling.

When pixels are sampled into the multisample buffer, each pixel is treated as an xzoom-by-yzoom square, which is then sampled just like a polygon.

For information about fast clears on RealityEngine, see the reference page for glTagSampleBufferSGIX().