![]() |
|Download Files|
![]() ![]() ![]() ![]() ![]() Next: 3.3 Points Up: 3 Rasterization Previous: 3.1 Invariance
3.2 AntialiasingAntialiasing of a point, line, or polygon is effected in one of two ways depending on whether the GL is in RGBA or color index mode.
In RGBA mode,
the R, G, and B values
of the rasterized fragment are left unaffected, but the A value
is multiplied by a floating-point value in the range
In color index mode,
the least significant b bits (to the left of the binary point)
of the color index are used for antialiasing;
The details of how antialiased fragment coverage values are computed are difficult to specify in general. The reason is that high-quality antialiasing may take into account perceptual issues as well as characteristics of the monitor on which the contents of the framebuffer are displayed. Such details cannot be addressed within the scope of this document. Further, the coverage value computed for a fragment of some primitive may depend on the primitive's relationship to a number of grid squares neighboring the one corresponding to the fragment, and not just on the fragment's grid square. Another consideration is that accurate calculation of coverage values may be computationally expensive; consequently we allow a given GL implementation to approximate true coverage values by using a fast but not entirely accurate coverage computation.
In light of these considerations,
we chose to specify the behavior of exact antialiasing in the prototypical
case that each displayed pixel is a perfect square of uniform intensity.
The square is called a fragment square and has lower left corner
A GL implementation may use other methods to perform antialiasing, subject to the following conditions:
![]() ![]() ![]() ![]() ![]() Next: 3.3 Points Up: 3 Rasterization Previous: 3.1 Invariance David Blythe Sat Mar 29 02:23:21 PST 1997
|