OpenGL 4.1 (Compatibility Profile) - July 25, 2010

3.3. ANTIALIASING 205
these b bits based on the fragment’s coverage value: the bits are set to zero for no
coverage and to all ones for complete coverage.
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
(x, y) and upper right corner (x+1, y +1). We recognize that this simple box filter
may not produce the most favorable antialiasing results, but it provides a simple,
well-defined model.
A GL implementation may use other methods to perform antialiasing, subject
to the following conditions:
1. If f 1 and f 2 are two fragments, and the portion of f 1 covered by some primitive
is a subset of the corresponding portion of f 2 covered by the primitive,
then the coverage computed for f 1 must be less than or equal to that computed
for f 2 .
2. The coverage computation for a fragment f must be local: it may depend
only on f’s relationship to the boundary of the primitive being rasterized. It
may not depend on f’s x and y coordinates.
Another property that is desirable, but not required, is:
3. The sum of the coverage values for all fragments produced by rasterizing a
particular primitive must be constant, independent of any rigid motions in
window coordinates, as long as none of those fragments lies along window
edges.
In some implementations, varying degrees of antialiasing quality may be obtained
by providing GL hints (section 5.8), allowing a user to make an image quality
versus speed tradeoff.
OpenGL 4.1 (Compatibility Profile) - July 25, 2010