15.3 Coverage Unit

The coverage value calculation, as described previously, produces a 4-bit number for each pixel that indicates how much of the pixel was covered by a primitive. For example, a value of 8 (1.0) indicates the pixel was fully covered. A value of 1 (0.125) indicates only one subpixel was covered. An example of the coverage calculation is shown in Figure 15-4.

Figure 15-4 Coverage Calculation

Note that it is very important that primitives sharing an edge have complementary subpixel masks, otherwise cracks may appear between edges. In the RCP, if primitives use the same vertex to create the primitive, then the pixel mask will be complementary. There are, however, cases where bad modelling can lead to cracks, as in Figure 15-5, below. These cases can occur when (incorrectly) fractalizing terrain or (incorrectly) generating triangles from NURBs surfaces, for example.

Figure 15-5 Complementary Edges