Once the occluders have been selected they are rendered to the framebuffer with lighting and texturing disabled. The polygons are colored white to produce values near or equal to 1.0 in opaque areas. OpenGL implementations which do not support some form of antialiasing will have pixels values that are either 0.0 or 1.0. A hierarchy of reduced resolution maps is created by copying this map to texture memory and performing bilinear texture filtering on the image to produce an image 1/4 the size. Additional maps are created by repeating this process. The size of the highest resolution map and the number of hierarchy levels created is a compromise between the amount of time spent rendering, copying, and reading back the images and the accuracy of the result. Some of the lower resolution images may be more efficiently computed on the host processor as the amount of overhead involved in performing copies to framebuffer or pixel readback operation dominates the time spent producing the pixels.
