| OCR Text |
Show 14 Figure 2.3. The Nusselt analog: projection onto hemisphere and circle. A significant part of the radiosity solution computation time is spent calculating form factors. For a scene described by n surface polygons, O(n2 ) form factor values are calculated for the radiosity solution. Cohen and Greenberg have developed a method for calculating the necessary form factors [9], which involves several steps. A hemi-cube (right half cube) is placed in the interior of a polygon, with its surface broken into rows and columns of pixel-like squares (Figure 2.4). The polygon closest to the hemi-cube along the direction from the center of the hemi-cube to the center of a given square on the hemi-cube surface is recorded, through a process similar to raster scan conversion. After the projection onto the hemi-cube is completed for all the polygons in the scene, the projected areas on the hemi-cube are converted into form factor values. The factors used to map the hemi-cube areas to form factor contributions are called delta form factors. Figure 2.5 is a summary of the hemi-cube form factor calculation algorithm. Form factor calculation is costly. Citing two cases, Cohen and Greenberg report |
