Faster pixel bit mask generation exploiting point-to-point coherence of scanlines

Everyone who is willing to generated high quality images is faced with aliasing problems. Prefiltering of the image is a recognized method to soften the symptoms of aliasing. One simple filter is the integration of the visible intensities over the area of a pixel. An exact pixel integrater has to clip all polygons found in a pixel against each other and the pixel window. Pixel bit masks is a convenient approximation to represent the visible area of polygons at a subpixel level. These masks reduces the full polygon clipping to simple boolean exclusive- or operations. The efficient generation of such pixel bit masks plays an important role in the time performance of the synthesis process. The idea of pixel bit masks is not new, but exploiting the point-to-point coherence of scanlines, it was possible to improve the performance over the original algorithm.