The raster buffers add one-pixel border the frame buffer

I.9 A FAST BOUNDARY GENERATOR FOR COMPOSITED REGIONS

A fast method to generate the boundary for a group of regions blends techniques from computer graphics and image processing. This algorithm consists of two basic steps in the finite resolution of a frame buffer. First, for each closed primitive, draw the region filled and clipped to the frame buffer limits. Even though we wish to display only the envelope of many

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Erosion of the filled regions to their boundary outline looks at each pixel and its nearest four connected neighbors in the frame buffer. A filled pixel value or color will be erased to the erosion value only when all of its four connected neighbor pixels are also filled with the same value. Erosion to the background value will leave just the boundary pixels drawn in the original fill color. However, eroding to a different color will leave the outline in the original fill value and refill the interior with the new erosion value. Three raster buffers are used for this testing process to avoid replacing pixels in the frame buffer later needed for evaluating other pixels or requiring rendering from one frame buffer to another. In the C code of Appendix II, the raster buffers add a one-pixel border to the frame buffer. Filling this border with the object’s original pixel-fill value will leave an open edge where the boundary is clipped at the frame buffer’s edge. When the border is set to the background value (zero in the code), a closed edged envelope is drawn where frame-edge clipping

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