A General-Purpose Hidden Surface and Hidden Line Algorithm
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There are many different types of hidden line and/or surface algorithm. One variety has a rectangular array that represents the totality of pixels on the screen. We imagine rays of light that leave the eye through each of the pixels on the screen. These rays naturally pass through objects in our scene and we can note the coordinates of these points of intersection. The array will hold the ‘z-coordinate’ (initially infinity) of the nearest point of intersection. So we build up a picture by adding new objects, finding where the rays cut the object, and changing the array values (and the pixel colour on the screen) whenever the latest point of intersection is nearer the eye than the corresponding value stored in the array. This technique (ray tracing) is very useful if we wish to shade-in areas in subtly differing tones of a given colour. It does, however, have enormous storage requirements and needs a very powerful computer, well beyond the capacity of microcomputers. Because we must work with only four colours and have limited storage we give another general algorithm which works on the ‘back to front’ principle mentioned earlier.
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