2 From Real Coordinates to Pixels
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Throughout the two-dimensional and three-dimensional sections of this book we shall divide the television screen into a text window (of two lines at the top of the screen) and a graphics window (the rest of the screen). As already discussed, on the Model B the graphics window (or graphics frame) consists of a rectangular matrix of coloured pixels; the size and number of the pixels and the number of possible colours depend on the MODE setting of the computer (0 to 7). Unless otherwise stated, two-dimensional and three-dimensional geometrical programs will be run in MODE 1. This means that access to these pixels is MODE-dependent and hence we shall consider the graphics frame to be made up of the rectangular matrix of addressable points (points for short) that are stacked in NXPIX (= 1280) vertical columns and NYPIX (= 960) horizontal rows. Each pixel on the screen corresponds to a number of different points; although the exact correspondence is naturally MODE-dependent, this need not concern us since the operating system deals with this problem of relationship. Unfortunately the word ‘pixel’ has several different meanings (as do ‘point’ and ‘dot’), so it must be remembered that in this book ‘pixel’ refers to a MODE-dependent group of addressable points on the television screen (see the last column of table 1.1).
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