From Real Coordinates to Pixels
Ian O. Angell, Brian J. Jones · 1983
We have seen that Spectrum imagines its graphics frame to be a rectangular matrix of addressable points or pixels. These pixels are stacked in NXPIX (=256) vertical columns and NYPIX (=176) horizontal rows. Individuals from the set of NXPIX by NYPIX pixels can be uniquely identified by a bracketed pair of integers; these are sometimes called a pixel vector (I, J), where 0 ⩽ I ⩽ NXPIX — 1 and 0 ⩽ J ⩽ NYPIX — 1, the vector specifying the position of the pixel in the I th column and J th row: the vector (0, 0) identifies the bottom left-hand corner pixel of the frame. The Spectrum has its own set of BASIC instructions that enable users to operate on the matrix of pixels, treating them as points of light that can be switched off or on. This enables the operator to approximate lines, or polygons and other special types of area, with a series of coloured dots (the pixels).