We begin by looking at the way in which TV images are fed into the visual sub-system. The human eye is often likened to a camera in that light from a scene passes through a lens and is cast onto a light sensitive surface -- the film in the camera or the retina in the eye. We bring this analogy up to date for the MTG and use a TV camera connected to a box known as a video digitizer which converts the video picture into a stream of numbers which are fed directly to the computer. Each number, or grey-level, is a measure of the light intensity over a small square patch of the picture, known as a pixel, and a complete image is built up from a large number of adjoining pixels arranged into rows and columns; like a jigsaw puzzle with square pieces of the same size.
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Figure: An array of numbers corresponding to the Underground sign in
figure 8.1.
Figure 8.3 shows the array of numbers produced by our video digitizer for a small part of the image shown in figure 8.1 in the vicinity of the Underground sign. Each number is a measure of the light intensity at the pixel in the corresponding position in the TV picture. The numbers range from 0 to 99; 0 is the darkest and 99 is the lightest -- intermediate values correspond to the greys in between.
We shall not address all of the technical problems involved in setting up the video digitizer and connecting it to the computer and assume instead that the grey-levels of figure 8.3 are available as an array of numbers inside the computer.
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