Science, Maths & Technology

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# 4.6 Greyscale

If this seems inadequate – it does seem rather an impoverished range of shades – all we need to do is allocate more bits. Three bits per pixel will give us eight shades, from black to white; four bits per pixel gives us 16 shades; and so on. This mapping of shades of grey between black and white in a black and white bitmap is known as greyscale. The range of numbers to which a pixel can be mapped is termed the pixel amplitude.

Let's try a simple example. I'm going to take the first of the two images in Figure 17. Once again, I can lay a grid over the image as in Figure 18, which I've enlarged a bit, to demarcate the pixels. Note that I'm only working with a section of the picture, near the nose of the aircraft.

Figure 18 Picture of aircraft divided into pixels

Now I need to inspect each pixel and decide on the closest shade of grey to represent what appears in the square. For simplicity, I've decided to use six shades of grey, as well as black and white. The result is shown in Figure 19. It's not brilliant, but that can be put down to a fairly large pixel size and a limited choice of greys.

Figure 19 A greyscale image of the aircraft

Finally, I can map each pixel in the image onto one of the eight binary numbers between 000 and 111 inclusive, depending on whether the image at that pixel is black (111), white (000) or some shade of grey between. The mapping in the small area I've marked will look like this:

The sequence of binary numbers

which is tedious enough to demonstrate just how simplistic the interior world of the computer is.