THE ANALYSIS OF TONE Our long-established system of photographic imaging has a beautiful smooth transfer from black to white. That smooth tonal range in an image can be likened to a natural hill - the valley floor denoting black, the summit white, with an infinite number of shades of grey in between. Just like the Grand Old Duke of York's men in the nursery rhyme, we can march up the hill and stop when we are neither up nor down but half way. We do this by stopping when we sense that we are the same distance from the top as from the bottom - half way up represents mid grey - mid way between black and white. Try it. Walk half way up a hill, put a mark down on the ground, then go back to the bottom and walk up again until you sense that you are half way. Now look down to see how close you are to your mark. Is it in front of you, behind you - or are you really lucky, discovering that it is right under your feet? Hardly likely, for it's impossible to know precisely where you are on a slope. That's the way it is in our analogue world. STEP RIGHT UP A staircase is a kind of man-made hill. It's much easier to know when you are half way up a flight of stairs. Take a short flight of seven steps and count them as you climb. One - two - three - four, stop. Half way represents mid grey. Could we be on to something useful here? The difference between analogue and digital is rather like the difference between a slope and a staircase. Cut steps into a hillside and you can count the steps as you climb, giving you the ability to know precisely where you are in relationship to the bottom and top. Sample an image, reducing it to a number of steps of grey, and you can define each tone precisely. But how many different tonal steps do you need? A staircase of seven steps gives us eight clearly defined levels between ground (level zero) and landing (the seventh step and, thus, the eighth level). If applied to every point on an image, would eight steps be enough to represent a continuous tone image? Let's see what describing an image in eight tonal steps gives us.     With black, white and just six shades of grey between, the image looks posterised. Eight steps is clearly too few. So how many do we need?   BENEDICTINE STEPS Have you ever climbed the 199 steps - a 'round' 200 including the bottom level - up to Whitby Abbey, on the north Yorkshire coast? Maybe those Benedictine monks knew what was coming, digitally, because 200 steps of grey from black to white are just about enough to give the impression of continuous tone. In digital imaging we use 256 steps or levels. Why 256 if 200 will do?   You can just see each individual step of a 'staircase' of 256 steps, but look at the screen from further away, and the steps merge to give the impression of a smooth slope.   A BIT LIMITED A 'bit' is the smallest piece of data which a computer can process. Just like a light switch, that bit can only be 'off' or 'on'. If we use one computer bit to describe our image, we can use those 'off' and 'on' conditions to represent just black and white, with nothing in between. No greys. Our image would look like this: A One Bit Image The Spirit of the Age is a true 'one bit' image - black and white with no tones. Using the 'scraper-board' technique, the black surface of the board was scraped away, revealing the white beneath. The work was inspired by the threat of doom brought about by the dawn of the atomic age. Copyright © 1949 by John Henshall Snr. A BIT MORE If we devote two computer bits to the description of each point of the image, we have four possibilities for the state of those bits. Let's see why using our light switches. Bits one and two can both be off, or bit one can be on while bit one is off, or bit one can be on while bit two is off, or both can be on - a total of four combinations from two bits. This enables us to describe our image in four tones: black, dark grey, light grey and white. A Two Bit Image   GET THE PICTURE? If we devote three bits to describing each point of the image, we get eight combinations. A Three Bit Image Four bits gives 16 combinations. Five gives 32, six gives 64 and 7 gives 128 combinations. You can draw the switches! Hopefully, you too are getting switched on to the way it works. Each time we devote one more computer bit to describing each point on our image, we double the total number of combinations. The total number of combinations equals the number of levels, or steps, of grey we can use to describe the image.   ONE SMALL STEP Unfortunately, 128 levels are not quite enough, so we double up from there to 256 levels, which takes eight bits. Now the steps are so small that the human eye cannot distinguish between their adjacent levels. This is what is referred to as a 'photo-realistic', or 'eight bit', monochrome image - the digital equivalent of continuous tone. Eric Morcambe would have loved it: you can't see the joins. An Eight Bit Image DOZENS OF BITS "But wait a minute, what's this I keep hearing about 'twenty four bits'?" Twenty four bits come about because eight bits are required to describe each of the primary colours -- red, green and blue -- which combine to make a colour image. Three times eight is twenty four. If your brain is not hurting too much already, the number of combinations of any one of 256 levels of red, together with any one of 256 levels of green, together with 256 levels of blue is 256 x 256 x 256. That comes to a staggering total of 16,777,216 colours. Don't worry about being challenged to name them all, it's easy: red, green and blue - the other 16,777,213 are simply a matter of proportion. So there we are. That's why you need 'twenty four bit colour' on your monitor to display photo-realistic digital images in full colour. A BIT TOO FAR I've also heard mention of 48 bit colour, what's that?" ... Steady on, now - a bit at a time please. All text, photographs, graphics and backgrounds in this Website are Copyright © 1996, 1997 & 1998 John Henshall at Electronic Photo-Imaging. All rights reserved. Material may only be downloaded for personal non-commercial use. Please safeguard the future of online publishing by respecting this copyright and the rights of all other authors of material on the Internet. Comments and re-publication enquiries are welcome. Please eMail us.