Number of Colors:

Well we’ve sure got lots of choices! We can, in pre-selected steps, use between 16 and MILLIONS of colors to paint our digital images.

We’ll start back at the monitor since I think that’s where we see digital images most often. Most monitor and driver board combinations that are in use now have the capability to display 256 colors. This just happens to be the number of counts that can be represented in one byte of data (0 to 255). Color is created on the screen by using varying amounts of the three primary colors Red, Green, and Blue (RGB) in a 256 color system the amounts of Red, Blue, and Green required for each color are stored on a color palette. Including shades of gray, black, and white the palette stores a total of 256 colors numbered from 0 to 255. Using this 256 color palette system, each pixel can be represented by a single byte of data that points to the entry in the palette where the actual color data for the pixel is located. Naturally for this to be effective the palette is stored in image files along with the pixel pointers for the image. As the computer reads and image file it uses the pixel pointers to find the color from the palette that should be actually be displayed on the screen for each pixel. This system has the advantages that the image files are small compared to other systems and 256 colors can do a nice job of rendering many kinds of images.

The 256 color system has its limitations. Since it relies on a palette, the palette must be available and usable. There are a variety of circumstances that can sometimes lead to an image being rendered with the wrong palette or with a corrupted palette. In these cases there is usually no doubt on the part of the person viewing the image that something is very wrong. In addition, in images where there are large flat areas, (areas of color that vary only slightly in color or tone) like a picture that included a gray sky, the 256 color system runs out of colors quickly. There just aren’t enough colors or shades of colors to properly represent the subtle changes happening in the image. This is when we observe an effect known as contouring. There will be sharp delineations in tone as you look across the flat area. this is caused by the fact that the image contained colors or tones the fell between those provided on the palette. The boundaries between the areas of different tone are caused by the fact the two choices in the palette weren’t close enough for the boundary to by invisible.

At the other end of the digital color world is the "millions of colors" system. In this system there are three bytes of data for each pixel; One byte is for Red, one for Green and one for Blue. Here, the actual mix of the primary colors needed for each individual pixel are stored as the pixel data. So in this system each pixel’s mix of primary colors (R, G, and B) can have a value between 0 and 255 or a total of 256 intensities (or densities) for each primary color. so the number of colors that can be created is 256 X 256 X 256 = 16,777,216 (millions). Are they all really different colors though? In fact many are tone variations of the same color. If you start with a given set of RGB values and then just scale them so the ratio between them doesn’t change the only difference is in the intensity of the color. The color (Hue) hasn’t actually changed. With this system contouring is much less of a problem, but you pay for it in files that are three times bigger per pixel stored. The images used to illustrate the equipment section are, all but one, the 256 color variety. You will likely be able to observe contouring in these images. I used the 256 color system for these because I felt that would serve the purpose and they load from the server a lot faster.

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