Making Bitmaps with the RGB Colour System, Palettes and Look Up Tables

As an image is captured and a grid of colour samples built up, each grid location’s colour needs to be given a representative number. Yes, we’re finally getting back to numbers again! Like a painter, when dealing with images the computer uses a palette. The palette contains the colour information for the picture, and, like the painter’s palette, forms the basis for creating the image. The image data can be developed from one of many existing predefined computer colour palettes, or, as in this example, a palette of colours can be constructed from the colours found in the image, ascribing each colour a numerical value as we go. In this second method, variously called index mapping or colour mapping, the colour map or CLUT (colour look up table) is embedded in the image data itself. GIF images are a common example of an image file type that uses colour mapping. Matching Colour Palettes? There are many standard palettes in computing, for instance the 216 web safe colours, the Windows palette, and the 16 bit RGB true colours. If necessary, a palette can be uniquely defined for a particular image, but the same palette must be used when displaying the image as used when creating the image. If the palettes match, the result will be the same on any computer – at least in theory. Unfortunately, it’s unlikely. Because of complex imaging techniques in use such as dithering and anti-aliasing, extra pixel information such as transparency, the limitations imposed by the hardware on any particular computer system, and even the brightness and contrast settings on a given monitor, it’s impossible...

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