Images tend to have large areas that are a solid color, so that there are many pixels in a row with the same value. In Run-Length Encoding, each such set of pixel values are replaced by the run count and the run value.
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AAAAAAAAAbbbbTTTTTTT becomes 8A3b6T
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where the run count is the length of the run - 1.
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This scheme is easy to implement and fast to both compress and decompress. It achieves good rates of compression on simple pictures.
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It is not as successful on complex pictures, such as color photographs, where there are more gradations of color.
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