 While, in principle, any general-purpose lossless compression algorithm ("general-purpose" meaning that they can accept any bitstring) can be used on any type of data, many are unable to achieve significant compression on data that are not of the form for which they were designed to compress.Many of the lossless compression techniques used for text also work reasonably well for indexed images.These techniques take advantage of the specific characteristics of images such as the common phenomenon of contiguous 2-D areas of similar tones.