Block Truncation Coding

Block Truncation Coding, or BTC, is a type of lossy image compression technique for grayscale images. It divides the original images into small sub-images and then using a quantizer, which adapts itself according to the image statistics, to reduce the number of gray levels in the image. It is an early predecessor of the popular hardware DXTC technique, although BTC compression method was first adapted to color long before DXTC using a very similar approach called Color Cell Compression .

Compression procedure

A 256 x 256 pixel image is divided into blocks of 4x4 pixels. For each block two values are computed: The Mean and the Standard Deviation, these values change from block to block. Then a two level quantization on the block is made as follows: If a pixel value is equal to or greater than the mean it is assigned the value "1", otherwise "0". This 16 bit block is stored along with the values of Mean and Standard Deviation. Reconstruction is made with two values "a" and "b" which preserve the mean and the standard deviation. BTC was first proposed by Professors Mitchell and Delp at Purdue University. Another variation of BTC is Absolute Moment Block Truncation Coding or AMBTC, in which instead of using the standard deviation the first absolute moment is preserved along with the mean. AMBTC is computationally simpler than BTC. AMBTC was proposed by Lema and Mitchell.