Block Truncation Coding

Block Truncation Coding (BTC) is a type of lossy image compression technique for greyscale images. It divides the original images into blocks and then uses a quantizer to reduce the number of grey levels in each block whilst maintaining the same mean and standard deviation. 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.[1] BTC has also been adapted to video compression.[2]

BTC was first proposed by Professors Mitchell and Delp at Purdue University.[3] 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 and also typically results in a lower Mean Squared Error (MSE). AMBTC was proposed by Maximo Lema and Robert Mitchell.[4]

Using sub-blocks of 4×4 pixels gives a compression ratio of 4:1 assuming 8-bit integer values are used during transmission or storage. Larger blocks allow greater compression ("a" and "b" values spread over more pixels) however quality also reduces with the increase in block size due to the nature of the algorithm.

The BTC algorithm was used for compressing Mars Pathfinder's rover images.[5]

  1. ^ Liou, D. -M.; Huang, Y.; Reynolds, N. (1990). "A new microcomputer based imaging system with C/sup 3/ technique". IEEE TENCON'90: 1990 IEEE Region 10 Conference on Computer and Communication Systems. Conference Proceedings. p. 555. doi:10.1109/TENCON.1990.152671. ISBN 0-87942-556-3. S2CID 62015990.
  2. ^ Healy, D.; Mitchell, O. (1981). "Digital Video Bandwidth Compression Using Block Truncation Coding". IEEE Transactions on Communications. 29 (12): 1809. Bibcode:1981ITCom..29.1809H. doi:10.1109/TCOM.1981.1094938.
  3. ^ Delp, E.; Mitchell, O. (1979). "Image Compression Using Block Truncation Coding". IEEE Transactions on Communications. 27 (9): 1335. Bibcode:1979STIA...8011525D. doi:10.1109/TCOM.1979.1094560.
  4. ^ Lema, M.; Mitchell, O. (1984). "Absolute Moment Block Truncation Coding and akhand Its Application to Color Images". IEEE Transactions on Communications. 32 (10): 1148. doi:10.1109/TCOM.1984.1095973.
  5. ^ "Rover Camera Instrument Description". NASA. Retrieved 2021-05-18.