Optimisation of multi-level block truncation coding

Block truncation coding (BTC) is very attractive for real-time image coding at moderate bit-rate due to its low computation and storage demands. One major artefact of this image compression technique is edge raggedness. Modifications are mainly in two areas: bit-rate reduction and improvement of the quality of reconstructed image. Our investigation is in the latter area. In our integrated multi-level BTC scheme, the visual quality is substantially improved, the bit-rate is maintained the same as standard BTC and the computational demand is still kept low. We exploit various algorithms and derive new techniques to optimise the performance of multi-level BTC. Dynamic range tuning (DRT) is used to tackle 2-level BTC. Middle group settlement is used to carry out binary classification for tri-modal distribution. For 4-level BTC, an iterative DRT is derived. Based on our experience on 2- and 4-level BTCs, an optimised 3-level BTC is derived. Extensive testing has been carried out on 30 images. The resultant visual quality is nearly perfect and the PSNR may be boasted by up to 7 dB. © 2001 Elsevier Science B.V. All rights reserved.