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 7dB.
block truncation coding image compression multi-level quantization