Haar wavelet processor for adaptive on-line image compression
F. Javier Díaz, Angel M. Buron, Jose M. Solana · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2005
An image coding processing scheme based on a variant of the Haar Wavelet Transform that uses only addition and subtraction is presented. After computing the transform, the selection and coding of the coefficients is performed using a methodology optimized to attain the lowest hardware implementation complexity. Coefficients are sorted in groups according to the number of pixels used in their computing. The idea behind it is to use a different threshold for each group of coefficients; these thresholds are obtained recurrently from an initial one. Parameter values used to achieve the desired compression level are established "on-line", adapting their values to each image, which leads to an improvement in the quality obtained for a preset compression level. Despite its adaptive characteristic, the coding scheme presented leads to a hardware implementation of markedly low circuit complexity. The compression reached for images of 512x512 pixels (256 grey levels) is over 22:1 (≈0.4 bits/pixel) with a rmse of 8-10%. An image processor (excluding memory) prototype designed to compute the proposed transform has been implemented using FPGA chips. The processor for images of 256x256 pixels has been implemented using only one general-purpose low-cost FPGA chip, thus proving the design reliability and its relative simplicity.