From Lossy to Lossless Wavelet Image Coding in a Tree-Based Encoder with Resolution Scalability

For a lossy encoder, it is important to be able to provide also lossless compression with little or no modification of the usual algorithm, so that an implementation of that algorithm can work in lossy or lossless mode, depending on the specific application, simply by varying the input parameters. In this paper, we evaluate the capability of the Lower Tree Wavelet (LTW) image encoder to work in lossless mode. LTW is a fast and multiresolution wavelet image encoder, which uses trees as a fast mode to group coefficients. In addition, general details on how to implement efficiently (i.e., with only shift and addition/subtraction operations) a reversible integer-to-integer wavelet transform are also given, as a requirement to implement a wavelet-based lossless encoder. Numerical results show that despite being general purpose (i.e., both lossy and lossless) and lacking of complex techniques (such as high-order context and predictive coding), the LTW performs as well as JPEG 2000 in lossless mode, and only 5% below LOCO-I, a specific lossless algorithm.