Adaptive RD Optimal Sparse Coding With Quantization for Image Compression
In image and video compression for many multimedia applications, an image/frame is divided into component blocks or patches and is then encoded using some type of transform. Traditional transforms use a complete dictionary of basis functions. A recent technique of growing interest is signal approximation using a linear combination of basis functions from an overcomplete dictionary. The result is a sparse set of coefficients that can represent the original signal and is called sparse coding. This is an NP-hard problem. Orthogonal matching pursuit is a greedy algorithm that is effectively used to address this problem. Keeping in mind the iterative nature of this algorithm, in a recent conference publication, we proposed a rate distortion optimization (RDO) method to select the best sparse representation among iterations up to a target sparsity level. In this paper, we expand the work and consider an adaptive coding scheme that takes advantage of both discrete cosine transform (DCT) and sparse coding. This scheme shows a better performance over plain DCT or sparse coding schemes. We further propose a scheme to increase the coding efficiency of sparse coding by quantizing the sparse coefficients. We investigate an RDO method to select the value of the quantization parameter from a range, balancing distortion, and bit rate. Based on experimental results, we provide a comparison between conventional DCT-based coding, sparse coding scheme, our mixed coding scheme, and the proposed method that includes quantization of the sparse coefficients.