An Efficient Re-Indexing Algorithm for Color-Mapped Images

Abstract

Abstract-The efficiency of lossless compression algorithms for fixed-palette images (indexed images) may change if a different indexing scheme is adopted. Many lossless compression algorithms adopt a differential-predictive approach. Hence, if the spatial distribution of the indexes over the image is smooth, greater compression ratios may be obtained. Because of this, finding an indexing scheme that realizes such a smooth distribution is a relevant issue. Obtaining an optimal re-indexing scheme is suspected to be a hard problem and only approximate solutions have been provided in literature. In this paper, we restate the re-indexing problem as a graph optimization problem: an optimal re-indexing corresponds to the heaviest Hamiltonian path in a weighted graph. It follows that any algorithm which finds a good approximate solution to this graph-theoretical problem also provides a good re-indexing. We propose a simple and easy-to-implement approximation algorithm to find such a path. The proposed technique compares favorably with most of the algorithms proposed in literature, both in terms of computational complexity and of compression ratio.Index Terms-Hamiltonian path, indexed images, NP completeness, re-indexing, traveling salesman problem (TSP).

An adaptive palette reordering method for compressing color-indexed image

“…Finally, the reordered index maps were encoded with JPEG-LS. Table 1 lists the compression ratios achieved by JPEG-LS when encoding the outputs of various reordering methods [4][5][6][7][8][9]12]. The proposed method boosts up more compression performance than the others.…”

“…Some of them assign indices to palette colors based on the attributes of the palette colors [4] or the distance among the palette colors [5]. Some of them assign indices to palette colors based on the number of occurrences of having two particular palette colors in two spatially adjacent pixels [6][7][8][9]. All of them can effectively improve the compression rate when their outputs are encoded with JPEG-LS.…”

“…PROPOSED ALGORITHM Unlike conventional palette reordering methods [4][5][6][7][8][9] in which the palette is reordered based on some global properties of the input image, the proposed approach reorders the palette adaptively according to the current statistics and local characteristics of the image such that more spatial redundancy can be removed in the produced index map.…”

The 1.5D sieve algorithm

“…Hamiltonian paths have also been used in various areas of image processing, such as ordering or indexing (Bar-Joseph and Cohen-Or, 2003;Battiato et al, 2004), as well as mathematical morphology (Lezoray et al, 2007). Fig.…”

A low-complexity palette re-indexing technique based on sampling-swapping

“…), which is too high and in practical encourages the use of lower-cost sub-optimal solutions. For this purpose, several palette re-indexing schemes [1] [2] [3] [4] [5] [6] have been developed. A detailed survey was reported in [7] and recently, a comparison between two effective reindexing methods was made in [8].…”