Color gradient vectorization for SVG compression of comic image

•The proposed CGV can identify CG in the comic image and embed it in SVG syntax.•The proposed post-processing can remove redundant points in SVG paths.•The reusability of SVG syntax can further reduce the file size.•It provides not only good perceptual quality but also small file size.

In past years, different raster-to-vector methods were proposed to convert bitmap images to SVG format. However, they did not consider the color gradient (CG) that frequently appears in the comic image. Their results needed multiple divided color regions to represent a single CG region. It produced poor perceptual quality and large SVG size. In this paper, we propose the CGV (CG vectorization) method to resolve this problem. CGV first applies a linear-time algorithm to identify the CG vector for representing the color and the direction of CG in each region. Then, we merge neighboring regions those have the same CG vector as a large CG region and represent it by a single path of SVG with linear gradient syntax. Experimental results show that our method outperforms other state-of-the-art SVG vectorization systems in terms of not only SVG size but also perceptual quality. For example, the averages SSIM are 0.85 by Autotrace, 0.92 by Autotrace+Merge, 0.88 by SWaterG, 0.91 by Vector Magic and 0.94 by our method. It is about 57.23% for our average space saving. Moreover, comparing with other systems’ results, our SVG files take the shortest time (about 0.12 s in average) of rendering on handheld devices.