In images, the representation of glossiness, translucency, and roughness of material objects (Shitsukan) is essential for realistic image reproduction. To date, image coding has been developed considering various indices of the quality of the encoded image, for example, the peak signal-to-noise ratio. Consequently, image coding methods that preserve subjective impressions of qualities such as Shitsukan have not been studied. In this study, the authors focus on the property of glossiness and propose a method of glossiness-aware image coding. Their purpose is to develop an encoding algorithm that produces images that can be decoded by standard JPEG decoders, which are commonly used worldwide. The proposed method consists of three procedures: block classification, glossiness enhancement, and non-glossiness information reduction. In block classification, the types of glossiness in a target image are classified using block units. In glossiness enhancement, the glossiness in each type of block is emphasized to reduce the amount of degradation of glossiness during JPEG encoding. The third procedure, non-glossiness information reduction, further compresses the information while maintaining the glossiness by reducing the information in each block that does not represent the glossiness in the image. To test the effectiveness of the proposed method, the authors conducted a subjective evaluation experiment using paired comparison of images coded by the proposed method and JPEG images with the same data size. The glossiness was found to be better preserved in images coded by the proposed method than in the JPEG images.
Subband coding is a powerful means for highly efficient image compression. In order to improve the coding performance of subband image coding, we recently have proposed the optimum space-frequency partition coder (OSFP) that optimizes the following three factors in the rate-distortion sense: the frequency band partition with a small number of subbands, quantization and the spatial segmentation to exclude redundant pixels. However, an encoded image obtained by OSFP is not necessarily optimal in subjective image quality because the three factors are optimized to minimize the mean square error (MSE). In this paper, we present a new OSFP that obtains a high quality coded image subjectively by optimizing the three factors so that MSE weighted by considering both the human visual sensitivity and a region-of-interest of human is minimized. Experimental results show that the quality of encoded images obtained by the proposed method has higher subjectively than them of both the conventional OSFP and JPEG2000 by the mean opinion score (MOS).