A JPEG backward-compatible image coding scheme for high dynamic range images

Recently, the camera industry began the transition from low dynamic range (LDR) images, using the traditional 8 bits per RGB color representation, to high dynamic range (HDR) images, which use higher bit depths to represent each RGB color. The legacy-JPEG (L-JPEG) standard (ISO/IEC 10918) dominates the LDR image coding market, but offers no support for HDR images. To solve this problem, the JPEG committee (ISO/IEC JTC1 SC29 WG1) initiated a new HDR image coding standardization called JPEG XT (ISO/IEC 18477), which provides L-JPEG backward compatibility. In JPEG XT, HDR images are encoded into base-layer and residual-layer codestreams. An input HDR image is tone-mapped to an LDR image, which is compressed by the L-JPEG encoder, and the base-layer codestream is constructed to provide L-JPEG backward compatibility. The residual-layer codestream allows a residual-layer decoder to reconstruct an HDR version of the LDR codestream. In this paper, we propose a new JPEG backward-compatible image coding method for HDR images. We generate the residual data in the discrete cosine transform (DCT) domain, unlike existing JPEG XT profiles that generate their residual images in the spatial domain. The proposed encoder predicts the DCT coefficients of the input HDR image based on those of the L-JPEGencoded LDR version. Therefore, the inverse DCT process is not required in our encoding structure, which reduces the encoding time. Experimental results using various sample images show the superiority of the proposed method in terms of encoding time and objective and subjective evaluations. (C) 2017 Elsevier Inc. All rights reserved.