Improvement of JPEG XT Floating-Point HDR Image Coding Using Region Adaptive Prediction

Displays capable of representing high-dynamic range (HDR) images were recently released in the digital consumer electronics market, and consumers have become increasingly interested in HDR images. However, the majority of todays digital imaging devices are still low dynamic range (LDR) using an 8-bit representation for each RGB color components; hence, a backward-compatible HDR image format with an existing LDR-based imaging environment is required. Accordingly, JPEG XT, a new HDR image coding standard providing JPEG backward compatibility was established by the JPEG committee based on the market needs. JPEG XT consists of a base and a residual layer. Both layers are independently encoded by a legacy JPEG encoder. Most importantly, the base layer contains a tone-mapped LDR version of the HDR image for compliance with a legacy JPEG decoder. To date, three profiles have been defined for JPEG XT to encode HDR images, which are represented using floating-point values. This paper proposes a new image-coding scheme that uses a region adaptive prediction method with modified current specifications on JPEG XT Part 7. The proposed method adaptively predicts the HDR pixel value using different prediction information for each block based on the ratio between the LDR and HDR blocks, which is invariant to the tone-mapping operator, while the existing JPEG XT profiles globally predict the HDR values using an inverse transformation of the LDR values. Experimental results using various sample images show the superiority of the proposed method in terms of objective and visual evaluations.