Error equalization for high quality LDR images in backward compatible HDR image coding

This report introduces an error equalization to increase quality of low dynamic range (LDR) images in a backward compatible high dynamic range (HDR) image coding. To utilize a currently standard source coding algorithm such as the JPEG encoder, a data compression friendly mapping such as the power function is applied before compression. In a receiver side, the decoded image is once again mapped based on a visually proper function such as the Hill function to generate the LDR image. In this report, we investigate how the errors added in the data compression process are magnified through the inverse power function and the Hill function, and show that the errors in dark pixels of the LDR image are extremely high. Based on this, we equalize probability density function of the error in the LDR image to increase its quality. As a result of experiments, it was observed that the PSNR of the LDR was increased by approximately 3 to 4 [dB] in the rate distortion curves.