Appearance-based Image Splitting for HDR Displays

High dynamic range (HDR) displays that incorporate two optically-coupled image planes have recently been developed. The existence of such displays requires HDR image splitting algorithms that send appropriate signals to each plane to faithfully reproduce the appearance of the HDR input. In this paper we introduce a new appearance-based HDR image splitting algorithm that incorporates the iCAM06 image appearance model to do image enhancement. We compare its performance to the widely used luminance square root algorithm and report the results of image quality experiments that compare the two algorithms with respect to contrast, color, sharpness, naturalness, and overall quality. We find that the overall quality, color, and naturalness of the images produced by the new algorithm is superior to those produced by the square root method. The new algorithm provides a principled and effective approach for presenting HDR images on dual imager HDR displays. Introduction Real-world scenes encompass a 10 log unit range of luminance levels, from below 0.001 cd/m to over 100,000 cd/m [1]. The 4 to 6 log unit (10,000 to 1,000,000 to 1) luminance dynamic range found in many scenes vastly exceeds the ranges that can be captured or reproduced by conventional imaging systems. For example, images captured by conventional digital cameras typically have dynamic ranges between 100 or 1000 to 1 around a level set by aperture and shutter speed. Conventional display systems are similar in that their output dynamic ranges are on the order of 100 to 1 with maximum luminance between 100 and 400 cd/m [2]. Over the past fifteen years, new technologies have been developed for overcoming this dynamic range bottleneck to produce image capture and display systems capable of recording and reproducing high dynamic range (HDR) images. High dynamic range image capture systems have been developed for both still images [3,4,5,6,7] and video recordings [8]. For displaying HDR images, several alternative systems have been developed, including both softcopy [9,10] and hardcopy [11,12] designs. HDR display systems typically reproduce high dynamic range images by using two standard dynamic range (SDR) imagers that are optically coupled. The basic principle is that one imager (such as a projector or LED array) provides spatially varying illumination for a second imager (for example a transmissive LCD or reflective print), allowing HDR image values to be reproduced. This dual image plane design requires that a given HDR input image be split into two complementary SDR components that drive the coupled systems. The widely used square root HDR splitting algorithm first converts an input HDR image to XYZ tristimulus values, then takes the square root of the Y channel and sends this achromatic signal to one image plane. A color signal is created by composing Y with its corresponding X and Z channels to the other. Under ideal conditions, this approach will reproduce the original luminance range of the HDR input, but faithful color reproduction is not considered. To take a more principled approach to the HDR image splitting problem, we have developed a new algorithm based on the iCAM06 image appearance model [13]. The algorithm first uses iCAM06 to create a SDR color image that is sent to one plane, then calculates a luminance residual that is sent to the other plane to reproduce the HDR luminance range [14]. The goal of the algorithm is to create displayed HDR images that better reproduce the visual appearances of HDR scenes. In the following sections, we first describe prior work on HDR displays and the luminance square root HDR image splitting algorithm, we then describe our new appearance-based HDR image splitting algorithm, and report on a series of experiments designed to evaluate and compare the appearance of HDR images displayed using the two algorithms.