The Reproduction of Specular Highlights on High Dynamic Range Displays

Recent advances in the design of high dynamic range (HDR) monitors enable the display of images having a large dynamic range, close to that encountered in the real world. As their usage will increase, we will be confronted with the problem of rerendering images that have been mapped to standard dynamic range (SDR) displays so that they look natural on HDR monitors. We address this issue for SDR images representing original HDR scenes. We propose a tone scale function that takes advantage of the increase in dynamic range of HDR monitors to recreate the brightness of specular highlights, which were clipped or compressed by the capturing and rendering process to SDR. We validate the use of such functions with a psychovisual experiment conducted on an HDR display, where the observers’ task was to judge pairs of tone-scaled images. The result of the experiment shows that using part of the extension of dynamic range provided by HDR displays to enhance the brightness of specular highlights leads to more natural looking images. Introduction HDR monitors capable of displaying simultaneously bright highlights and dark shadows have just started to come on the market. The development of these monitors raise new questions about how to re-render the large amount of legacy images that are already mapped to SDR displays. Specular highlights are often badly reproduced in images rendered to SDR displays. This is due to a strong luminance compression and/or clipping taking place during the image capturing and rendering process. As they offer important visual cues about three dimensional shapes and increase the sense of realism [1, 4], it would be beneficial to use part of the extended dynamic range of HDR displays to enhance their representation. The presence of specular highlights in an image suggests that the original scene had a high dynamic range, as specular highlights can be several orders of magnitude brighter than diffuse highlights [7]. We suggest the use of a tone scale function that expands the luminance range allocated to the specular parts of an image with the goal of recovering the natural look of the original HDR scene. In a psychovisual experiment, we test different tone scale functions by varying the display luminance range allocated to specular highlights. We prove that allocating some of the additional display range provided by an HDR monitor to specular highlights leads to a more natural displayed image than using a simple linear scaling of code values. In addition, the proposed tone scale prevents the re-rendered image to look too bright, which is likely to happen when applying just a linear scaling (illustrated in Figure 1). This article is structured as follows: First, we describe the Figure 1. Problem of re-rendering SDR images to HDR displays. If a simple linear scaling is applied, the image can appear too bright. construction of the tone scale function. Then, we explain the generation of the stimuli used in the psychovisual experiment. The experiment procedure is presented followed by a statistical analysis of the collected data. A discussion of the results concludes the article. The Tone Scale Function The tone scale function is applied to the luminance channel of a linearly-encoded input image. It is a piece-wise linear function composed of two slopes (Figure 2). Here, we only describe aspects of the tone scale function that are necessary to understand the psychovisual experiment. Implementation details are published elsewhere [5]. The shape of the tone scale is entirely defined by ω , the normalized code value of the maximum diffuse white in the image, and ρ , the percentage of the maximum display luminance allocated to ω .