Timo Kunkel

Calibrated image appearance reproduction

Managing the appearance of images across different display environments is a difficult problem, exacerbated by the proliferation of high dynamic range imaging technologies. Tone reproduction is often limited to luminance adjustment and is rarely calibrated against psychophysical data, while color appearance modeling addresses color reproduction in a calibrated manner, albeit over a limited luminance range. Only a few image appearance models bridge the gap, borrowing ideas from both areas. Our take on scene reproduction reduces computational complexity with respect to the state-of-the-art, and adds a spatially varying model of lightness perception. The predictive capabilities of the model are validated against all psychophysical data known to us, and visual comparisons show accurate and robust reproduction for challenging high dynamic range scenes.

Preference limits of the visual dynamic range for ultra high quality and aesthetic conveyance

A subjective study was conducted to investigate the preferred maximum and minimum display luminances in order to determine the dynamic ranges for future displays. Two studies address the diffuse reflective regions, and a third study tested preferences of highlight regions. Preferences, as opposed to detection thresholds, were studied to provide results more directly relevant to the viewing of entertainment or art. Test images were specifically designed to test these limits without the perceptual conflicts that usually occur in these types of studies. For the diffuse range, we found a display with a dynamic range having luminances between 0.1 and 650 cd/m2 matches the average preferences. However, to satisfy 90% of the population, a dynamic range from 0.005 and ~3,000 cd/m2 is needed. Since a display should be able to produce values brighter than the diffuse white maximum, as in specular highlights and emissive sources, the highlight study concludes that even the average preferred maximum luminance for highlight reproduction is ~4,000 cd/m2.

HDR and wide gamut appearance-based color encoding and its quantification

Color coding efficacy is critical to the success of any image compression method. The initial quantization step that happens during encoding limits the accuracy and directly affects compression performance. If too many code words are spent in areas of the color space that make little difference to perception, compression is taxed. If other areas are under-sampled, contour (banding) and color shifting artifacts can result. Legacy color encodings such as 24-bit sRGB do not easily extend to wide color gamuts and high dynamic range due to perceptual non-uniformity issues. In this paper, we compare a number of different color encodings and bit depths for suitability in HDR and wide gamut applications using a new metric for evaluating encoding efficacy, Number of Distinguishable Colors.

Influence of Screen Size and Field of View on Perceived Brightness

We present a study into the perception of display brightness as related to the physical size and distance of the screen from the observer. Brightness perception is a complex topic, which is influenced by a number of lower- and higher-order factors—with empirical evidence from the cinema industry suggesting that display size may play a significant role. To test this hypothesis, we conducted a series of user studies exploring brightness perception for a range of displays and distances from the observer that span representative use scenarios. Our results suggest that retinal size is not sufficient to explain the range of discovered brightness variations, but is sufficient in combination with physical distance from the observer. The resulting model can be used as a step toward perceptually correcting image brightness perception based on target display parameters. This can be leveraged for energy management and the preservation of artistic intent. A pilot study suggests that adaptation luminance is an additional factor for the magnitude of the effect.

Perceptual Design for High Dynamic Range Systems

Establishing a design process for a high dynamic range and wide color gamut signal representation that is based on perceptual limits, as opposed to image capture or display hardware restrictions, is beneficial from a fidelity, efficiency, and economic point of view. Such a signal representation will enable displays and cameras, as well as systems for image synthesis, manipulation, and transport, to improve without having their development hindered by limits imposed by the signal representation, as occurs today. This chapter describes the perceptual behavior of the human visual system and how this knowledge can be efficiently aligned with engineering concepts in order to design a perceptually accurate imaging pipeline. Key concepts that are discussed cover spatial and temporal perception, and how it affects quantization of dynamic range and, when combined with chromatic components, the extent and behavior of a system’s color volume.

A high bit depth digital imaging pipeline for vision research

In order to achieve accurate results in user studies in the fields of Psychophysics, Experimental Psychology, Ophthalmology and clinical studies there are high demands towards an imaging pipeline presenting these stimuli in an experiment (as illustrated in Figure 1). For example, display stability and repeatability, both short term and long term are crucial when conducting research leading to robust results. Further important factors are the perceptual limits of a graphics pipeline. Here, two important elements are the achievable dynamic range and the color gamut, which would ideally approximate or exceed the capabilities of the human visual system (HVS). In an optimal solution, those stimulus dimensions would be displayed with continuous intensity levels between their respective extrema (e.g. from dark to light) when presenting them to participants.