Ahmet Oguz Akyuz

Ghost Removal in High Dynamic Range Images

High dynamic range images may be created by capturing multiple images of a scene with varying exposures. Images created in this manner are prone to ghosting artifacts, which appear if there is movement in the scene at the time of capture. This paper describes a novel approach to removing ghosting artifacts from high dynamic range images, without the need for explicit object detection and motion estimation. Weights are computed iteratively and then applied to pixels to determine their contribution to the final image. We use a non-parametric model for the static part of the scene, and a pixels membership in this model determines its weight. In contrast to previous approaches, our technique does not rely on explicit object detection, tracking, or pixelwise motion estimates. Ghost-free images of different scenes demonstrate the effectiveness of our technique.

Do HDR displays support LDR content?: a psychophysical evaluation

The development of high dynamic range (HDR) imagery has brought us to the verge of arguably the largest change in image display technologies since the transition from black-and-white to color television. Novel capture and display hardware will soon enable consumers to enjoy the HDR experience in their own homes. The question remains, however, of what to do with existing images and movies, which are intrinsically low dynamic range (LDR). Can this enormous volume of legacy content also be displayed effectively on HDR displays? We have carried out a series of rigorous psychophysical investigations to determine how LDR images are best displayed on a state-of-the-art HDR monitor, and to identify which stages of the HDR imaging pipeline are perceptually most critical. Our main findings are: (1) As expected, HDR displays outperform LDR ones. (2) Surprisingly, HDR images that are tone-mapped for display on standard monitors are often no better than the best single LDR exposure from a bracketed sequence. (3) Most importantly of all, LDR data does not necessarily require sophisticated treatment to produce a compelling HDR experience. Simply boosting the range of an LDR image linearly to fit the HDR display can equal or even surpass the appearance of a true HDR image. Thus the potentially tricky process of inverse tone mapping can be largely circumvented.

Color appearance in high-dynamic-range imaging

When viewing images on a monitor, we are adapted to the lighting conditions of our viewing environment as well as the monitor itself, which can be very different from the lighting conditions in which the images were taken. As a result, our perception of these photographs depends directly on the environment in which they are displayed. For high-dynamic-range images, the disconnect in the perception of scene and viewing environments is potentially much larger than in conventional film and photography. To prepare an im- age for display, luminance compression alone is therefore not suffi- cient. We propose to augment current tone reproduction operators with the application of color appearance models as an independent preprocessing step to preserve chromatic appearance across scene and display environments. The method is independent of any spe- cific tone reproduction operator and color appearance model (CAM) so that for each application the most suitable tone reproduction op- erator and CAM can be selected.

Noise reduction in high dynamic range imaging

A common method to create high dynamic range (HDR) images is to combine several different exposures of the same scene. In this approach, the use of higher ISO settings will reduce exposure times, and thereby the total capture time. This is advantageous in certain environments where it may help minimize ghosting artifacts. However, exposures taken at high sensitivity settings tend to be noisy, which is further amplified by the HDR creation algorithm. We present a robust and efficient technique to significantly reduce noise in an HDR image even when its constituent exposures are taken at very high ISO settings. The method does not introduce blur or other artifacts, and leverages the wealth of information available in a sequence of aligned exposures.

Perceptual evaluation of tone-reproduction operators using the Cornsweet--Craik--O'Brien illusion

High dynamic-range images cannot be directly displayed on conventional display devices, but have to be tone-mapped first. For this purpose, a large set of tone-reproduction operators is currently available. However, it is unclear which operator is most suitable for any given task. In addition, different tasks may place different requirements upon each operator. In this paper we evaluate several tone-reproduction operators using a paradigm that does not require the construction of a real high dynamic-range scene, nor does it require the availability of a high dynamic-range display device. The user study involves a task that relates to the evaluation of contrast, which is an important attribute that needs to be preserved under tone reproduction.

Selective local tone mapping

When preparing high dynamic range images (HDR) for display on standard monitors, it is often necessary to make a choice between global and local tone mapping. While the former is simple and efficient, it may fail to reproduce details in high contrast image regions. Although, the latter can better reproduce details in such regions, it often comes at the cost of increased complexity and computational time. In this paper, we present an algorithm that combines the best of both approaches. We perform local tone mapping only in high frequency image regions where the visibility of details can be an issue. In low frequency regions, we employ global tone mapping to save computational resources without degrading quality. Our algorithm is most suitable for tone mapping operators (TMOs) that utilize the concept of local adaptation luminances.

Color appearance models and dynamic range reduction

When viewing photographs on a monitor, we are adapted to the lighting conditions of our viewing environment, which can be very different from the lighting conditions in which the photograph was taken. As a result, our perception of these photographs depends directly on the environment in which they are displayed. For high dynamic range images, the disconnect in perception of scene and viewing environments is potentially much larger than in conventional film and photography. To prepare an image for display, luminance compression alone is therefore not sufficient. We propose to augment current tone reproduction operators with application of color appearance models as an independent preprocessing step in order to preserve chromatic appearance across scene and display environments. The method is independent of any specific tone reproduction operator and color appearance model (CAM) so that for each application the most suitable tone reproduction operator and CAM can be selected.