Xiao-fan Feng

Decontouring: prevention and removal of false contour artifacts

Contone imagery usually has eight bits per pixel for each of the three primaries in typical displays. However, there are often points in the imaging pipeline that constrain this number for cost reasons. Conversely, higher quality displays seek to achieve 9-10 bits/pixel/color, though there may be system bottlenecks limited at 8. In both cases, a goal is to achieve a higher perceived bit-depth quality than is afforded by the imaging system. The two main artifacts caused by reduced bitdepth are contouring and loss of low amplitude detail. Prevention of these distortions can be accomplished by applying a dithering process before the bit-depth limitation. A technique for achieving bit-depth extension via spatiotemporal dithering has been previously been presented [1]. In applications where it is only possible to affect the image after the bit-depth losses have already occurred, it is impossible to accurately restore the loss of low-amplitude detail. However, it is possible to remove the false contours. Of the several approaches used to remove false contours, we will discuss predictive cancellation and its dependence on the spatial frequency localization and masking properties of the visual system. We discuss the key visual properties that arose while investigating these two applications, which include the optical transfer function (OTF) of the eye, masking by noise, and contour integration.

LCD motion blur modeling and analysis

Compared to cathode-ray tube (CRT) devices, the current liquid crystal display (LCD) devices have advantage on many aspects but disadvantage on displaying motion contents. LCD motion blur is caused jointly by the slow LCDs temporal response and the hold-type LCDs rendering method together with the motion pursuing function of human visual system (HVS). Although LCD motion blur has been addressed extensively in the literature, there is no systematic analysis. In this paper, a general LCD motion blur model, based on the sampling and reconstruction theory, is developed. This model, covering LCD and HVS, provides a systematic tool for quantitatively analyzing the LCD motion blur problem associated with any current and emerging LCD rendering technologies.

51.4: Quantitative Analysis of LCD Motion Blur and Performance of Existing Approaches

LCD Motion blur is a well known problem. Although many solutions have been proposed, some fundamental questions have not been answered yet. In this paper, we try to answer such questions. Specifically, we calculate the waveform and its blur width of a moving edge perceived on LCD screen for current LCD and the proposed four solutions of hold-type motion blur. We found that the slow response of current LCD is not a dominant factor of motion blur. The slow response of current LCD only contributes to 30% of the motion blur, while the hold-type rendering mode of LCD contributes to 70%. Therefore, fast LCD such as OCB itself does not significantly reduce motion blur. Fast LCD, on the other hand, is critical to the proposed three solutions of hold-type blur to avoid the ghosting artifact. With fast LCD, black data insertion and frame rate doubling can provide 50% reduction of motion blur. With both fast response LCD and fast backlight, backlight flashing can provide much higher reduction of motion blur.

Comparisons of motion-blur assessment strategies for newly emergent LCD and backlight driving technologies

— Compared to the conventional cathode-ray-tube TV, the conventional liquid-crystal TV has the shortcoming of motion blur. Motion blur can be characterized by the motion-picture response-time metric (MPRT). The MPRT of a display can be measured directly using a commercial MPRT instrument, but it is expensive in comparison with a photodiode that is used in temporal-response (temporal luminance transition) measurements. An alternative approach is to determine the motion blur indirectly via the temporal point-spread function (PSF), which does not need an accurate tracking mechanism as required for the direct “spatial” measurement techniques. In this paper, the measured motion blur is compared by using both the spatial-tracking-camera approach and the temporal-response approach at various backlight flashing widths. In comparison to other motion-blur studies, this work has two unique advantages: (1) both spatial and temporal information was measured simultaneously and (2) several temporal apertures of the display were used to represent different temporal PSFs. This study shows that the temporal method is an attractive alternative for the MPRT instrument to characterize the LCDs temporal performance.

Bit-depth extension using spatiotemporal microdither based on models of the equivalent input noise of the visual system

Continuous tone, or “contone”, imagery usually has 24 bits/pixel as a minimum, with eight bits each for the three primaries in typical displays. However, lower-cost displays constrain this number because of various system limitations. Conversely, high quality displays seek to achieve 9-10 bits/pixel/color, though there may be system bottlenecks limited at 8. The two main artifacts from reduced bit-depth are contouring and loss of amplitude detail; these can be prevented by dithering the image prior to these bit-depth losses. Early work in this area includes Roberts’ noise modulation technique, Mista’s blue noise mask, Tyler’s technique of bit-stealing, and Mulligan’s use of the visual system’s spatiotemporal properties for spatiotemporal dithering. However, most halftoning/dithering work was primarily directed to displays at the lower end of bits/pixel (e.g., 1 bit as in halftoning) and higher ppi. Like Tyler, we approach the problem from the higher end of bits/pixel/color, say 6-8, and use available high frequency color content to generate even higher luminance amplitude resolution. Bit-depth extension with a high starting bit-depth (and often lower spatial resolution) changes the game substantially from halftoning experience. For example, complex algorithms like error diffusion and annealing are not needed, just the simple addition of noise. Instead of a spatial dither, it is better to use an amplitude dither, termed microdither by Pappas. We have looked at methods of generating the highest invisible opponent color spatiotemporal noise and other patterns, and have used Ahumada’s concept of equivalent input noise to guide our work. This paper will report on techniques and observations made in achieving contone quality on ~100 ppi 6 bits/pixel/color LCD displays with no visible dither patterns, noise, contours, or loss of amplitude detail at viewing distances as close as the near focus limit (~120 mm). These include the interaction of display nonlinearities and their role of generating a low-spatial frequency flicker from mutually high-pass spatial and temporal noise, as well as the temporal response symmetries.

Two approaches to derive LED driving signals for high‐dynamic‐range LCD backlights

Abstract— The LED-array backlight technique dramatically enhances the dynamic range of an LCD and hence extends its ability to present images with high reality. This is achieved by modulating LEDs individually, thus providing an area-adaptive backlight for the display. The spatial overlap of light from the LED (crosstalk) occurs due to the diffusion screen placed between the backlight and LCD layer. However, the crosstalk is not only a blessing for supplying high brightness but is also a curse for causing potential artifacts, making the derivation of an LED driving signal a challenging task. This paper formulates the problem into two mathematical models: an iterative de-convolution approach and a linear optimization approach. Algorithms for solving these two models are provided. The first approach provides instantaneous and satisfactory results except for high-intensity highlights in the image. The linear optimization method conquers this drawback, but requires much more computation, possibly requiring preprocessing of the target, and also introduces undesired artifacts. These two approaches are extensively evaluated by building an image database composed of 161 high-dynamic-range images.

66.1: Distinguished Student Paper: Deriving LED Driving Signal for Area-Adaptive LED Backlight in High Dynamic Range LCD Displays

LED backlight technique dramatically enhances the dynamic range of an LCD display and hence extends its ability to present images with high reality. This is achieved by modulating LEDs individually, thus providing an area-adaptive backlight for the display. However, crosstalk of LEDs is not only a blessing for supplying high brightness but is also a curse for causing potential artifacts, making the derivation of LED driving signal a challenging task. This paper formulates the problem in two mathematical models: an iterative de-convolution approach and a linear optimization approach. Algorithms for solving these two models are provided. An image database composed of 161 high dynamic range images is built to evaluate the performances of these two approaches.

54.2: LCD Motion Blur Analysis and Modeling Based on Temporal PSF

In this paper, an LCD motion blur model using CRT as a reference is discussed. This model, covering both LCD and HVS characteristics, shows that the temporal point spread function (temporal PSF) of an LCD device determines its motion blur, and the normalized blurred edge width (N-BREW) in MPRT describes the effective width of temporal PSFs, while response time (RT) describes the rising edge of the temporal PSF of fast hold-type LCDs. We also theoretically derive the temporal PSFs of many old and new LCD temporal rendering technologies in this paper. Finally, this model is verified by a subjective experiment.

Dynamic gamma: Application to LCD motion-blur reduction

— A metric, “dynamic gamma,” to quantitatively evaluate the dynamic temporal response of an LCD device is proposed. Dynamic gamma, associated with 2-D plots, is more suitable for quantitatively characterizing the dynamic characteristics of an LC panel. The dynamic gamma metric was applied to improve the temporal response of LCDs. From dynamic gamma data, overdrive tables can be derived. Dynamic gamma can also be used to evaluate the effectiveness of overdrive. With a second-order dynamic gamma, the performance of different overdrive algorithms can be quantitatively assessed. The dynamic gamma metric was also applied to backlight flashing and developed a time adaptive overdrive algorithm. The new algorithm reduces the ghosting artifact due to the timing mismatch between LCD driving and backlight flashing. Experimental results from a simulated tracking camera confirms the advantages of the new algorithm designed using dynamic gamma.

P-27: Adaptive Display Color Correction Based on Real-time Viewing Angle Estimation

We propose a method for adaptive LCD color correction through manipulating the pixel values to be displayed based on automatically detected viewing angle. The viewing angle detection is achieved through gaze/eye/face detection and tracking in the image/video captured by a camera that is fixed on the display. The proposed approach does not require physical modifications to the LCD display.