17.1: Comparison of Motion-Blur Measurement Methods


 Motion blur is a significant display property for which accurate, valid, and robust measurement methods are needed. Recent motion blur measurements of a set of eight displays by a set of six measurement devices provided an opportunity to evaluate techniques of measurement and analysis. We found significant discrepancies between instruments, and variability within instruments. 1 Objective and Background Many modern display technologies, notably LCD, are subject to motion blur. Motion blur arises when the eye tracks a moving image, while the display presents individual frames that persist for significant fractions of a frame duration, or longer. As a result, the image is smeared across the retina during the frame duration. There have been a number of attempts to characterize motion blur in a systematic and meaningful way. Most of these involve estimating the width of an edge subjected to motion blur. This edge can be captured in any of three ways[1, 2]. The first method employs a pursuit camera that tracks a vertical edge (between two graylevels) as it moves horizontally across the screen. The camera is simulating the eye as it pursues the moving edge. The result, after averaging over time, is a picture of the blurred edge. After averaging over the vertical dimension (orthogonal to the motion), a one-dimensional waveform representing the cross-section of the blurred edge can be obtained. It describes relative luminance (a number proportional to luminance) as a function of horizontal position in pixels. We will call this the Moving Edge Spatial Profile (MESP). When recorded at several speeds of edge motion, the waveforms are usually found to correspond when the horizontal scale is divided by the speed. Therefore it is conventional to rescale the horizontal axis of the profile (pixels) by dividing by the speed (pixels/frame) to obtain a waveform that is a function of time (frames). We call this the Moving Edge Temporal Profile (METP). It is also conventional to characterize the width of the METP in terms of the time interval between 10% and 90% points of the curve. This quantity is called the Blur Edge Time (BET) and is reported in msec. The second method employs a stationary high-speed camera. With a sufficiently high frame rate, it is possible to capture a sequence of frames, that, with appropriate shifting and adding, can also yield a record of the MESP and thereby the METP. The highspeed camera avoids the mechanical challenges of the pursuit camera. The third method employs a fixed non-imaging detector such as a photodiode which measures the luminance over time as the display is switched from one graylevel to another. This temporal step response is then convolved with a pulse of duration equal to the hold time (for an LCD, typically one frame), to obtain another version of the METP [3]. This last method relies on an assumption that all pixels are independent. It has been demonstrated to be accurate in many cases [1, 2], but may fail when motion-dependent processing is present. Outstanding questions remain regarding the accuracy and agreement among these various methods, and also regarding the analysis of the METP. In February 2008 a unique opportunity was provided to address some of these questions. Under the auspices of the International Committee on Display Measurement [4], an experiment was conducted in which eight flat panel displays were measured by six different motion blur measuring instruments. The testing facility was provided by Samsung, Inc, in Seoul, Korea. The objective of this report is to discuss preliminary results from that experiment. The primary focus will be on the degree of agreement among methods, and on the analysis of the METP.