Joyce E. Farrell

Resolution and light sensitivity tradeoff with pixel size

When the size of a CMOS imaging sensor array is fixed, the only way to increase sampling density and spatial resolution is to reduce pixel size. But reducing pixel size reduces the light sensitivity. Hence, under these constraints, there is a tradeoff between spatial resolution and light sensitivity. Because this tradeoff involves the interaction of many different system components, we used a full system simulation to characterize performance. This paper describes system simulations that predict the output of imaging sensors with the same dye size but different pixel sizes and presents metrics that quantify the spatial resolution and light sensitivity for these different imaging sensors.

Color image quality metric S-CIELAB and its application on halftone texture visibility

We describe experimental tests of a spatial extension to the CIELAB color metric for measuring color reproduction errors of digital images. The standard CIELAB /spl Delta/E metric is suitable for use on large uniform color targets, but not on images, because color sensitivity changes as a function of spatial pattern. The S-CIELAB extension includes a spatial processing step, prior to the CIELAB /spl Delta/E calculation, so that the results correspond better to color difference perception by the human eye. The S-CIELAB metric was used to predict texture visibility of printed halftone patterns. The results correlate with perceptual data better than standard CIELAB and point the way to various improvements.

Short- and long-range processes in visual apparent movement

SummaryThe minimum stimulus-onset asynchrony required for perception of beta apparent movement was measured with point stimuli separated by visual angles ranging from about 0.1· to 5· and viewing distance as a parameter. For each viewing distance, the threshold for beta movement was a monotonic function of the visual angle with a strong linear increase over angles less than 0.25· (short-range function) and a much weaker linear increase over angles greater than 1.5· (long-range). The short- and long-range functions were differentially affected by increase in viewing distance: the long-range function increased in slope, but the extrapolated zero-intercept was constant; the short-range function changed in intercept, but not in slope. The results provide strong evidence for separate short- and long-range processes in visual motion perception.

Image capture: simulation of sensor responses from hyperspectral images

This paper describes the design and performance of an image capture simulator. The general model underlying the simulator assumes that the image capture device contains multiple classes of sensors with different spectral sensitivities and that each sensor responds in a known way to irradiance over most of its operating range. The input to the simulator is a set of narrow-band images of the scene taken with a custom-designed hyperspectral camera system. The parameters for the simulator are the number of sensor classes, the sensor spectral sensitivities, the noise statistics and number of quantization levels for each sensor class, the spatial arrangement of the sensors and the exposure duration. The output of the simulator is the raw image data that would have been acquired by the simulated image capture device. To test the simulator, we acquired images of the same scene both with the hyperspectral camera and with a calibrated Kodak DCS-200 digital color camera. We used the simulator to predict the DCS-200 output from the hyperspectral data. The agreement between simulated and acquired images validated the image capture response model and our simulator implementation. We believe the simulator will provide a useful tool for understanding the effect of varying the design parameters of an image capture device.

The relationship between image fidelity and image quality

Image fidelity (inferred by the ability to discriminate between two images) and image quality (inferred by the preference for one image over another) are often assumed to be directly related. We investigated the relationship between the perceived image fidelity and image quality of halftone textures. Subjects were asked to rank order a set of printed halftone swatches on the basis of smoothness. They were then asked to reduce the contrast of each pattern until it was at threshold, thus providing an estimate of the patterns perceptual strength and its discriminability from a non-textured swatch. We found only a moderate correlation between image fidelity and image quality.

Efficient method for paired comparison

This paper describes a more efficient paired comparison method that reduces the number of trials necessary for converting a table of paired comparisons into scaler data. Instead of comparing every pair of samples (the complete method), a partial method is used that makes more comparisons between closer samples than between more distant samples. A sorting algorithm is used to effi- ciently order the samples with paired comparisons, and each com- parison is recorded. When the sorting is completed, more trials will have been conducted between closer samples than between distant samples. Regression is used to scale the resulting comparison ma- trix into a one dimensional perceptual quality estimate.

Preferred color spaces for white balancing

When rendering photographs, it is important to preserve the gray tones despite variations in the ambient illumination. When the illuminant is known, white balancing that preserves gray tones can be performed in many different color spaces; the choice of color space influences the renderings of other colors. In this behavioral study, we ask whether users have a preference for the color space where white balancing is performed. Subjects compared images using a white balancing transformation that preserved gray tones, but the transformation was applied in one of the four different color spaces: XYZ, Bradford, a camera sensor RGB and the sharpened RGB color space. We used six scenes types (four portraits, fruit, and toys) acquired under three calibrated illumination environments (fluorescent, tungsten, and flash). For all subjects, transformations applied in XYZ and sharpened RGB were preferred to those applied in Bradford and device color space.

Digital camera simulation

We describe a simulation of the complete image processing pipeline of a digital camera, beginning with a radiometric description of the scene captured by the camera and ending with a radiometric description of the image rendered on a display. We show that there is a good correspondence between measured and simulated sensor performance. Through the use of simulation, we can quantify the effects of individual digital camera components on system performance and image quality. This computational approach can be helpful for both camera design and image quality assessment.

Visible persistence of moving objects.

A single line was presented in a succession of orientations, each orientation separated by a fixed angle and by a fixed interval of time, and subjects reported the number of successive lines that appeared to rotate together. The perceived number of rotating lines increased linearly with the rate of stimulus presentation, with a slope that was proportional to the spatial separation. The linear functions obtained in this first experiment predicted the results of a second experiment in which subjects adjusted the spatial and temporal variables to a discrimination threshold for seeing two rotating lines. If the slope of the linear functions is considered to be an estimate of the duration of visible persistence, then these results suggest that the visible persistence of a briefly presented stimulus increases with the distance separating that stimulus from other stimuli.

Visual Apparent Movement: Transformations of Size and Orientation

Sequential alternation between same-shaped stimuli differing in size (size ratio s) and orientation (angular difference v) produced a visual illusion of translation in depth and concurrent rotation. The minimum stimulus-onset asynchrony required for the appearance of a rigidly moving object was approximately a linearly increasing function of (s− 1)/(s + 1) for simple translation in depth and a linearly increasing function of v for simple rotation. The extrapolated zero intercept was lower for translation than for rotation, but estimated transformation times were additive in combined transformations. The results suggest that (a) the processes of apparent translation in depth and apparent rotation are individually sequential-additive in structure, and (b) apparent translations and rotations are combined by fine-grained alternation of steps of apparent translation and steps of apparent rotation. Similar principles account for recent data on imagined spatial transformations of visual size and orientation.