Yousun Bang

Gradient approach to quantify the gradation smoothness for output media

We aim to quantify the perception of color gradation smoothness using objectively measurable properties. We propose a model to compute the smoothness of hardcopy color-to-color gradations. It is a gradient-based method that can be determined as a function of the 95th percentile of second derivative for the tone-jump estimator and the fifth percentile of first derivative for the tone-clipping estimator. Performance of the model and a previously suggested method were psychophysically appreciated, and their prediction accuracies were compared to each other. Our model showed a stronger Pearson correlation to the corresponding visual data, and the magnitude of the Pearson correlation reached up to 0.87. Its statistical significance was verified through analysis of variance. Color variations of the representative memory colors-blue sky, green grass and Caucasian skin-were rendered as gradational scales and utilized as the test stimuli.

Printer Resolution Measurement Based on Slanted Edge Method

Printer resolution is an important attribute for determining print quality, and it has been frequently referred to hardware optical resolution. However, the spatial addressability of hardcopy is not directly related to optical resolution because it is affected by printing mechanism, media, or software data processing such as resolution enhancement techniques (RET). The international organization ISO/IEC SC28 addresses this issue, and makes efforts to develop a new metric to measure this effective resolution. As the development process, this paper proposes a candidate metric for measuring printer resolution. Slanted edge method has been used to evaluate image sharpness for scanners and digital still cameras. In this paper, it is applied to monochrome laser printers. A test chart is modified to reduce the effect of halftone patterns. Using a flatbed scanner, the spatial frequency response (SFR) is measured and modeled with a spline function. The frequency corresponding to 0.1 SFR is used in the metric for printer resolution. The stability of the metric is investigated in five separate experiments: (1) page to page variations, (2) different ROI locations, (3) different ROI sizes, (4) variations of toner density, and (5) correlation with visual quality. The 0.1 SFR frequencies of ten printers are analyzed. Experimental results show the strong correlation between a proposed metric and perceptual quality.

Quantification of perceived macro-uniformity

Macro-uniformity refers to the subjective impression of overall uniformity in the print sample. By the efforts of INCITS W1.1 team, macro-uniformity is categorized into five types of attributes: banding, streaks, mottle, gradients, and moiré patterns, and the ruler samples are generated with perceptual scales. W1.1 macro-uniformity ruler is useful for judging the levels of print defect, but it is not an easy task to reproduce the samples having the same perceptual scales at different times in different places. An objective quantification method is more helpful and convenient for developers to analyze print quality and design printing system components. In this paper, we propose a method for measuring perceived macro-uniformity for a given print using a flat-bed scanner. First, banding, 2D noise, and gradients are separately measured, and they are converted to the perceptual scales based on subjective results of each attribute. The correlation coefficients between the measured values of the attributes and the perceptual scales are 0.92, 0.97, and 0.86, respectively. Another subjective test is performed to find the relationship between the overall macro-uniformity and the three attributes. The weighting factors are obtained by the experimental result, and the final macro-uniformity grade is determined by the weighted sums of each attribute.

New measurement method of banding using spatial features for laser printers

The techniques of one-dimensional projection in the spatial domain and contrast sensitivity function (CSF) are generally used to measure banding. Due to the complex printing process of laser printers, hardcopy prints contain other 2D nonuniformities such as graininess and mottle besides banding. The method of 1D projection is useful for extracting banding, but it induces the confounding effect of graininess or mottle on the measurement of perceived banding. The appearance of banding in laser printers is more similar to the sum of various rectangular signals having different amplitudes and frequencies. However, in many cases banding is modeled as a simple sinusoidal signal and the CSF is frequently applied. In this paper, we propose new measurement method of banding well correlated with human perception. Two kinds of spatial features give a good performance to banding measurement. First the correlation factor between two adjacent 1D signals is considered to obtain banding power which reduces the confounding effect of graininess and mottle. Secondly, a spatial smoothing filter is designed and applied to reduce the less perceptible low frequency components instead of using the CSF. By using moving window and subtracting the local mean values, the imperceptible low frequency components are removed while the perceptible low frequency components like the sharp edge of rectangular waves are preserved. To validate the proposed method, psychophysical tests are performed. The results show that the correlations between the proposed method and the perceived scales are 0.96, 0.90, and 0.95 for black, cyan, and magenta, respectively.

Device calibration method for optical light modulator

Due to subtle misalignment of optical components in the fabrication process, images projected by an optical light modulator have severe line artifact along the direction of the optical scan. In this paper, we propose a novel methodology to calibrate the modulator and generate the compensate image for the misaligned optical modulator in order to eliminate the line artifact. A camera system is employed to construct Luminance Transfer Function (LTF) that characterizes the optical modulator array. Spatial uniformity is obtained by redefining the dynamic range and compensating the characteristic curvature of the LTF for each optical modulator array element. Simulation results show significant reduction in the visibility of line artifact.

Hardcopy global color correction

When time, temperature or an external environment change, a laser electrophotographic printer produces quite different color tones from original ones. To achieve consistent color reproduction, many researchers have tried to characterize printer tone curves and developed methods to correct color tones. Color channel independent methods are most widely used, and there are two approaches in color channel independent method: (1) Instrument-based correction and (2) visual correction. Two approaches provide some trade-offs between cost and accuracy. In this paper we propose a methodology which combines the strengths of these two approaches. We describe how we design a calibration page and how we characterize lightness variation of a reference patch. We then present the procedure of our global tone correction method based on visual appearance match of end-users as well as the predetermined reference lightness model. We simulate tone distortion state by varying hardware parameters, and perform visual appearance match experiments to subjects. Our experimental results show that our method can significantly reduce color difference between the original print and the print at the distortion state. This suggests that we can reliably estimate the distortion parameter, and correct tones close to an original state.