Haisong Xu

Reconstructing spectral reflectance by dividing spectral space and extending the principal components in principal component analysis

Principal component analysis (PCA) is widely used to reconstruct the spectral reflectance of surface colors. However, the estimated spectral accuracy is low when using only one set of three principal components for three-channel color-acquisition devices. In this study, the spectral space was first divided into 11 subgroups, and the principal components were calculated for individual subgroups. Then the principal components were further extended from three to nine through the residual spectral error of the reflectance in each subgroup. For each target sample, the extended principal components of the corresponding subgroup samples were used in the common PCA method to reconstruct the spectral reflectance. The results show that this proposed method is quite accurate and outperforms other related methods.

Assessing total differences for effective samples having variations in color, coarseness, and glint

Effect coatings have the unique property of large change of appearance under different viewing conditions. This results in quality control problems of related products. In this letter, samples of metallic panels with effect coatings are visually assessed and measured. Based on experimental results, we propose formulae to predict precisely the total differences of effective samples in terms of variations in color, coarseness, and glint. Under diffused illumination, the total difference formula includes color difference and coarseness difference. Under directional illumination, the total difference formula includes color difference and glint difference.

Image quality evaluation for smart-phone displays at lighting levels of indoor and outdoor conditions

The image quality of two active matrix organic light emitting diode (AMOLED) smart-phone displays and two in-plane switching (IPS) ones was visually assessed at two levels of ambient lighting condi- tions corresponding to indoor and outdoor applications, respectively. Naturalness, colorfulness, brightness, contrast, sharpness, and overall image quality were evaluated via psychophysical experiment by catego- rical judgment method using test images selected from different applica- tion categories. The experimental results show that the AMOLED displays perform better on colorfulness because of their wide color gamut, while the high pixel resolution and high peak luminance of the IPS panels help the perception of brightness, contrast, and sharpness. Further statis- tical analysis of ANOVA indicates that ambient lighting levels have signif- icant influences on the attributes of brightness and contrast.

Testing CIELAB-based color-difference formulae using large color differences

Three advanced CIELAB-based color-difference formulae, CMC, CIE94, and CIEDE2000, together with the basic CIELAB equation, were tested using large color-difference visual data (maximum average size was 12 CIELAB ΔE units) produced in this study. The color-difference comparison experiment was carried out at CIE Gray and Blue centers by a panel of 6 normal color-vision observers using CRT-generated stimuli based on the psychophysical method of constant stimuli. The experimental data, processed via probit analysis, were well fitted to chromaticity ellipses with a high reliance according to the observer accuracy in terms ofPF/3 measure. A detailed comparison was performed to analyze the agreement between predicted color differences from all formulae and their corresponding visual scales in all measurement planes of CIELAB space. The results show that the CIEDE2000 marginally outperformed the others at all color centers while CIE94 was the worst in original formulae or with optimizedkL value, but the CIELAB performed worst when the parametric factors ofkL,kc, andkH were all optimized, with the CMC always lying between these extremes.

Camera-based model to predict the total difference between effect coatings under directional illumination

A camera-based model is established to predict the total difference for samples of metallic panels with effect coatings under directional illumination, and the testing results indicate that the model can precisely predict the total difference between samples with metallic coatings with satisfactory consistency to the visual data. Due to the limited amount of testing samples, the model performance should be further developed by increasing the training and testing samples.

Investigation of perceptual attributes for mobile display image quality

Large-scale psychophysical experiments are carried out on two types of mobile displays to evaluate the perceived image quality (IQ). Eight perceptual attributes, i.e., naturalness, colorfulness, brightness, contrast, sharpness, clearness, preference, and overall IQ, are visually assessed via categorical judgment method for various application types of test images, which were manipulated by different methods. Their correlations are deeply discussed, and further factor analysis revealed the two essential components to describe the overall IQ, i.e., the component of image detail aspect and the component of color information aspect. Clearness and naturalness are regarded as two principal factors for natural scene images, whereas clearness and colorfulness were selected as key attributes affecting the overall IQ for other application types of images. Accordingly, based on these selected attributes, two kinds of empirical models are built to predict the overall IQ of mobile displays for different application types of images.

Novel spectral characterization method for color printer based on the cellular Neugebauer model

The cellular Yule-Nielson spectral Neugebauer (CYNSN) model to characterize a typical CMYK (i.e., cyan, magenta, yellow, and black) 4-ink color printer is presented. By reconstructing spectral reflectance, models with high accuracy of spectral and colorimetric predictions are built. A novel cell-searching algorithm is proposed and used together with the iteration method to invert the cellular Neugebauer model efficiently. Large numbers of high quality hardcopy samples are produced to evaluate model performance and prove the feasibility of the algorithm. The spectral-based model performs better compared with the usual model based on CIE1931 XYZ tristimulus values.

Comprehensive model for predicting perceptual image quality of smart mobile devices

An image quality model for smart mobile devices was proposed based on visual assessments of several image quality attributes. A series of psychophysical experiments were carried out on two kinds of smart mobile devices, i.e., smart phones and tablet computers, in which naturalness, colorfulness, brightness, contrast, sharpness, clearness, and overall image quality were visually evaluated under three lighting environments via categorical judgment method for various application types of test images. On the basis of Pearson correlation coefficients and factor analysis, the overall image quality could first be predicted by its two constituent attributes with multiple linear regression functions for different types of images, respectively, and then the mathematical expressions were built to link the constituent image quality attributes with the physical parameters of smart mobile devices and image appearance factors. The procedure and algorithms were applicable to various smart mobile devices, different lighting conditions, and multiple types of images, and performance was verified by the visual data.

Automatic geometrical calibration for multiprojector-type light field three-dimensional display

Abstract. An automatic geometrical calibration approach has been developed to calibrate a multiprojector-type light field (LF) display automatically and accurately. The calibration framework based on image mapping is detailed, which transfers the calibration of three-dimensional (3-D) scene into the calibration of two-dimensional image in the diffuser interface. A multiprojectors-type LF display prototype is applied to implement the experimental calibration. Comparison results of the reconstructed 3-D scene before and after calibration show that a better overall performance is obtained through the proposed calibration approach.

Color appearance and visual measurements for color samples with gloss effect

We assess the color appearance of the samples with different inks on glossy substrates, five kinds of paper with different gloss levels. The color samples are measured using spectrophotometers under different illuminating/viewing geometries and visually estimated using the psychophysical method of magnitude estimation. The results of the two approaches are compared through the color appearance model of CIECAM02. The experimental data analysis indicates that the 0/45 and 15/0 geometries can be used to describe the two major aspects of gloss effect, the enlargement of color gamut, and the reduction of lightness. The agreement for hue attribute between instrumental measurement and visual assessment is better than those for colorfulness and lightness.