Edul N. Dalal

The effect of gloss on color

A model was developed for the effect of gloss on reflected color, taking the measurement geometry of the color measuring device into account. It considers the total front surface reflectance of a given sample to be independent of gloss and determined only by its refractive index and the angle of the incident beam. Although the total front surface reflectance is fixed, its specular component increases (and the diffuse component decreases) with increasing sample gloss. Therefore, the diffuse reflectance factor measured by a spectrophotometer decreases with sample gloss. In this article, a defined curve was fitted to the measured reflectance factor of black xerographic print samples having a range of gloss values for the 0/45 and diffuse spectrophotometer geometries, and this curve was used to predict the CIELAB color values for samples of the other colors at various gloss levels. The predicted color was compared to the measured color for cyan, magenta, yellow, red, green, and blue images that spanned a wide range of gloss. The RMS color error between the predicted and measured color of all of these images was found to be about 3 CIELAB ΔE* units.

Method for quantifying the color gamut of an output device

A technique is proposed for estimating the surface of the color gamut of an output device, in 3D colorimetric space. The method employs a modified convex hull algorithm. This approach is shown to be more general, and more accurate, than existing techniques. Simple numerical metrics are derived from this surface description: namely the gamut volume in 3D space; and the percentage of colors from the Pantone Matching System which fall within the gamut.

Predicting customer preference from objective image quality metrics for monochrome document products

This paper describes a regression model for predicting customer preference from objective image quality metrics for black and white printers, copiers and multifunction systems. In order to quantify customer preference for monochrome images the quantitative preference system was previously developed. Using this system, a preference survey with five different customer-type documents was used to obtain the preference data. Objective image quality metrics were obtained from a scanner-based measurement system. Using this regression model, typically 80% or more of the variation of the overall preference can be explained by six objective image quality metrics: Relative TRC Error; Mottle; Visual Noise; Visual Structure; Streaks and Bands; and Relative Dynamic Range Reduction. The results also provide the relative significance of these attributes for the different kinds of customer images.

Perception-based line quality measurement

It is well-known that many sub-attributes of line quality, such as edge raggedness, line waviness, etc., contribute to the perception of the overall line quality. But the relative importance of these sub-attributes is not clear, nor is there a method available for combining them into one representative number for overall line quality. To address these issues, we have designed and conducted a series of psychophysical experiments, which explore the shape of the human visual transfer functions (VTF) relevant to the perception of these sub-attributes. Based on this, we have proposed an approach to assess overall line quality. In our method, we first pre-process the line image acquired (for example from a scanner) and extract certain profiles relevant to line quality measurement. A set of corresponding VTF’s is then applied to these profiles to calculate the various sub-attributes. Finally, overall line quality is determined by the weighted combination of these individual sub-attributes. Our preliminary results show that this measurement correlates well with human perception of overall line quality, for the sub-attributes studied.

Recent progress in the development of ISO 19751

A small number of general visual attributes have been recognized as essential in describing image quality. These include micro-uniformity, macro-uniformity, colour rendition, text and line quality, gloss, sharpness, and spatial adjacency or temporal adjacency attributes. The multiple-part International Standard discussed here was initiated by the INCITS W1 committee on the standardization of office equipment to address the need for unambiguously documented procedures and methods, which are widely applicable over the multiple printing technologies employed in office applications, for the appearance-based evaluation of these visually significant image quality attributes of printed image quality. 1,2 The resulting proposed International Standard, for which ISO/IEC WD 19751-13 presents an overview and an outline of the overall procedure and common methods, is based on a proposal that was predicated on the idea that image quality could be described by a small set of broad-based attributes.4 Five ad hoc teams were established (now six since a sharpness team is in the process of being formed) to generate standards for one or more of these image quality attributes. Updates on the colour rendition, text and line quality, and gloss attributes are provided.

Update on the INCITS W1.1 standard for evaluating the color rendition of printing systems

The color rendition ad hoc team of INCITS W1.1 is working to address issues related to color and tone reproduction for printed output and its perceptual impact on color image quality. The scope of the work includes accuracy of specified colors with emphasis on memory colors, color gamut, and the effective use of tone levels, including issues related to contouring. The team has identified three sub-attributes of color rendition: (1) color quantization -- defined as the ability to merge colors where needed, (2) color scale -- defined as the ability to distinguish color where needed, and (3) color fidelity -- defined as the ability to match colors. Visual definitions and descriptions of how these sub-attributes are perceived have been developed. The team is presently defining measurement methods for these, with the first of the sub-attributes considered being color quantization. More recently, the problem of measuring color fidelity has been undertaken. This presentation will briefly review the definitions and appearance of the proposed sub-attributes. The remainder of the discussion will focus on the progress to date of developing test targets and associated measurement methods to quantify the color quantization and color fidelity sub-attributes.

INCITS W1.1 standards for perceptual evaluation of text and line quality

INCITS W1.1 is a project chartered to develop an appearance-based image quality standard. This paper summarizes the work to date of the W1.1 Text and Line Quality ad hoc team, and describes the progress made in developing a Text Quality test pattern and an analysis procedure based on experience with previous perceptual rating experiments.

INCITS W1.1 development update: appearance-based image quality standards for printers

In September 2000, INCITS W1 (the U.S. representative of ISO/IEC JTC1/SC28, the standardization committee for office equipment) was chartered to develop an appearance-based image quality standard.(1),(2) The resulting W1.1 project is based on a proposal(3) that perceived image quality can be described by a small set of broad-based attributes. There are currently six ad hoc teams, each working towards the development of standards for evaluation of perceptual image quality of color printers for one or more of these image quality attributes. This paper summarizes the work in progress of the teams addressing the attributes of Macro-Uniformity, Colour Rendition, Gloss & Gloss Uniformity, Text & Line Quality and Effective Resolution.

Recent Progress in the Development of INCITS W1.1, Appearance-Based Image Quality Standards for Printers

In September 2000, INCITS W1 (the U.S. representative of ISO/IEC JTC1/SC28, the standardization committee for office equipment) was chartered to develop an appearance-based image quality standard.(1),(2) The resulting W1.1 project is based on a proposal(4) that perceived image quality can be described by a small set of broad-based attributes. There are currently five ad hoc teams, each working towards the development of standards for evaluation of perceptual image quality of color printers for one or more of these image quality attributes. This paper summarizes the work in progress of the teams addressing the attributes of Macro-Uniformity, Color Rendition, Text and Line Quality and Micro-Uniformity.

Optimal patch code design via device characterization

In many color measurement applications, such as those for color calibration and profiling, “patch code” has been used successfully for job identification and automation to reduce operator errors. A patch code is similar to a barcode, but is intended primarily for use in measurement devices that cannot read barcodes due to limited spatial resolution, such as spectrophotometers. There is an inherent tradeoff between decoding robustness and the number of code levels available for encoding. Previous methods have attempted to address this tradeoff, but those solutions have been sub-optimal. In this paper, we propose a method to design optimal patch codes via device characterization. The tradeoff between decoding robustness and the number of available code levels is optimized in terms of printing and measurement efforts, and decoding robustness against noises from the printing and measurement devices. Effort is drastically reduced relative to previous methods because print-and-measure is minimized through modeling and the use of existing printer profiles. Decoding robustness is improved by distributing the code levels in CIE Lab space rather than in CMYK space.