The image quality of reprinted documents that were scanned at a high resolution may not satisfy human viewers who anticipate at least the same image quality as the original document. Moiré artifacts without proper descreening, text blurred by the poor scanner modulation transfer function (MTF), and color distortion resulting from misclassification between color and gray may make the reprint quality worse. To remedy these shortcomings from reprinting, the documents should be classified into various attributes such as image or text, edge or non-edge, continuous-tone or halftone, color or gray, and so on. The improvement of the reprint quality could be achieved by applying proper enhancement with these attributes. In this paper, we introduce a robust and effective approach to classify scanned documents into the attributes of each pixel. The proposed document segmentation algorithm utilizes simple features such as variance-to-mean (VMR), gradient, etc in various combinations of sizes and positions of a processing kernel. We also exploit each direction of gradients in the multiple positions of the same kernel to detect as small as 4-point text. Experimental results show that our proposed algorithm performs well over various types of the scanned documents including the documents that were printed in a resolution of low lines per inch (LPI).
Histogram equalization is one of the well-known methods for contrast enhancement. However, the conventional contrast enhancement methods based on histogram equalization still show some problems such as washed out appearance and gradation artifact. To overcome such drawbacks, we propose a novel dynamic histogram equalization method based on gray level labeling method. The main contribution of the proposed method is to expand the dynamic range of the subhistogram up to the entire dynamic range of the input image while the intensity orders of adjacent pixels are preserved. The proposed method first decomposes the image histogram into a number of sub-histograms based on gray level labeling method. A full dynamic range of input gray level is assigned to each sub-histogram and each transform function is calculated based on the bi-histogram equalization method. Finally, a contrast enhanced pixel value is the weighted average of the results from each transform function. Experimental results show that the proposed method produces better contrast enhanced images than several histogram equalization based methods without introducing several side effects.
An image taken under the backlight condition shows that a main object or foreground appears very dark, but
a background appears relatively bright since the exposure time of the main object or foreground is relatively
shorter than the one of the background due to high luminance from the background. The determination of
the backlight image is generally done by luminance histogram analysis since it is believed that the distinct
characteristic of the backlight image is a large luminance difference between the foreground and background.
However, this conventional detection method may not be adequate for video images since it generally targets
on still images. Furthermore, the detection of the backlight image would not be performed well if there are
abrupt changes in light, motion, or scenes. Inaccurate detection leads to unnecessary compensation that makes
images over-highlighted or flickered, especially when consecutive frames of video have different illumination
modes. Since an image taken under normal light conditions may also have the similar luminance characteristics
of the backlight image, it would not be sufficient to discriminate between the normal light and backlight image
using only luminance information. Therefore, the analysis of chrominance of images is introduced to detect the
backlight image more accurately.
The electrophotographic process depends on a complex interplay between electrostatically charged toner particles,
the developer roller, and the organic photoconductor during development; and between the toner particles, the
organic photoconductor, and the paper during transfer. The task of controlling the imaging process is made
even more challenging by the fact that colorant planes are developed independently and in succession. At high
colorant levels, toner particles for a given colorant plane may be strongly repelled by toner that has already been
deposited for previously developed colorant planes. The result is scattering of toner away from the edges of thin
lines and character strokes. In previous work, we have proposed a coring method to reduce the occurrence of the
toner scatter, and conducted psychophysical experiments to determine the preferred level of coring as a function
of line width and colorant level. In this paper, we apply the edge transition width (ETW) metric to physically
measure the impact of toner scatter on the sharpness of edges of lines and character strokes. We consider ETW
both with and without coring, and compare it to the results from our earlier psychophysical experiments.
The use of color electrophotographic (EP) laser printing systems is growing because of their declining cost. Thus, the print quality of color EP laser printers has become increasingly important. Since text and lines are indispensable to print quality, many studies have proposed methods for measuring these print quality attributes. Toner scatter caused by toner overdevelopment in color EP laser printers can significantly impact print quality. A conventional approach to reduce toner overdevelopment is to restrict the color gamut of printers. However, this can result in undesired color shifts and the introduction of halftone texture. Coring, defined as a process where the colorant level is reduced in the interior of text or characters, is a remedy for these shortcomings. The desired amount of reduction for coring depends on line width and overall nominal colorant level. In previous work, these amounts were chosen on the basis of data on the perception of edge blur obtained from softcopy simulation of the blurring. We describe psychophysical studies to directly establish optimal coring values as a function of line width and nominal colorant level. For each line width and nominal colorant level, this is done by asking human subjects to choose the minimum amount of coring that is necessary to eliminate the perception of toner scatter. We conduct four separate psychophysical studies to address different aspects of this question.
The use of color electrophotographic (EP) laser printing systems is growing because of their declining cost.
Thus, the print quality of color EP laser printers is more important than ever before. Since text and lines are
indispensable to print quality, many studies have proposed methods for measuring these print quality attributes.
Toner scatter caused by toner overdevelopment in color EP laser printers can significantly impact print quality.
A conventional approach to reduce toner overdevelopment is to restrict the color gamut of printers. However,
this can result in undesired color shifts and the introduction of halftone texture in light regions. Coring, defined
as a process whereby the colorant level is reduced in the interior of text or characters, is a remedy for these
shortcomings. The desired amount of reduction for coring depends on line width and overall nominal colorant
level. In previous work, these amounts were chosen on the basis of data on the perception of edge blur that was
published over 25 years ago.