JIST - Society for Imaging Science and Technology

Journal of Imaging Science and Technology® 51(5): 402–406, 2007.
© Society for Imaging Science and Technology 2007
Human Perception of Contour in Halftoned Density Step
Image
Phichit Kajondecha, Hongmei Cheng and Yasushi Hoshino
Department of Systems Engineering, Nippon Institute of Technology, Miyashiro, Saitama, 354-8501, Japan
E-mail: s3054602@sstu.nit.ac.jp
Abstract. Contour is an important element of an image, usually
expressed by the difference in density between two areas of an
image. Objects are usually recognized from observation of their contours.
In some cases, emergence of a false contour is a problem in
image coding and decoding. Halftoning is an essential image process
for digital printing of continuous tone. Contour perception is
studied experimentally under various halftoning conditions, observation
distances, and illumination conditions. The results show that in
the human visual system, contour perception becomes easier with a
decreasing stimulus of dots, which serve as the component of a
halftone image. © 2007 Society for Imaging Science and
Technology.
DOI: 10.2352/J.ImagingSci.Technol.200751:5402
INTRODUCTION
In image processing, contours or boundaries in an image are
important factors because our perception and recognition of
an object depend on contour. In a sense, therefore, our eyes
can be said to search for contour in an image. 1 Contour is
the boundary between two areas of different density, color,
or texture. If the dots are sufficiently small, the eye cannot
detect the dot pattern 2 and we can recognize contour. In
order to achieve high-quality image printing, the contour
expression characteristics of an image are considered important.
One way to understand and evaluate image quality is
use of the human visual system (HVS). 3–5
Digital halftoning is essential for processing in digital
printing and can be achieved by various methods. A conventional
halftone is a fundamental method that has a wide
application in printing. Digital halftoning has been evaluated
from various viewpoints, but we have not yet gained a sufficient
understanding of the relationship between the ease of
perceiving contour, which is one of most fundamental elements
of an image, and half-toning methods.
In this study, images with contours of two different density
values are processed to halftoned images of various halftone
cycles and halftone angles of 0° and 45°. These
halftoned images are observed under several conditions of
illumination and observation distance. With respect to halftone
frequency, images of relatively large cycles are prepared
for accurately expressing the density of the image. If the
observation is conducted at long distances, the condition of
view angle from the eye is the same as that of a small cycle.
The perception of contour is examined under various conditions,
and the dependence of perception ratio on the difference
in density is obtained. As the stimuli of the halftone
cycle decrease relative to the density difference between regions,
the contour is perceived more easily.
EXPERIMENTAL
As shown in Figure 1, two types of images were produced,
each of which differed with respect to the direction of the
contour line: (a) Images with perpendicular contour lines
and (b) images with diagonal contour lines. The images were
printed at a size of 10 cm10 cm with a resolution of
300 dpi. The image printing conditions are shown in Table
I, and as can be seen in the table, in some cases the density
level of area A was fixed at 192 while the density level of area
B was varied between 191 and 180. In other cases, the density
level of area A was fixed at 64 while that of area B was
varied between 63 and 52.
Figure 2 illustrates the halftone dot arrangement, screen
dot shape, and screen angle. The screen dot shape of the
halftone is circular and produced at two screen angles (0°
and 45°). As shown in Figure 3, the cycle of halftone dots
was prepared in four levels: 1mm, 2mm, 3mm, and
4mm.
The observation experiments are carried out as illustrated
in Figure 4. The halftoned images were presented under
three illumination conditions at four different observation
distances. The perception ratios are obtained as the
ratio of testees who perceived contour (responded “yes”)
verses total testees. Testees who could not perceive contour
responded “no.”
RESULTS AND DISCUSSIONS
The perception ratios obtained in the experimented conditions
are plotted against the density difference between areas
Received Feb. 2, 2007; accepted for publication May 7, 2007.
1062-3701/2007/515/402/5/$20.00.
Figure 1. Example density difference patterns: a perpendicular contour
pattern and b diagonal contour pattern.
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