Elektronika 2009-11.pdf - Instytut Systemów Elektronicznych

Considering human visual perception
for determination of distortion
One of the indicators defining the quality of images and the
level of introduced distortions is a coefficient PSNR (Peak Signal
to Noise Ratio). Good quality images with hidden messages
have the coefficient of PSNR ≥ 30 [4]. The
disadvantage of this index is low correlation with visual perception
of a recipient. In studies of human vision for evaluation
of the quality of vision two measures are used: visual acuity
and contrast threshold [5].
Visual acuity, called VISUS determines the size of the smallest
character called optotype, which is correctly recognized by
human from a certain distance. This measure determines the
fraction of Sandella, which is the quotient of the distance from
the test chart to the distance from which the optotype is recognized
at 5 minutes angle. Visus measure equals 5/5 signifies
correct human sight. If Sandella fraction is 5/15 it means person
with poor sight. [5]. If the visual acuity is 1 (6/6) then the spatial
frequency is 20/20 cycle/degree. If the visual acuity is 0.1 (6/60)
then the spatial frequency is 20/200 cycle/degree.
Examination of visual acuity is held at maximum contrast,
where optotype having a black color is placed on a white
background. Because the conditions of the observation usually
do not allow the observations at maximum contrast therefore
more important measure for determining the quality of
sight is the contrast [5].
Contrast characterizes the degree of distinguishing of the
subject from the background and the clarity of its details. Contrast
measure can be calculated based on relation (Michaelson’s):
(1)
2.5%, 1.25%, 0.6%. In this way optotype chart is used for
examining the acuity as a chart for examining the sensitivity of
an eye to changes of contrast. If recognized contrast is 2.5%
then contrast sensitivity is 100/2.5 = 40 and if it is 0.6% equals
100/0.6 = 167 respectively.
The changes of contrast are noticeable between the acuity
values of 0 (high optotype symbols) and 1 (small optotypes).
All changes in contrast above 1 (20/20 cycle/degree)
are imperceptible. Hiding information in the area above
1 cycle/degree does not lead to visible distortions in the resulting
image. The biggest changes are noticeable in the area
of visual acuity for optotypes 0.03 (20/600) and 0.1 (20/200),
where the contrast sensitivity is the greatest.
A modified form of the determination of invisibility to the
human eye of the fact of steganographic hidden messages in
pictures is the utilization of standardized contrast sensitivity
function CSF proposed by Manos and Sakrisona [8]. This
function is defined in formula:
This function indicates the sensitivity of the human eye to different
frequency of visual impulses, which expresses the contrast.
The higher frequency of impulse changes, the worse
pattern recognition. Recognizing patterns is also aggravated
in case of rare impulse changes. Maximum sensitivity is additionally
dependent on viewing angle. If the viewing angle will
decrease, extreme of the function moves slightly towards
higher frequencies. In case of increase of viewing angle, the
extreme moves in the other direction [8]. Contrast sensitivity
chart is shown in Fig. 1.
(3)
where: I max means an object with maximum brightness and
I min the object with minimum brightness. Contrast equals
1 means a black object on a white background. Contrast with
lower values means gray object on a gray background. When
K = 0 then the object is imperceptible from the background.
The ability to recognize complex objects determines the sensitivity
to contrast function. This function determines the contrast
scale at what certain optotype is recognizable. Contrast
sensitivity relation is expressed in a formula:
C=1/K p (2)
where: K p defines the contrast threshold. It sets the limit of
discrimination of the observed object from the background [5].
The relation between sensitivity and contrast and the scale of
optotype represents the contrast sensitivity function. This
function is presented in a logarithmic scale in regard of its high
value of contrast sensitivity and non-linear characteristics. It
allows estimation of a degree of deterioration in sight efficiency
at reduced contrast. Its shape determines the test
charts with optotypes of different contrast against background.
Contrast sensitivity can be examined using an optotypes chart
with use of two charts [6]:
• with symbols of the same size, but with variable contrast
(Pelli-Robson chart [7]),
• with symbols of the same level of contrast, but with a variable
size of letters.
Chart (2) for examining the contrast sensitivity is created
on the basis of visual acuity examination charts by taking rows
of all sizes of letters to a relevant level of contrast. Contrast
levels for optotype chart are as follows: 100%, 25%, 10%, 5%,
Fig. 1. Optotype chart to examine the sense of contrast
Rys. 1. Tablica optotypów do badań poczucia kontrastu
Fig. 2. The graph on the contrast sensitivity function [5]
Rys. 2. Wykres funkcji wrażliwości na kontrast [5]
42 ELEKTRONIKA 11/2009