Elektronika 2009-11.pdf - Instytut Systemów Elektronicznych

a)
Fig. 4. Source image
Rys. 4. Obraz źródłowy
can cause visible distortions. Images with the added noise are
shown in Fig. 5.
After adding Gaussian noise received information that:
• number of blocks of recognizable changes of contrast:
5562,
• number of blocks of invisible changes of contrast: 6726.
By adding Poisson noise to the source image obtained information
that:
• number of blocks of recognizable change of contrast: 5733,
• number of blocks of invisible changes of contrast: 6555.
In both cases the number of blocks, in which visible
changes can be seen significantly increased, what reflecting
the reality of distortions introduced by the noise.
It is also worth noting that usage as the sensitivity parameter
of visibility criterion to changes in contrast allows controlling
the strength of inserting messages. This can be done
by local increase or reduction of data amount hidden in selected
piece of the image in a way that the distortion level was
below human eye sensitivity to local changes of contrast. Introduction
of changes by adding noise to the source image
(hiding messages) may not cause visible changes within a single
block, but it can cause changes in comparison to the
image of adjacent blocks in resulted image.
Summary
b)
Steganographic hiding information in images should not cause
visible distortions to the human eye. The fact of hidden message
can be regarded as noise, which is different for particular
algorithms. Application of quality evaluation methods of the
images based on the human eye perception, such as contrast
threshold and contrast sensitivity function allows the determination
of the quality of the resulting images and the steganographic
method used whether it ensures required level of
invisibility of hidden message.
In addition by evaluating the quality of the source image it
can be judged whether a certain picture is useful for hiding the
information in or which areas are not suitable for this purpose.
References
Fig. 5. The image of the added noise: a) Gaussian noise, b) Poisson
noise with λ = 0.1
Rys. 5. Obraz z dodanym szumem: a) szumem Gaussa, b) szumem
Poissona z λ = 0,1
For example for a typical image with the town view shown
in Fig. 4, with a size of 1024 x 768 pixels it can get data:
• number of blocks of recognizable changes of contrast: 3742,
• number of blocks of invisible changes of contrast: 8546.
Number of blocks of imperceptive contrast to the human
eye signifies the uniform areas. In these areas it should not
make changes because the steganographic algorithm action
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44 ELEKTRONIKA 11/2009