International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004 
4. EXPERIMENTS 
In this paper, a new method based on DCT has been proposed 
in order to achieve hidden communication over the 
communication networks. The experiments conducted on the 
images given in Figures 1-4. The image given in Fig.1 has been 
used as cover image and all of the other images have been 
embedded into the cover image. The success of the hiding 
process has been evaluated both objectively and subjectively. 
The well known MSE value has been used as objective 
evaluation measure for hiding performance. And the given 
images have been used as objective evaluation measure of the 
hiding performance of the proposed method. 
The information hiding process has been given below step-by- 
step: 
l. Compute the frequency domain coefficients of the 
cover image by using the DCT with for 8x8 pixels 
sized unoverlapping blocks. 
2. Use the numerical values of lower sub-triangle of the 
DCT blocks in order to hide message information(s). 
3. Recode the message information(s) by using the 
appropriate tools such as, 
a. Rescaling intensity coding, 
b. Sampling intensity values, 
c. Using block-coding techniques (i.e., run- 
length coding). 
4. Use one of the computationally secure cryptographic 
methods (i.e., position permuting, color shifting, and 
vigenerre-image coding) for images in order to 
increase the security level of the message. 
5. Replace the numerical values, which have been 
determined at Step-3, with the values of the 
information coefficients, which are still in spatial 
domain. 
6. Send to stego-object to audiences over the 
communication net. 
The proposed method has some advantages and disadvantages 
that are listed below, 
1. The proposed method is a computationally secure; 
therefore, the minimum size of the cover image 
determines the level of the security of message 
information. 
2. The proposed method is NOT an information lossless 
coding technique until audiences uses frequency 
domain. 
3. The information hiding capacity of the proposed 
method depends on to the methods that are used in 
Step-3 of the proposed method and inherit 
specifications of the message information(s) images. 
4. The proposed method is not based on the bit-change 
method; therefore it can produce artificial impulses if 
stego-message converted into spatial domain. 
In the experiments, The Fig.2, Fig.3, and Fig.4 message images 
have been embedded in to first, second, and third band of the 
message image, respectively. The stego-message image, Fig.5, 
has been converted into spatial domain for visualization. 
328 
  
  
Figure 2: The elevation image of the Earth (2400x1200 pixels 
sized). 
  
Figure 3: The lights at the night over the Earth image. 
(2400x1200 pixels sized). 
4.1 Image Quality Measures 
The well-known image quality measures; Mean Squared Error 
(MSE), Pearson Correlation Coefficient, and Peak Signal To 
Ratio (PSNR) have been used in order to objectively evaluate 
the performance of the proposed method. The quality measure 
of PSNR is defined with; 
A 
z 
p 
PSNR=10 lo — Max — IdB 9) 
E10 T MSE 
where /max is equal to 255 for 8 bit gray scale images. The 
MSE is calculated by using the Eq. (2) given below: 
MN 
] 2 
MSE= MN > > ( i78) (10) 
i=l j=l 
Interna 
erre 
Where 
horizot 
represe 
the p 
Correl: 
Corr 
where 
Figure 
  
Figur 
Table 
Stego 
Provi 
becar 
comn 
infori