INTRODUCTION
Block truncation coding is a simple and effective method for image compression
(Delp and Mitchell, 1979). It was also called the momentpreserving
block truncation because it preserves the standard mean and standard deviation
of each image block. Less computational complexity provided a widely applications
for BTC (Lv and Lu, 2011; Mohammad
et al., 2011). To reduce the bitrate of BTC, various modifications
to BTC have been proposed in the past few decades. Lema
and Mitchell (1984) presented a simple and fast variant of BTC, named Absolute
Moment BTC (AMBTC) that preserves the higher mean and lower mean of a block.
In a comparative study, Kumar and Singh (2011) presented
a BTC scheme called EBTC which has higher PSNR than BTC and AMBTC (Absolute
moment BTC). Kamel et al. (1991) proposed a variable
BTC algorithm with optimal threshold. It brought a reduction of the error in
the reconstructed images by almost 40%. Wu (2002) presented
probability based BTC to reduce the bit plane overhead. Hu
(2004) employed twodimensional prediction technique, the bit map omission
technique and the bit map coding with edge patterns to cut down the bitrate
of moment preserving block truncation coding. Han et
al. (2008) proposed a BTC based on the vector quantizer for the color
image compression which can get high compression ratio and good visual quality.
Wang and Chong (2010) proposed an adaptive multilevel
BTC. The scheme adaptively selected 2level or 4level BTC according to the
edge property of blocks. Guo (2010) proposed an Ordered
Dither Block Truncation Coding (ODBTC) by using a dither array Look Up Table
(LUT). Somasundaram and Vimala (2010) developed an efficient
block truncation coding by exploiting the feature of interpixel correlation.
Choi and Ko (2011) devised a novel DPCMBTC. The scheme
derived a bivariate quadratic function to represent the mean squared error (MSE)
between the original block and the block reconstructed in the DPCM (differential
pulse code modulation) framework and adopted a nearoptimal quantizer to prevent
the rapid increase of the quantization error. It improved peak signaltonoise
ratio performance compared with the common DPCMBTC method without optimization.
Natarajan and Rao (2011) proposed two for modified BTC
algorithms by using the ratio of moments. It also coded the ratio values and
the bitplane to reduce the bitrates. Vimala et al.
(2011) improved adaptive block truncation coding method by exploiting the
feature of interpixel redundancy and it reduced the bitrate further while
retaining the quality of the reconstructed images.
However, many existing BTC schemes didn’t fully exploiting features of
image subblocks and can’t obtain good tradeoff between low bitrate and
high quality of reconstructed images. To solve this problem, we combine texture
masking characteristics of image subblocks and Least Significant Bits substitution
approach to design an adaptive BTC scheme. It can reach optimal balance between
low bitrate and high PSNR values.
DATA HIDING BASED ADAPTIVE BTC SCHEME
Encoding algorithm for adaptive BTC: In order to get an efficient tradeoff between high visual quality and low bitrate, visual perception of image subblocks should be considered during BTC. We mainly exploited the texture sensitivity of image subblocks. Generally, highly textured blocks contain more reach structural information than smooth blocks. This means smooth blocks can be encoded with fewer bits than texture blocks during BTC. Denotes the lower mean and higher mean of the current subblock as x_{l} and x_{h} during BTC, respectively. For simplicity, the texture compression factor λ of an image subblock can assumed to be directly proportional to the absolute of lower and higher mean. So λ can be calculated as follows:
The detailed encoding process of BTC is described as below:
• 
Input: original image X 
• 
Output: BTC code stream C 
Step 1: 
Input the original image X and divide original image X into
nonoverlapping subblocks of size nxn 
Step 2: 
Calculate the mean ,
lower mean x_{1l} and higher mean x_{1h} for the current
nxn block X_{1} 
Step 3: 
Compute the texture factor λ using Eq. 1 
Step 4: 
Given a threshold value T_{1}, if λ<T_{1} then
encode the current nxn block X_{1} with the mean .
For decoding purpose, binary bit sequence ‘00’ is used as the
indicator bits of current block. At the same time, the indicator bits are
hidden into the mean
by applying LSB substitution as shown in Eq. 2 and go
to step 14 else continue with step 5: 
where mod (a, b) returns a modulo b and the number 0 is the decimal value of the binary bit sequence ‘00’.
Setp 5: 
Divide the nxn block into four nonoverlapping blocks X_{2} with
size n/2xn/2 
Step 6: 
Compute the mean ,
lower mean x_{2l}, higher mean x_{2h} and texture factor
λ for the current block X_{2} 
Step 7: 
Determine the second threshold value T_{2}, if λ≤T_{2}
then encode the current block X_{2} with the mean .
Hide indicator bits ‘01’ into the mean
for decoding purpose else go to step 9 
Step 8: 
Repeat the steps 6 and 7 until all the four n/2xn/2 subblocks and go
to step 14 
Step 9: 
Further to divide current n/2xn/2 block X_{2} into four nonoverlapping
blocks X_{3}of size n/4xn/4 
Step 10: 
Compute the mean ,
lower mean x_{3l}, higher mean x_{3h} and texture factor
λ for current block X_{3} 
Step 11: 
If λ≤T_{3} then encode the current block X_{3}
with the mean
and embed the indicator bits ‘10’ into the mean
using LSB method 
Step 12: 
Else encode the current block X_{3} with the lower mean x_{3l},
higher mean x_{3h} and bit plane B and dented as a trio (x_{3l},
x_{3h}, B), use ‘11’ as the indicator bits for decoding
purpose and hide them into the lower mean x_{3l} with LSB scheme 
Step 13: 
Repeat steps 1012 until all the four n/4xn/4 subblocks and go to step
8 
Step 14: 
Go to step 2 until all the image subblocks are processed and the BTC
code stream C is generated 
Decoding strategy for adaptive BTC: The decoding strategy is very simple,
which consists of the following steps:
• 
Input: BTC code stream C 
• 
Output: Reconstructed image I 
Step 1: 
Read BTC code stream C 
Step 2: 
Obtain the quantizing level x_{l} or mean value from compressed
code stream C 
Step 3: 
Extract the 2 LSBs from the quantizing level x_{l} or mean value
as the indicator bits 
Step 4: 
Reconstruct image blocks with the following strategy 
If the indicator bits are ‘00’ then replace the nxn block X_{2}
with the block mean .
If the indicator bits are ‘01’ then recover the n/2xn/2 block X_{2}
with the block mean .
If the indicator bits are ‘10’ generate the n/4xn/4 block X_{3}
with only the block mean .
If the indicator bits are ‘11’ then reconstruct the n/4xn/4 X_{3}
with the trio (x_{3l}, x_{3h}, B).
Step 5: 
Repeat step 2step 4 until all the BTC code bits are processed
and finally the reconstructed image I is generated 
EXPERIMENTS AND ANALYSIS
The proposed algorithm has been implemented in MATLAB7.0. In our experiments,
some standard images of size 256x256 are used to test our BTC algorithm. Figure
1ad show four test images as examples and the corresponding
BTC compressed images are shown in Fig. 2 under n = 8 and
threshold T_{1}, T_{2}, T_{3} value 0.05, 0.05, 0.10,
respectively.
In Fig. 2ad, it is observed that compressed
images generated by the proposed BTC method have satisfactory visual quality
with PSNR values over 30.56 dB.
We have applied our image compression schemes with different combination of
threshold values and image subblock size. Table 1 lists the
tested results of the proposed scheme under different threshold values for n
= 8. From Table 1, we can know that our BTC scheme can obtain
high PSNR values and low bitrate. Especially it achieves 27.9527 dB PSNR at
0.7912 bpp and 31.3547 dB at 1.5029 bpp on average.

Fig. 1(ad): 
Four test images of size 256x256. (a) Opera, (b) Lena, (c)
Boast and (d) Peppers 
Moreover, the reconstructed images have high weighted PSNR values over 39.24
dB at low bitrate with 0.7912 bpp.
Finally, The PSNR values and the bit rate obtained with the proposed scheme
are compared with that of the existing EBTC algorithm (Kumar
and Singh, 2011) and the comparison results are shown in Fig.
3.

Fig. 2(ad): 
BTC compressed images generated by our scheme. (a) Opera,
(b) Lena, (c) Boast and (d) Peppers 

Fig. 3: 
PSNR values and bitrate obtained with different BTC schemes 
Table 1: 
PSNR and bpp values under different threshold values for
n = 8 

Figure 3 show that the PSNR value achieved by the presented
algorithm is about 4.0 dB higher than that by the EBTC method when the bitrate
is roughly the same.
CONCLUSION
BTC is one of simple and fast image compression algorithms but common BTC schemes have high bitrate. In this paper an adaptive BTC algorithm is developed by fully considering texture masking characteristics of original image blocks. During BTC, the texture sensitivity is exploited to recognize the image subblocks in terms of perception feature. At the same time, the LSB substitution method is employed to hide the indicator bits of each image subblock. The proposed BTC algorithm outperforms other existing BTC schemes in terms of bitrate and PSNR values. It can be applied to realtime image communications via the Internet.
ACKNOWLEDGMENT
This study was supported by National Natural Science Foundation of China (61073191, 61170287), Hunan Provincial Natural Science Foundation of China (10JJ6090), Scientific Research Fund of Hunan Provincial Science and Technology Department, China (2011GK3140, 2010GK3049, 2011GK3139), Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province ([2010]212).