Cryptography Reference
In-Depth Information
where the dummy image should be compressed before being transmitted, we
propose a steganography technique based on JPEG2000 lossy compression
scheme [13] and BPCS steganography. In JPEG2000 compression, wavelet
coe
cients of an image are quantized into a bit-plane structure and there-
fore BPCS steganography can be applied in the wavelet domain. The pro-
posed JPEG2000-BPCS steganography provides a significant integration of
JPEG2000 lossy compression scheme and BPCS steganography, and a solution
to the aforementioned problem associated with bit-plane-based steganography
methods.
8.5.1 JPEG2000 Compression Standard
JPEG2000 encoder consists of several fundamental components: pre-processing,
discrete wavelet transform (DWT), quantization, arithmetic coding (tier-1
coding), and bit-stream organization (tier-2 coding) [13] (see the left part of
Fig. 8.12). Pre-processing includes intercomponent transformation for multi-
component images, typically color images. After the DWT is applied to each
component, wavelet coe
cients are quantized uniformly with deadzone. After
the quantization step, an optional step to realize a functionality called region
of interest (ROI) can be added. The ROI is realized by scaling up the wavelet
coe
cients in the relevant regions. The quantized wavelet coe
cients are then
bit-plane encoded by arithmetic coding.
In JPEG2000, each subband of the wavelet transformed image is encoded
independently of the other subbands. Furthermore, each subband is parti-
tioned into small blocks called codeblocks, and each codeblock is indepen-
dently encoded by EBCOT algorithm [14]. This procedure is absolutely dif-
ferent from other well-known embedded wavelet coders such as EZW [15] and
SPIHT [16]. The independent encoding of codeblocks provides many advan-
tages such as localized random access into image, improved error resilience,
e
cient rate control, and flexible bit-stream ordering. The quantized wavelet
coe
cients in a codeblock are bit-plane encoded by three passes with arith-
metic coding: significance propagation pass, refinement pass, and cleanup pass.
The compressed data from the codeblocks are organized into units called
packets and layers in tier-2 coding. A precinct is a collection of spatially con-
tiguous codeblocks from all subbands at a resolution level. The compressed
data from the codeblocks in a precinct constitutes a packet. A collection of
packets from all resolution levels constitutes a layer. Therefore a layer cor-
responds to one quality increment for the entire full resolution image. In
JPEG2000, different types of progression orders are possible and a typical
one is layer-resolution-component-position progression. Once the entire image
has been compressed, a rate-distortion optimized bit-stream is generated for
a target file size (bit rate).
