1 Image Transcoding in the block DCT Space Jayanta Mukhopadhyay Department of Computer Science & Engineering Indian Institute of Technology, Kharagpur, 721302, India [email protected]
Dec 18, 2015
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Image Transcoding
in the block DCT Space
Jayanta MukhopadhyayDepartment of Computer Science & Engineering
Indian Institute of Technology, Kharagpur, 721302, India
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h'
g'g
h 2
2
2
2
)(0 nx )(~0 nx)(1 na
)(1 nd
Analysis Synthesis
Forward DWT and Inverse DWT
Discrete Wavelet Transform
4
2-D DWT
1D DWT applied to vertical and horizontal direction. For Multilevel DWT:-
The LL band is recursively decomposed, vertically and horizontally.
Filtering is performed in time domain.
Image in spatial domain
LH
HL
HH
2LL 2HL
2LH 2HHLL
LH
HL
HH
Decomposition 1st Level 2nd Level
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DCT domain Upsampling with Zero Insertion
Type-II DCT of upsampled signal as obtained throughzero insertion of signal x(n) is computed by:
Note:- DCT obtained is referred as upsampled DCT.
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A typical conversion matrix4x4 block to 8x8 upsampled type-II DCT
For even sample xoxoxoxo…
For odd sampleoxoxoxox…
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Computation of upsampled DCT
W aveletcoe fficient
Block
DCT domainupsamplingEntire band
UpsampledType-II
DCT blockof
wavelet band
1. Upsampling entire wavelet bandM ethod
W aveletcoefficient
Block
UpsampledType-II
DCT blocks8 x 8
DCT domainupsampling4 x 4 blocks
UpsampledType-II
DCT blockof
wavelet band
CompositionDCT
blocks
2. Upsampling using composition of smaller DCT blocksM ethod
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Wavelet synthesis in the DCT domain
Result is transcoded type-II DCT coefficients.Operation is equivalent to IDWT + DCT.
Type-IIupsampled
DCT block ofwavlet
coefficientband
Type-IDCT block
ofSynthesis Filter
ImpulseResponse DCT
domainFiltering
Type-IIDCT block
ofFiltered
Signal/image
),({2 nmxC uee )},({)},({ 12 nmhCnmxC euee)},( nmh
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Transcoding in the DCT domain
2
2
D C T
)(1 na
)(1 nd
)(ny)(kY
)(' ng
)(' nh
W a ve le ts ubba nds
D C TB lo c k s
D W T to D C T T ra ns c o ding
S ynthe s isfilte ring
U ps a m pling
U ps am ple dType - II
D C T blo c k
T yp e-ID C T o f
F ilter
)(1 na
P o intw is e
m ultip ly
)(' nh
T ra ns c o de dD C TB lo c k
Appr o xi m at i o n s ubbands ynthe s i s i n D C T do m ai n
W a ve le ts ubba nd
S ynthe s isfilte r
Composite Operation
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Wavelet to DCT Transcoding (WDT)
LL Filteredbloc k
H L Filteredbloc k
HH Filteredbloc k
LH Filteredbloc k
Point wiseAddition
TranscodedDCT Block
LL HL
LH HH
Ups am plingand Filtering
Ups am plingand Filtering
Upsamplingand Filtering
Upsamplingand filter ing
W avelet bands
WWfT
B 22
Viswanath, Mukherejee and Biswas (2009), Springer Journal on SIVP,
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WDT computes DCT blocks of larger size. By decomposing, 8x8 blocks are obtained. Cost of transcoding increases. To reduce the cost, smaller blocks are to be considered.
WBDT:
* Three blocks of NxN size are used in a
wavelet subband of size WxW.
* Blocks are Upsampled, composed, filtered and
decomposed.
* The transcoded blocks of size 2Nx2N.
Note:- N=4 for JPEG based applications.
Wavelet to Block DCT Transcoding (WBDT)
This work is published in Springer Journal on SIVP, Jan 2009
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WBDT computes DCT blocks of size 8x8, Uses composing and decomposing of DCT blocks. Requires Type-I DCT of filter and Type-II DCT of data.
Linear Filtering in the DCT domain: Requires only Type-II DCT.
Blocks are processed separately and added together.
The WBDTL,
* Three adjacent NxN size subband blocks are used.
* Upsampled blocks filtered separately and the results are added.
* Computes the transcoded blocks of size 2Nx2N.
Note:- For JPEG based applications, N=4.
Wavelet to Block DCT Transcoding by Linear Filtering (WBDTL)
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Where
and
are the even upsampled DCT blocks of wavelet approximation subband blocks and DCT matrices of the lowpass synthesis filter.
are the odd upsampled DCT blocks of wavelet detail subband blocks and DCT matrices of the highpass synthesis filter.
Where
and
Wavelet to Block DCT Transcoding by Linear Filtering (WBDTL)
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Transcoding with Multilevel DWT
LH
HL
HHLH
HL
HH
2LL 2HL
2LH 2HHLL
2LL 2HL
2LH 2HH
2nd level Transcoding
LH
HL
HH
LL
1st level Transcoding Reconstructed image DCT Blocks
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For a tile size of 64 x64In JPEG2000
For blocks of size 8x8
Cost of Multilevel Transcoding
Hybrid Approach
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JPEG2000 9/7 wavelet filters
Analysis Filter Coefficients
Synthesis Filter Coefficients
i Lowpass H(i)
Highpass G(i)
Lowpass H’(i)
Highpass G’(i)
0 0.6029 -1.1151 1.1151 -0.6029
0.2669 0.5913 0.5913 0.2669
-0.0782 0.0575 -0.0575 0.0782
-0.0169 -0.0913 -0.0913 -0.0169
0.0267 -0.0267
1234
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Results: Quantization of wavelet subbands
For Lena image
Large number of coefficients
are zeros
No. of zero coefficients
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Observations
The proposed new algorithms for transcoding the DCT coefficients from wavelet coefficients are computationally efficient.
Both the approaches (WBDT and WBTL) are found to be equivalent.
Proposed transcoding achieves PSNR performances as same as the spatial domain technique.
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Block DCT to wavelet transcoding
The technique is based on the combined step of block filtering and downsampling directly in the DCT domain.
Filtering matrices are computed using wavelet analysis filters.
Three adjacent DCT blocks are used in transcoding.
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2
2
ID C T
)(nh
)(ng
)(nx)(kX
)(1 na
)(1 nd D C TB lo c k s
W a ve le ts ubba nds
D C T to D W T T ra ns c o ding
A na lys isfilte ring
D o w ns a m pling
F ilte r ing a ndD ow n
s a m pling
T yp e-ID C T o f
the F ilter)(1 naID C T
( )h n
D C TB lo c k s
Appr o xi m at i o n s ubbandtr ans c o di ng i n D C T do m ai n
W a ve le ts ubba nd
Block DCT to wavelet transcoding
Composite operation
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Block DCT to Wavelet Transcoding (BWT)
HL and HH are computed from Type-I DCT of the analysis filters.
In 1-D:
In 2-D:
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Block DCT to Wavelet Transcoding with Linear Convolution (BWTL)
This work is Accepted in IEEE ICIP 2009
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Results: DCT to Wavelet transcoding
PSNR close to
300 dBRef: ST subbands
Almost equal
Almost equal
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Quantization effect on transcoded subbands
More degradation of quality in lower frequency subbands
This work is under revision in Springer Journal on SIVP
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Spatial transcoding and the BWTL transcoding techniques perform at par.
However, the BWT technique perform the best among them.
This is due to the fact that the rounding errors are accumulating in spatial domain transcoding and the BWTL techniques.
But the BWT technique operates with three blocks simultaneously, and thereby rounding errors are reduced.
Observations
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JPEG2000 Compression Standard
JPEG2000 is an emerging image compression standard Uses Discrete wavelet transform (DWT) as transform . Transform coefficients are quantized an encoded.
SourceImage
ForwardWavelet
TransformQuantization
EntropyEncoding
Com pressedIm a ge da ta
....01010001....
ReconstructedImage
InverseWavelet
TransformDequantization
EntropyDecoding
Com pressed Im a ge da ta
....01010001....
Compression
Decompression
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Transcoding JPEG2000 to JPEG
Encoding of JPEG2000 usually gives 2-4 dB higher PSNR values at the same compression level.
Average decoding time for JPEG2000 is almost five times higher compared to JPEG
Current web browsers do not support JPEG 2000 natively.
Since JPEG is the most common compression standard and has very low complexity, transcoding to JPEG provides a smooth upgrade path.
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Transform Domain Transcoder
SourceImage
ForwardWavelet
TransformQuantization
EntropyEncoding
Com pressed Im a ge da ta
....01010001....
ReconstructedImage
Block DCT
DWT to DCTTranscoderDequantization
EntropyDecoding
Com pressedIm a ge da ta
....01010001....
Compression
Decompression Transcoding
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Since impulse responses of JPEG2000 wavelet filters are symmetric, DCT domain filtering can be used.
JPEG2000 encoded using 2-D wavelet transforms by incorporating 9-7 analysis filters.
The wavelet coefficients are encoded with different quantization levels to achieve different compression rate.
Tanscoding…
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Transform domain Transcoding
EntropyDecoding
Waveletbands
WaveletDCT
Transcoder
Q uantizeDCT
Blocks
JPEGEntropy
EncodingJPEG
JPEG2000
NxNDCT
Blocks
compressed Image
compressed Image
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Guaranteed PSNR(Mg): The minimum PSNR value with respect to the JPEG2000 decoded image to be achieved by the transcoder. Here we used a typical value of 40 dB.
Equivalent rate (ρeq): The minimum compression rate of the transcoded JPEG image providing at least the guaranteed PSNR with respect to the JPEG2000 decoded image. The equivalent rate is obtained at 40 dB point with respect to JPEG2000 decoded image.
Equivalent PSNR (Meq): The transcoder PSNR with respect to the original image at the equivalent rate.
We have defined three measures as follows: