MMC_07_Still image compression.pdf

8/10/2019 MMC_07_Still image compression.pdf

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Outlines

Bi-level image compression

G3/G4, JBIG, JBIG2

JPEG

JPEG-LS JPEG2000 (in other lectures)


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Bi-level Image Compression

Standards ITU G3/G4: Most widely used image compression standards for FAX

Cannot efficiently deal with digital haltfone image G3:

Non-adaptive, 1-D run length coding Last K-1 lines of each group of K lines can be optionally 2-D run-length coded. Compression ratio 15:1

G4: Only 2-D run length coding allowed 30% better compression by removing error recovery from G3

JBIG: 30% better than G4

Digital Half toning 8:1 compression Deal with digital halftones Adaptive arithmetic Coding Adaptive templates for halftones Lossless binary and low precision gray level (< 6 bits per pixel) Progressive transmission (pyramid transmission)

offers progressive encoding/decoding capability, the resulting bitstream contains a set ofprogressively higher resolution images

one bit/pixel using bit-plane coding

JBIG2 Allow lossy compression Model-based coding


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Color Image Compression

Intended to be blank


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JPEG-LS JPEG-LS is in the current ISO/ITU standard for lossless or near

lossless" compression of continuous tone images.

It is part of a larger ISO effort aimed at better compression ofmedical images.

Uses the LOCO-I (LOw COmplexity LOssless Compression for

Images) algorithm proposed by Hewlett-Packard.

Motivated by the observation that complexity reduction is oftenmore important than small increases in compression offered bymore complex algorithms.

Main Advantage: Low complexity!


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JPEG-LS The LOCO-I algorithm makes uses of context modeling.

The idea of context modeling is to take advantage of the structure within the input source { theconditional probabilities}.

Example for context model Binary source P(0) = 0.4, P(1) = 0.6 0th order entropy H(S) = -0.4log2(0.4) -0.6log2(0.6) = 0.97 if previous symbol is 0, probability of current symbol being 0 is 0.8 if previous symbol is 1, probability of current symbol being 0 is 0.1 For context 0, H(S1) = -0.8log2(0.8) -0.2log2(0.2) = 0.72 For context 1, H(S2) = -0.1log2(0.1) -0.9log(0.9) =0.47 Avg bit rate = 0.4*0.72+0.6*0.47 = 0.57

JPEG-LS Context model


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Why Do We Need Standards ? Image (and video) coding standards provide

interoperability between codecs built by different

manufacturers Basis for most products in communication technology

Standards based products can be build with common softwareand hardware tools

Only syntax and decoder specified

Standards provide state-of-the-art technology that isdeveloped by a group of experts in the field

Actual performance depends on implementation of standardregarding error resilience, delay, display

Encoder is not standardized and its optimization is left to themanufacturer


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The Scopeof Picture and Video Coding

Standardization

Only the Syntaxand Decoderare standardized:

Permits optimization beyond the obvious

Permits complexity reduction for implementability

Provides noguarantees of Quality

Pre-Processing EncodingSource

DestinationPost-Processing& Error Recovery

Decoding

Scope of Standard


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JPEG Joint Photographic Experts Group (ISO/JTC1/

SC2/WG10): Information Technology

Still (color) image coding

History: (Rao & Hwang, Chap. 8)

Nov.1986: JPEG, joined by CCITT and ISO,was established

June 1987: 10 proposals were evaluated.

Criteria : 3-stage progressive .25 bpp, .75bpp, 4bppThree classes of algorithms selected: Adaptive DCT,Adaptive DPCM, Generalized block truncation coding


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JPEG (cont.)

History: (cont.)

Jan.1988 : ADCT stood out in the secondround contest

Oct.1989 : Major technical points agreed

1989 -1991: Refinement Feb. 1991: Committee draft finished

July 1992 : International Standard

(IS)10918-1 established


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JPEG Operating Modes Sequential (baseline)

JPEG Compatible : support at least base line

Hierarchical Progressive

Lossless


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JPEG Features Resolution independence

Multiple of 8 or Padding

Precision DCT operations restricted to 8 or 12 bit for input sample

Lossless: 2~16 bpp

No bit rate target RD tradeoff by quantization

Luminance-Chrominance separability You can recover luminance only image

Extensible No bounds on # of progressive stages or low resolution

stages


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Common Mode of Operations DCT based process

Source image: 8-bit for each color component

Sequential operations

Huffman coding 2 AC and 2 DC tables

Decoders process scans with 1, 2, 3, 4 components E.g. CMYK

Interleaved and non-interleaved scans Non-interleaved scans

One 16x16 Y, 8x8 Cr, 8x8 Cb Y1, Y2, Y3 Y16, Cr1, Cr2, Cr3, Cr4, Cb1, Cb2, Cb3, Cb4

Interleaved scans

Y1, Y2, Y3, Y4, Cr1, Cb1, Y5, Y6, Y7, Y8, Cr2, Cb2


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JPEG Baseline Coder Sequential DCT coding

FDCT QDPCM

Zig-zagscan

DCHuffman

ACHuffman

8*8block

Quantmatrices

DC

AC

Codebooks

Codes

Codebooks

DC inv-Huffman

AC inv-Huffman

IDPCM

IQ IDCT

DC

AC

Quant

matrices

8*8recons

block

DCoffset

DC

offset


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Main Steps of JPEG Baseline Input data is 8-bits DC offset

Level shift by substracting 2n-1

For 8-bit, subtract 128 to remove DC level Only affects DC value, shift a neural gray intensity to zero

DCT Precision of DCT is unspecified IDCT precision: IEEE 1180.1 (withdrawed now)

DCT on 8x8 blocks Using blocks has the effect of isolating each block from its neighboring context.

This is why JPEG images look choppy blocky when a high compression ratio

Quantization Quantized DCT coefficients are restricted 11 bits

Zigzag scan VLC

DC: DPCM

AC: VLC


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Differential DC and Zig-zag Scan

wheres are quantized values.

- ZigZag scan to produce long run of zeros

),1()()( = iDCiDCiDiffDC

)(

DC


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QuantizationCustom quantization table: Visibility threshold

{Q (u, v )}, user specified, range [1 ~ 255]

Quantized coeffs.: (11 bits: [-128 x 8, 127 x 8])

Fq(u, v) = round ;

Inverse

R(u, v) = Fq(u, v) Q(u, v)

),(),(

vuQvuF


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Psychovisual Aspects Small variation in intensity is more visible in flat area

More visible in luminance


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Examples of Quantization Table ISO / IEC 10918-1 Annex K

Luminance table Chrominance table16 11 10 16 24 40 51 61 17 18 24 47 99 99 99 9912 12 14 19 26 58 60 55 18 21 26 66 99 99 99 9914 13 16 24 40 57 69 56 24 26 56 99 99 99 99 99

14 17 22 29 51 87 80 62 47 66 99 99 99 99 99 9918 22 37 56 68 109 103 77 99 99 99 99 99 99 99 9924 35 55 64 81 104 113 92 99 99 99 99 99 99 99 9949 64 78 87 103 121 120 101 99 99 99 99 99 99 99 9972 92 95 98 112 100 103 99 99 99 99 99 99 99 99 99


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Baseline Huffman Coding


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Symbol-1 Structure


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Huffman DC Coding (cont.) Amplitude

coded using variable-length-integer (VLI) simply a shifted binaryrepresentation of the size bits

(1s Complement form)

Size = 0, no additional bits are required Huffman table for DC can be user specified or default

If user specified, transmit the table in the file header


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Huffman AC Coding (cont.) Size: number of bits of the amplitude (11 categories)

Before coding, merge runlength and size information together.Then, (runlength, size) is coded using VLC. (Example in AnnexF) RLC (runlength, size) = [#-zeros-to-skip , next non-zero value]

Amplitude is coded using VLI.


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JPEG Default AC Huffman Table

(for Luminance) Symbol 1


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Symbol 2


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Examples After DCT and quantization

0

0

00001001

0001111100012123

012313520

[20,5,-3,-1,-2,-3,1,1,-1,-1,0,0,1,2,3,-2,1,1,0,0,0,0,0,0,1,1,0,1,EOB]

29: previous DC

-9 (0,5) (0,-3) (0,-1) (0,-2)

(0,-3) (0,1) (0,1) (0,1)

(0,-1) (0,-1) (2,1) (0,2)

(0,3) (0,-2) (0,1) (0,1)(6,1) (0,1) (0,1) (1,1) EOB

Zigzag scan

Run levelDPCM (101 0110/100 101/01 00/ /1010)


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Additional JPEG Coding Modes Sequential DCT mode Baseline

Progressive DCT mode

Spectral selection Low freq components of all blocks aretransmitted first; then, higher freq. components.

Successive approximation The most significant (ms) 1-bits of every coeff is transmitted; then, the next ms bit.

Hierarchical mode: Using up- and down-sampling to

increase/decrease spatial resolution. Lossless: DPCM Prediction errors are coded using VLC.

Arithmetic coding are used in other modes than baseline

More complex, but extra 5 ~ 10% bit rate reduction


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Transmission of DCT Coefficients


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Hierarchical Coding

L l C di (R l d b


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Loseless Coding (Replaced by

JPEG-LS)

2:1 compression

MMC_07_Still image compression.pdf

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