Image Compression Standards

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Image CompressionStandards : 

Image CompressionStandards By A.Raju NIT Warangal

What’s Image Compression? : 

What’s Image Compression? Reduction of the amount of data removal of redundant data Transforming a 2-D pixel array into a statistically uncorrelated data set

Why Compression? : 

Why Compression? Important in data storage and data transmission Examples: – Progressive transmission of images/videos – Video coding (HDTV, teleconferencing) – Digital libraries and image databases – Remote sensing – Medical imaging

Why Standardization? : 

Why Standardization? Compression is one of the technologies that enable the multimedia revolution to occur. However for technology to be effective there has to be some degree of standardization so that the equipment designed by different vendors can talk to each other. These standards are accepted by ISO (International Standards Organization)and ITU(International Telecommunications Union) once called CCITT (Consultative committee of the International Telephone and Telegraph).

Image Compression Standards : 

Image Compression Standards

Bi-Level Still Images : 

Bi-Level Still Images CCITT Group3:Designed for transmitting binary documents over telephone. (FAX) CCITT Group4:Simplified version of CCITT Group 3 JBIG or JBIG1:Joint Bi-level Image Experts Group, for lossless compression of bi level images JBIG2:For internet and FAX applications, internet, desktop TIFF:Tagged Image File Format. Flexible file format supporting variety of compression standards including JPEG, JPEG2000

Continuous Tone Still Images : 

Continuous Tone Still Images JPEG: Joint Photographic Experts Group. Popular for compression of images on Internet. JPEG-2000: Follow on to JPEG for increased compression of photographic quality images PDF: Portable Document Format. For representing 2-D documents PNG : Portable Network Graphic. For Losslessly compression of full color images.

Video : 

Video DV:Digital Video. For semiprofessional video production, camcorders H.261:2 way video conferencing standard. DCT compression mode. H.262:Designed for DVDs with 15Mb/s H.263:Enhanced version of H.261 for ordinary telephone modems. H.264:For video conferencing, Internet streaming, and TV Broadcasting Uses variable block size integer transforms, adaptive arithmetic coding MPEG-1:Motion Pictures Expert Group. For CD-ROM applications. MPEG-2:Extension for MPEG-1, for DVDs with higher transfer rate MPEG-4:Extension to MPEG-2, supports HDTV, variable block sizes MPEG-4 AVC:Uses Advanced Video Coding (AVC). Identical to H.264

Compression Scheme: : 

Compression Scheme: In any compression scheme there are: Step 1- Removal of redundancy based on implicit assumption about the structure in the data Step 2- Assignment of binary code words to the information deemed non redundant. Transformer Applies a one-to-one transformation to the input image data. Output of the transformer is an image representation which is more efficient compression than the raw image data.

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Quantizer Generates a limited number of symbols that can be used in the representation of the compressed image. Coder Coder assigns a code word, a binary bit stream , to each symbol at the output of Quantization. The coder may employ a Fixed-Length or Variable- Length codes.VLC, also known as Entropy Coding, assigns a codeword in such a way as to minimize the average length of the binary representation of the symbols

JPEG standard : 

JPEG standard JPEG : Joint Photographic Experts Group Standard for continuous tone still images Widely used standard New Standard : JPEG 2000 The standard describes 29 distinct coding systems for compression of images. Four Modes: Sequential (Baseline) JPEG Progressive JPEG Hierarchical JPEG Lossless JPEG Major Steps in JPEG Image Compression • Discrete Cosine Transform (DCT) / IDCT • Quantization / Dequantization • Differential Pulse Code Modulation (DPCM) • Entropy Coding of DC Coefficients • Run-Length Coding (RLC) • Entropy Coding of AC Coefficients

Block diagram for JPEG encoder. : 

Block diagram for JPEG encoder.

Block diagram for JPEG Decoder. : 

Block diagram for JPEG Decoder.

Baseline JPEG : 

Baseline JPEG DC Coefficient : First coefficient in every 8 x 8 block Represents the average value of pixels in block AC Coefficients : Remaining 63 coefficients in every 8 x 8 block DC Coefficients: treated separately from the AC Coefficients Differential values of DC coeffs. of all blocks are derived and encoded. The entries of Q(u,v) tend to have larger values towards the lower right corner. This aims to introduce more loss at the higher spatial frequencies

Example: : 

Example: Original 8x8 image Shifting and DCT Quantization matrix Q(u,v) Quantized DCT coefficient matrix 15

JPEG coefficient coding categories : 

JPEG coefficient coding categories

Suggested Huffman code for DC differences : 

Suggested Huffman code for DC differences JPEG Standard, Table K3 – Luminance

JPEG suggested AC code for luminance : 

JPEG suggested AC code for luminance

JPEG suggested AC code for luminance(cont.) : 

JPEG suggested AC code for luminance(cont.)

Slide 20: 

Progressive: Spectral Selection Spectral selection: Takes advantage of the “spectral” (spatial frequency spectrum) characteristics of the DCT coefficients: higher AC components provide detail information. Scan 1: Encode DC and first few AC components, e.g., AC1, AC2. Scan 2: Encode a few more AC components, e.g., AC3, AC4, AC5. ... Scan n: Encode the last few ACs, e.g., AC61, AC62, AC63.

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Progressive: Successive Approximation Successive approximation: Instead of gradually encoding spectral bands, all DCT coefficients are encoded simultaneously but with their most significant bits (MSBs) first. Scan 1: Encode the first few MSBs, e.g., Bits 7, 6, 5, 4. Scan 2: Encode a few more less significant bits, e.g., Bit 3. ... Scan n: Encode the least significant bit (LSB), Bit 0. Example: 150(10)=10010110(2) 144(10)=10010000(2) 144(10)=10010000(2) 148(10)=10010100(2) 150(10)=10010110(2) 150(10)=10010110(2)

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Hierarchical JPEG The encoded image at the lowest resolution is basically a compressed lowpass filtered image, whereas the images at successively higher resolutions provide additional details (differences from the lower resolution images). Provides Progressive Representations Provides Multiple Resolutions Requires more space

JPEG 2000 Family of Standard : 

JPEG 2000 Family of Standard JPEG(1992) lossy and lossless, DCT based JPEG-LS(1997) lossy and near-lossless prediction-based JPEG2002 part1(2002) lossy and lossless, wavelet-based JPEG2002 part2(2002) Provision for 3D extension JPEG (Started in 2002) Multicomponent and Volumetric images

The JPEG 2000 Standard : 

The JPEG 2000 Standard JPEG 2000 PART 1 Released in 2002 High coding efficiency ( 20% more than JPEG) Wavelet-based compression Seamless lossy-to-lossless compression Highly scalable ( in quality, resolution, component) Easy alteration of the progression order Region of interest coding Random access to code stream portions

The JPEG 2000 Standard : 

The JPEG 2000 Standard JPEG 2000 part 2 Supports arbitrary wavelet filters Supports multi component trasformations Multi component support: Linear block transform ( DCT,KLT,….) 3D wavelet transform Predictive coding Must specify inverse transform, then store 2D wavelet-compressed components

The JPEG-LS Standard : 

The JPEG-LS Standard Released in 1997 Based on non-linear prediction and context-based Golomb-Rice coding Has low complexity Lossless compression is better than JPEG 2000 Provides support for near-lossless compression Does not provide scalability Good choice for on-board compression

Various standards : 

Various standards Film 8mm, 16mm, 35mm, IMAX TV NTSC, PAL, SECAM, HDTV VideoTape VHS, S-VHS, Hi-8, Dig-8, DV Compression MJPEG, MPEG (1,2,3,4) File Formats AVI, MPG, VCD, DVD, MOV

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