Jpeg & Jpeg 2000

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A PPT about the most important image compression standards

Transcript of Jpeg & Jpeg 2000

JPEG & JPEG 2000

JPEG, JPEG 2000, SPIHTJPEGAcronym for Joint Photographic Experts Group.

It is an Image compression standard.

It employs lossy compression scheme.

Degree of compression can be adjusted allowing a trade off between size and quality.JPEG ENCODING

Basic steps in JPEG encodingStep 1 - Colour space transformationFirst, the image should be converted from RGB into a different colour space called YCBCR.

The YCBCR color space conversion allows greater compression without a significant effect on perceptual image quality

The compression is more efficient because the brightness information, which is more important to the eventual perceptual quality of the image, is confined to a single channel.

Step 2 Downsampling

Humans can see considerably more fine detail in the brightness of an image (the Y' component) than in the hue and color saturation of an image (the Cb and Cr components).

So in this step we reduce the spatial resolution of the Cb and Cr components (called "downsampling" or "chroma subsampling").

Step 3 -Block splitting

After subsampling, each channel must be split into 88 blocks.

This is called as block splitting.

Step 4 - Discrete cosine transformBefore computing the DCT of the 88 block, its values are shifted from a positive range to one centered around zero.

Next a two-dimensional discrete cosine transform (DCT) is performed.

DCT aggregates most of signal to one corner of result

Step 5 QuantizationThe JPEG algorithm uses uniform midtread quantization to quantize the various coefficients.The quantizer step sizes are organized in a table called the quantization table

Quantization output is obtained byExample-

Quantized output

Step 6 CodingThe differences between neighboring labels are encoded rather than the labels themselves.

These differences, can take on a large number of values.

A Huffman code for such a large alphabet would be quite unmanageable.

The JPEG recommendation resolves this problem by partitioning the possible values that the differences can take on into categories.

Thus, category 0 has only one member (0), category 1 has two members (1 and 1), category 2 has four members (3,2, 2, 3), and so on.

The category numbers are then Huffman coded.Decoding

Features of JPEGImage contents with a simple structure lead automatically to smaller files.

The quality of the representation is individually configurable.

A reliable reproduction of colour information, e.g. for background colours, is not guaranteed at larger compression rates.

The compression of monochromatic images (e.g. b/w graphics, technical drawings, etc.) result frequently in clearly perceptible compression artifacts.

JPEG 2000The current JPEG standard provides excellent performance at rates above 0.25 bits per pixel.

However, at lower rates there is a sharp degradation in the quality of the reconstructed image.

To correct this and other shortcomings, the JPEG committee initiated work on another standard, commonly known as JPEG 2000.Improvements over the 1992 JPEG standardSuperior compression performance

Choice of lossless or lossy compression.

Error resilience.

Flexible file format

Encoding of JPEG 2000Steps

Color components transformation

Tiling

Wavelet transform Reversible or irreversible

Quantization

Coding

Application AreasWherever we require image compression we use JPEG or JPEG 2000

Some of the areas are:Digital photography

Medical imaging

Digital cinema

Image archives

SurveillanceSPIHTSPIHT (Set Partitioning In Hierarchical Trees) is computationally very fast and among the best image compression algorithms.

SPIHT AlgorithmFigure1 Parent-child relationship in SPIHT

SPIHT AlgorithmIn the algorithm, three ordered lists are used to store the significance information during set partitioning. List of insignificant sets (LIS), list of insignificant pixels (LIP), and list of significant pixels (LSP) are those three lists.O(i, j) :Set of offspring of the coefficient (i, j),D(i, j) :Set of all descendants of the coefficient (i, j),L(i, j) :D(i, j) -O(i, j)

H :Roots of the all spatial orientation treesSPIHT Algorithm

SPIHT Algorithm

the output binary stream: 11100011100010000001010110000