khalil imagecompression

8/13/2019 Khalil ImageCompression

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o LosslessInformation preserving

Low compression ratios

o LossyNot information preserving

High compression ratios

Trade-off: image quality vs compression ratio


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oLossless algorithms do not change the content of a file. If you compress afile and then decompress it, it has not changed.The following algorithms are lossless:

Flate/deflate compressionHuffman compressionLZW compression

RLE compression

oLossy algorithms achieve better compression ratios by selectively gettingrid of some of the information in the file. Such algorithms can be used forimages or sound files but not for text or program data.The following algorithms are lossy :

JPEG compressionMPEG compression


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Run Length encoding (RLE).

Lample Zie welch (LZW). Flate / Deflate .

Huffman .

JPEG .


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RLE is probably the easiest compression algorithm there

is. It replaces sequences of the same data values within a

file by a count number and a single value. Suppose the

following string of data (17 bytes) has to be compressed:

ABBBBBBBBBCDEEEEF

Using RLE compression, the compressed file takes up 10bytes and could look like this:A*8BCD*4EF


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Input: string of characters

Internal: dictionary of (codewords, words)

Output: string of codewords and characters.Codewords are distinct from characters.

In algorithm, w is a string, cis character and

w+cmeans concatenation.

When adding a new word to the dictionary, anew code word needs to be assigned.


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Huffman compression belongs into a family of algorithms with a variable

codeword length. That means that individual symbols (characters in a text

file for instance) are replaced by bit sequences that have a distinct length.

So symbols that occur a lot in a file are given a short sequence while other

that are used seldom get a longer bit sequence.

A practical example will show you the principle: Suppose you want to

compress the following piece of data: ACDABA

Since these are 6 characters, this text is 6 bytes or 48 bits long. With

Huffman encoding, the file is searched for the most frequently appearing

symbols (in this case the character Aoccurs 3 times) and then a tree isbuild that replaces the symbols by shorter bit sequences. In this particular

case, the algorithm would use the following substitution table: A=0, B=10,

C=110, D=111. If these code words are used to compress the file, the

compressed data look like this:01101110100


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The JPEG algorithms performs its compression in four phases :

1) First, the JPEG algorithms first cuts up an image in separate blocks of 88

pixels. The compression algorithm is calculated for each separate block,

which explains why these blocks or groups of blocks become visible when

too much compression is applied.

2) The next step in the compression process is to apply a Discrete Cosine

Transform (DCT) for the entire block. DCT is a complex process that is let

loose on each individual pixel. It replaces actual color data for each pixelfor values that are relative to the average of the entire matrix that is being

analyzed. This operation does not compress the file, it simply replaces 88

pixel values by an 88 matrix of DCT coefficients.


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3) Once this is done, the actual compression can start. First the compression

software looks at the JPEG image quality the user requested (e.g. Photoshop

settings like lowquality, mediumquality,) and calculates two tables of

quantization constants, one for luminance and one for chrominance. Once thesetables have been constructed, the constants from the two tables are used to

quantize the DCT coefficients. Each DCT coefficient is divided by its

corresponding constant in the quantization table and rounded off to the nearest

integer. The result of quantizing the DCT coefficients is that smaller,

unimportant coefficients will be replaced by zeros and larger coefficients will

lose precision. It is this rounding-off that causes a loss in image quality.


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4) The resulting data are a list of streamlined DCT coefficients. The last step in

the process is to compress these coefficients using either a Huffman or

arithmetic encoding scheme. Usually Huffman encoding is used.


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By putting 2 compression algorithms on top of each other, JPEG achieves

remarkable compression ratios. The downside of JPEG compression is that the

algorithm is only designed for continuous tone images (remember that the P

in JPEG stands for Photographic). JPEG not does not lend itself for images with

sharp changes in tone.

JPEG Making image files smaller is a plus for transmitting files across networksand for archiving libraries of images. Being able to compress a 2 Mbyte full-

color file down to, say, 100 Kbytes makes a big difference in disk space and

transmission time! And JPEG can easily provide 20:1 compression of full-color

data. JPEG is that it stores full color information: 24 bits/pixel (16 million

colors).

After the above, we conclude that the best choice for image

compression is JPEG algorithm .

But!!!!! (thatscorrect before 2000)


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The JPEG2000 compression algorithm was released by an ISOstandardization committee in January 2000. It is based on the

existing JPEG specifications and adds important things to the

existing standard.

The JPEG 2000 is a new image coding system that uses state-of-

the-art compression techniques based on wavelet technology.


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o DWT= intra-component decorrelation

concentrate image energy in a small area

o No blocking artefacts at high compression ratios

o

Enables multi-resolution image representation


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Improved compression efficiency.

Lossy to lossless compression.

Single compression / Multiple decompression.

Scalability :

- Resolution.

- Quality (SNR scalability).

Region of Interest Coding (ROI). Error Resilience.


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JPEG JPEG2000


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Compression

Decompression

storage


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+

+

2.6 kBytes (1:100)


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1.6 %download

8.0 % download

2.2 % download

26.4 % download

100 % download


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Embedded error : 16 bytes set to 0 in the middleof the compressed file.

JPEG JPEG2000


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JPEG 2000 offers numerous advantages over the old JPEGstandard. One main advantage is that JPEG 2000 offers bothlossy and lossless compression in the same file stream, whileJPEG usually only utilizes lossy compression.

A second advantage of JPEG 2000 over JPEG is that JPEG 2000

is able to offer higher compression ratios for lossy compression. Another advantage of JPEG 2000 is its ability to display images

at different resolutions and sizes from the same image file. WithJPEG, an image file was only able to be displayed a single way,with a certain resolution.

SO , the best choice for image compression isJPEG2000 algorithm


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