COMPRESSION TECHNIQUES
1. INTRODUCTION
- There are many different compression techniques, and they will be
deployed in different situation depending on the nature of the data .
- A compression ratio gives an indication of the success of a compression
method in reducing size. It is calculated by dividing the size of the
original file.
DIFFERENT COMPRESSION TECHNIQUES
1. LOSSY COMPRESSION
- Lossy compression is a term used to describe compression techniques
that result to loss of data.
- That is data is permanently removed from the file. When decompressed,
the data that was removed types of file, is recreated from the data
that remains in the file. The re-created data will almost certainly not be
the same as the original data. Instead, it will be an approximation of the
original data.
- This may seeem a very strange concept, but for many types of files
and approximation of data is good enough.
- Many of the sound and image file formats uses lossy compression ;
for example JPEG and MP3 are both lossy formats
SOME LOSSY COMPRESSION ALGORITHM
I.
JPEG : DCT-Based image coding standard
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JPEG enables a compression technique which is able to do compressing
continous-tone image or data along with pixel having depth of 6 to 24
bits with the enough efficiency and speed.
A Discrete Cosine Transform (DCT) describes a fixed series of the
data points in the terms of sum of the cosine function fluctuate at various
frequencies DCTs are vital for various implementations in science and in
the engineering area, from the lossy compression of audio for example
MP3 and images like JPEG.
By using the cosine instead of a the sine function it is complicated in
the implementations for the compression which it returns found that the
cosine function is so much effective as mentioned here, some functions are
required to the exact a typical signal, while for the specific selection of the
boundary condition.
JPEG image compression performs in the part through rounding off
the non essential bits of the information. Here the is a relationship between
the information loss and the reductionof size. Various number of the
famouscompression techniques have achieved these intuitive differences,
consist of those that are used in the music files, video and images.
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DCT fragment the iùage into the parts of separate frequencies in which
less significant frequencies are cancelled through the quantization process
and more significant frequencies are used for retrieving the image while the
process of decompression .
II.
Image Compression by Walvelet Transform
- For various signals the technique of wavelet transform is very efficient
tool as compare to the fourrier transform technique. The technique of
wavelet transformed enables the multi-resolution represented by the
use of the set of analyzing functions or wavelets.
- The technique of wavelet transform found in various forms.
- These tranform can be represented in the figure below
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III.
ADVANTAGES AND DISADVANTAGES OF LOSSY
-Advantages :
- very small file size and lots of tools, plugins and softwares
support it.
-Disadvantages :
- Quality degrades with higer ratio of compression cannot get
original after compression
LOSSLESS COMPRESSION
- Lossless compression refers to techniques where no data is lost in the
compression process .
- When the file is decompressed, the contents are identical to the original
file(before it was compressed).
- Lossless compression is appropriate when data cannot be approximated.
If you produce an assignment and email it to a teacher, it is important
that when the file is opened it is exaclty the same as the original.
-
When a file is saved, compression is applied automatically to reduce the
file size. When the file is opened, the data is recreated so that it can be
viewed and worked with.
-
When you save a piece of work, you rely on the software to store the
data in the most appropriate form .
There are two types of lossless techniques :
- run length encoding
- (RLE) and Dictionary-Based method
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IV.
Run length Encoding (RLE)
- This is a lossless compression technique. It works by finding ‘runs’ of
repeated binary patterns and replacing them with a single instance of
the pattern and a number that specifies how many times the pattern is
repeated.
- If and image of image size 28 pixel x 28 pixel and it uses 5 Colours. So
3 bits are needed to define 5 colour code (23=8).
- The colour palettte is as follows ;
CODE
000
001
010
011
100
101
110
111
COLOR
White
Pink
Blue
Magenta
Navy
unused
Unused
unused
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NB:
1. Usually, runs are not processed row by row ; there may be long runs
that span several rows
2. The data does not have to be image data. However, this technique
is well suited to image data , especially files with smaller colour
palettes, as these images often have repeated blocks of colour
3. In most RLE schemes, the run is typically encoded into two bytes ;
one byte for the pattern and one byte for the number of repetitions
in a run
4. RLE will only achieve significant reductions in file size if ther are
long runs of data. In the extreme case of being no repeated
patterns, the file size could be double that of the original size.
V.
Dictionary Based method
This compresssion method also relies on finding pattern in data.
However, it does not look for repeated
runs of characters, as the
repetitions do not need to be adjacent to each other, Instead of storing the
pattern itself, a shorter code is substituted.
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VI.
ADVANTAGES AND DISADVANTAGES
-Advantages : no loss of quality slight decrease in image file size
-Disadvantages : Larger file than if you were using lossy compress
SOME SUMMARY
S/N Lossless data compression
Lossy data
compression
1
In lossless data compression there is
no lost of any data and quality
There is lost of data and
quality which is not
measurable
2
The file is restored in it original form
The file is not restored in it
original form
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It is mainly used to compress text,
sound and images
Mainly used to compress
audio video and images
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It holds more data
It hold less data
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High quality file data
File quality is low
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It supports RAW,BMP,PNG
Lossly support
GPEP,GIP and MP3
MP4
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CONCLUSION
There are many compression techniques under the lossly and
lossless method that are used inorder to compress data. Each method is
used inorder to optimise the compression technique.
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