WHY IMAGE COMPRESSION IS REQUIRED?

CONSIDER A STANDARD DEFINITION(SD) MOVIE USING 720×480×24 BIT PIXEL ARRAYS WITH 30 FPS.

A DIGITAL MOVIE IS A SEQUENCE OF VIDEO FRAMES OF WHICH EACH FRAME IS A STILL IMAGE.

FOR 30 FRAMES 30 FRAMES/SEC×(720×480) PIXELS/FRAME×3

BYTES/PIXEL=31104000 BYTES/SEC. FOR A 2 HR MOVIE 31104000 BYTES/SEC×(60)^2 SEC/HR×2 HRS=2.24×10^11 BYTES

 WHAT ARE IMAGE FORMATS,IMAGE CONTAINERS &IMAGE COMPRESSION STANDARDS?

The JPEG StandardJPEG is an image compression standard

which was accepted as an international standard in 1992.Developed by the Joint Photographic

Expert Group of the ISO/IECFor coding and compression of

color/gray scale imagesYields acceptable compression in the

10:1 range

The JPEG StandardJPEG is a lossy compression technique

Based on the DCTJPEG is a general image compression

technique independent ofImage resolutionImage and pixel aspect ratioColor systemImage complexity

A scheme for video compression based on JPEG called Motion JPEG (MJPEG) exists

BLOCK DIAGRAM OF JPEG

CONSTRUCTING N×N SUB IMAGES WITH DCT

FORWARD &INVERSETRANSFORMS

FOURIER TRANSFORMWALSH HADAMARD TRANSFORMDISCRETE COSINE TRANSFORM

The image is divided up into 8x8 blocks2D DCT is performed on each blockThe DCT is performed independently for each blockThis is why, when a high degree of compression is requested, JPEG gives a “blocky” image result

N

vy

N

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NDCT

N

y xy

N

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2

)12(cos

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1 1

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THE SIGNIFICANT FACTOR THAT EFFECTS TRANSFORM CODING ERROR AND COMPUTATIONAL COMPLEXITY IS SUB IMAGE SIZE WHICH ARE USUALLY 8×8 AND 16×16.

RECONSTRUCTION ERROR VERSUS SUB IMAGE SELECTION

BIT ALLOCATION THE RECONSTRUCTION ERROR ASSOCIATED WITH THE

TRUNCATED SERIES EXPANSION IS A FUNCTION OF NUMBER AND RELATIVE IMPORTANCE OF TRANSFORM COEFFICIENTS THAT ARE DISCARDED,AS WELL AS THE PRECISION THAT IS USED TO REPRESENT THE RETAINED COEFFICIENTS.

RETAINED COEFFICIENTS ARE SELECTED BASED ON TWO TYPES NAMELY

ZONAL CODING THRESHOLD CODING

BIT ALLOCATION CODINGSZONAL CODING IS DONE BASED ON THE

MAXIMUM VARIANCE.THRESHOLD CODING IS DONE BASED ON

THE MAXIMUM MAGNITUDE.

CONSIDER A 8×8 SUB IMAGE

LEVEL SHIFTING THE PIXELS

BY FORWARD DCT

NORMALIZATION OR QUANTIZATION MATRIX

TRANSFORMED ARRAY AFTER QUANTIZATION

ZIG-ZAG ORDERING OF JPEG IMAGE

DCT COEFFICIENT MATRIX

AFTER DENORMALIZATION

INVERSE DCT TRANSFORM

ADDING 128 TO THE ENTRY

DIFFERENCES BETWEEN ORIGINAL &RECONSTRUCTED SUB IMAGE

Examples of varying JPEG compression ratios

CONCLUSION BASED ON THE COMPRESSION RATIO

IMAGES WITH LESS MEMORY ARE OBTAINED.

THERE ARE JPEG COMPRESSIONS WHICH ARE LOSSLESS USED IN MEDICAL IMAGING , SPACE

&RESEARCH FIELDS.MEDIA COMPRESSION IS INDISPENSABLE

EVEN AS STORAGE AND STREAMING CAPACITIES INCREASE