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
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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