Refers to reducing the amount of data required to
represent a digital image.
Principle of compression:
reduce the redundant information, e.g.,
Coding redundancy is present when less than optimal code
words are used.
Interpixel redundancy results from correlations between
the pixels of an image.
Psychovisual redundancy is due to data that is
ignored by the human visual system
(i.e. visually non essential information).
Principal objective: To minimize
the number of bits required to
represent an image.
*Intended to yield a compact
representation of an image.
*Reducing the image storage.
Image data compression methods
fall into two common categories:
* A lossy compression method is one where
compressing data and then decompressing it
retrieves data that may well be different from
the original, but is close enough to be useful
in some way.
* Used to compress multimedia data (audio,
video, still images), especially in applications
such as streaming media and internet
Provide higher levels of data reduction
Result in a less than perfect reproduction
* Also called Information preserving
* Compress and decompress images without
*In this coding scheme, transforms such as DFT
DCT are used to change the pixels in the original
image into frequency domain coefficients (called
*These coefficients have the energy compaction
property i.e. energy of the original data being
concentrated in only a few of the significant
transform coefficients. Only those few significant
coefficients are selected and the remaining are
* Joint Photographic Experts Group created the
* Lossy Compression Technique based on use of
Discrete Cosine Transform (DCT)
Steps in JPEG Compression
*Divide Each plane into 8x8 size blocks.
*Compute DCT of each block
*Treat separately DC components of each
*Apply Quantization to discard values
*Separately encode DC components