Image Compression

Image Compression

by

Mohamed Hagras

2

Contents

What is Image Compression? Why Image Compression? Lossless and Lossy Techniques Bitmap Format Graphics Interchange Format Portable Network Graphics Format Tag Image File format JPEG JPEG 2000

22/05/2010 Image Compression

3

Contents

Video Compression PrincipalsFramesTypes of frames• I-frames• P-frames• B-frames• PB-frames• D-frames

Conclusion


4

What is Image Compression?

A technique used to reduce the volume of information to be transmitted about an image.


M

Width 1 Unit

M

1/2

D

Height 1 Unit

M=Number of Samples

D=Spacing Between Samples

5

Why Image Compression?

Requirements may outstrip the anticipated increase of storage space and bandwidth.

For data storage and data transmission:DVDReal time applicationsPrinter

Take advantage of :Spatial redundancyTemporal redundancy (videos)


6

Lossless And Lossy Compression

Lossless Compression

Lossy Compression

Definition There is no information loss. The image can be reconstructed exactly the same as the original.

Information loss is tolerable.

e.g RLE JPEG 2000

Applications

Medical imagery Archiving

Commercial distribution (DVD). Rate constrained environment where lossless methods can not provide enough compression ratio.22/05/2010 Image Compression

7

Bitmap Format

Files with .bmp extension Each pixel is represented by a fixed number

of bits (integral power of 2)


Advantages Disadvantages Simple Widely used Relatively well

documented Free of patents

Minimum or zero compression

8

Graphics Interchange Format

GIF supports transparency and interlacing Transparency is supported by specifying which pixels of

the image is unused and as such, to reuse the background color.

Interlacing creates the illusion of faster loading graphics as the image is presented to the viewer in a series of interlaced frames.

256 colors out of a palette of 224 colors are indexed.

Amongst the oldest formats available. Supports animations.22/05/2010 Image Compression

9

Portable Network Graphics Format PNG Developed to improve upon and replace GIF.

Supports: 2D Interlacing Transparency Variable Color Depth

Uses Lossless Data compression method called DEFLATE.

The only Disadvantage of png is that it doesn’t support Animations.22/05/2010

A PNG image with an 8-bit transparency channel

Image Compression

The same image is overlaid onto a

checkered

background

10

Tag Image File Format Developed by Aldus and Microsoft. Tags used to indicate how data is arrange

and compressed. e.g LZW/RLE compression


Advantages Disadvantages Highly flexible and

platform independent Widely used by

scanning,faxing, word processing

capable of describing bilevel, grayscale, palette-color, and full-color image data

TIFF image can’t have more than 4GB of data

Difficult to write TIFF readers which take advantage of all its features

11

JPEG

Joint Photographic Expert’s Group. International standard for photographs. Lossless/lossy. Based on the facts that :

Humans are more sensitive to lower spatial frequency components.

A large majority of useful image contents change relatively slowly across images.


12

JPEG Encoding


Cr

YCb

QuantizationFq(u, v)

DPCM

RLC

Entropy Coding

HeaderTables

Data

CodingTables

Quant…Tables

F(u, v)

8 x 8

DCTf(i, j)

8 x 8

Zig ZagScan

13

JPEG (Cont)


Advantages Disadvantages

Compression ratios of 20:1 are easily attained.

Doesn’t support transparency.

24-bits per pixel can be used leading to better accuracy.

Doesn’t work well with sharp edges.

Progressive JPEG(interlacing)

Almost always lossy and

No target bit rate

14

Comparative Analysis


a. 256 color bmp b. gif c. jpeg253 KB 4.08 KB 10.1 KB

d. Png e. tiff6.27 K 16.2 KB

15

JPEG 2000 Wavelet based image compression standard


Compressed Image Data

Entropy Encodin

g

Quantization

Forward Transfor

m

Entropy Decodin

g

Inverse Quantizati

on

Inverse Transfor

m

Compressed Image Data

SourceImage Data

Reconstructed

Image Data

16

Wavelet Transform Of An Image

Advantages:

Lossless and lossy compression.

Progressive transmission by pixel accuracy and resolution.

Region-of-Interest Coding. Random codestream

access and processing. Robustness to bit-errors. Content-based

description. Side channel spatial

information (transparency).


17

JPEG vs. JPEG 2000


512x512 image reconstructed after compression of 0.2 bpp using JPEG and JPEG 2000 respectively

18

Moving Images (Videos) Any video can be thought up of as a sequence

of digitized pictures or frames.


Types Of Frame Coding

Intra-Frame Coding(Spatial Redundancy)

Inter-Frame Coding(Temporal

Redundancy) I-Frame P-Frame

B-Frame

JPEG algo applied to each frame independently moving JPEG or MJPEG Typical compression ratios bw 10:1 and 20:1 Not large enough to produce the needed compression ratios

Only a small portion of each frame is involved with any motion, so only info related to those segments is send


I-Frame P-Frame B-Frame PB-Frame Each frame is

coded independently.

Y (luminance) and Cb, Cr (chrominance) matrices encoded separately using JPEG algo.

I-frames frequently used in order to tackle corruption.

N = GOP = no. of frames bw successive I-frames, range(3,12).

Encoded using a combination of either a preceding I-frame or P-frame.

No. of P-frames limited as errors propagate from one p-frame to another.

M = prediction span = no. of frames bw a P-frame and the immediately preceeding I-frame or P-frame.

Their contents are predicted using both past and future frames

Allows for occasional fast moving objects

Provides better motion estimation.

Provides highest level of compression.

As they are not involved in coding of other frames they do not propagate errors.

Two neighboring P- and B-frames are encoded as if they were a single frame.

It increases the frame rate without increasing the resulting bit rate required.

20

Encoding Operation


21

D-Frame

Used in movie / video-on-demand applications.

User may wish to rewind or fast-forward the video.

This requires the compressed video to be decompressed at much higher speeds.

D-frames inserted at regular intervals throughout the stream.

By using only the encoded DC coefficients of each block of pixels in the periodically inserted D-frames, a low resolution sequence of frames is provided at much higher speeds.


22

Performance

Compression ratios for I-frames is similar to that obtained with JPEG and is typically between 10:1 and 20:1 depending on the complexity of the frame contents.

Compression ratios for P-frames and B-frames are higher and depend on the search algorithm used. Typical figures are 30:1 for P-frames and 30:1 through 50:1 for B-frames.


23

Conclusion

Every format has its own advantages and disadvantages.

Depending upon the application the appropriate format should be used.

Bmp is simple but heavy. Only redeeming feature of gif : supports

animations. Png better than the rest in overall terms. Tiff is appropriate where flexibility is needed. JPEG is good for photography. JPEG 2000 is an all encompassing standard.


Thanks For ListeninG

Image Compression

Documents

compression image compression

se image compression

features image compression

png image

512x512 image

kbimage compression

rateimage compression

frame jp