2.1Si23_03

SI23Introduction to Computer

Graphics

SI23Introduction to Computer

Graphics

Lecture 2 - Images

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Course OutlineCourse Outline

ImageDisplay

URLGIMP

colour

2D vectorgraphics

URL SVGViewer

lines,areas

graphicsalgorithms

interaction

VRMLviewer

3DGraphics

URL

surfaces

viewing, shading

GraphicsProgramming

OpenGLAPI

animation

2.3Si23_03

Course Outline – Images and Colour

Course Outline – Images and Colour

Image creation, manipulation and display

Colour

URLGIMP

colour

ImageDisplay

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Graphics Display Hardware -Cathode Ray

Tube

Graphics Display Hardware -Cathode Ray

Tube

phosphorcoatedscreen

electron beam

heated cathode

focus deflectionplates

Basic technology forworkstation screens is thecathode ray tube

Beam scans acrossscreen in seriesof lines - 60 times per second

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Colour Raster Scan DisplayColour Raster Scan Display

Colour CRTred

greenblue

metal mask

Colour composed ascombination of RED, GREEN, BLUE

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Colour Frame BuffersColour Frame Buffers

1

110

11

0Today most colour displayshave 24bit frame buffersor pixmaps.

So each pixel has a 24bitcolour value associatedwith it - 8 bits for R,G,B.

01101101 11011001 10011100

red gungreen gunblue gun

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Colour Look Up Table (CLUT)

Colour Palette

Colour Look Up Table (CLUT)

Colour Palette

8-bit frame buffer 1 0 1 0 0 1 1 1 Pixel value (=167)

167

.

.

11001100 10110110 11011100

.

...

.

.

.

...

.

...

red green blue

24-bit colour table

Pixel value acts as an index into the look up table. Thisallows 256 (=28) on-screen colours from a palette of 16.7 million

To save on memory costs, some displays use a look-up table …

2.8Si23_03

Gamma CorrectionGamma Correction

For CRT displays, brightness is not a linear function of the drive signal

Instead it is related by:Brightness = (Signal)

Typical value for is 2.2 Thus displays will provide ‘gamma

correction’ of the red, green and blue signals to give the expected uniform effect

Note: assumes signal in range [0,1]

2.9Si23_03

HomeworkHomework

How does colour work for LCD displays??

2.10Si23_03

ImagesImages

We can generate images in a variety of ways…

… and we can subsequently process them in order to …

The GIMP will help us do many of these things

Fill in the answers!

2.11Si23_03

Image FormatsImage Formats

Large number of formats have emerged over the years

Why? Which are in

common use? Use a couple of

test pictures

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Silicon Graphics RGB Format

Silicon Graphics RGB Format

File begins with a header which has the following information:

– xsize, ysize - dimensions of image– zsize : 1 for 8-bit (colour index); 3 for 24-bit

colour– colour map flag: indicates image or colour

map

The rest of the file contains the data– for 8-bit, each pixel represented as 1 byte, so

there are ysize scan lines, each of xsize bytes– for colour, three single images (for R, G, B)

are stored, one after the other.

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Image SizesImage Sizes

The picture of Bank’s Peninsula is 2472 by 1704 pixels

– as RGB image it is 12 Mbytes

The diagram with the square is 256 by 256 pixels

– As RGB image it is 193k bytes

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Compression - RLECompression - RLE

Consider the triangleon right.Suppose 0 representsblack and 1 representswhite.

The image file will look like, for each scan line:000000000000111111111111110000000000

It would be more efficient to store as ‘runs’ of pixel values:12*0,14*1,10*0

This is known as Run Length Encoding (RLE)and is an option in the RGB image file specification.

2.15Si23_03

RLE CompressionRLE Compression

The NZ photo reduces from 12 Mbytes to 9.5 with RLE compression

Is this what you’d expect?

The square reduces from 193k to 13k with RLE compression

Is this what you’d expect?

2.16Si23_03

Image FormatsImage Formats

RGB is one of a large number of image formats – MS Windows bitmap for PC applications

(BMP format)– GIF, JPEG, PNG are widely used across

different platforms RLE is just one of a number of

compression techniques - different formats use different compression methods

2.17Si23_03

Image Formats - GIFImage Formats - GIF

GIF (Graphics Interchange Format)– probably the most common image format – developed by CompuServe Inc – 8 bit colour indices point to entries in a

colour table of size 256– thus in uncompressed form, we have

header comprising colour table, followed by rows of data, one byte per pixel

– rows stored as 0,8,16,..; 4,12,20,..; 2,6,10,..; 1,3,5,... this interlacing gives progressive image display

– 24 bit colour images not supported– Animated GIFs a very useful feature (used

with care)

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LZW CompressionLZW Compression

GIF uses the LZW (Lempel-Ziv-Welch) compression technique which has been subject of frequent legal battles -UNISYS and CompuServe claim to have patented it (so not supported fully in Windows GIMP)

Dictionary-based– frequently occurring patterns of data are

identified and given code words which are shorter than the original pattern

For normal images, typically better than RLE; not good on noisy images (neither is RLE)

For file size comparison, see PNG later

2.19Si23_03

Image Formats - PNGImage Formats - PNG

PNG = Portable Network Graphics

Created as a successor to GIF, free of legal restrictions

Defined as a Web standard by W3C

.. And better than GIFs!

Key features:– Transparency– Gamma correction– Text– Interlacing– Filtering– Compression

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PNG FeaturesPNG Features

Transparency– Alpha channel in

addition to RGB

Gamma Correction

– Store the gamma value at creation

– Correct at display time

– Can you work out the correction formula?

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PNG FeaturesPNG Features

Text– Why would it be

useful to associate text with an image?

Interlacing– 2d interlacing– 7 passes (Adam7)– Why is this

useful?

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PNG FeaturesPNG Features

Filtering– Simple operator

to promote good compression

– Eg sub filter replaces pixel values on scan line, with differences from predecessor

Compression– Uses LZ77

approach (recall from SI11)

– Plus Huffman encoding (also from SI11)

– Used in gzip

Encoding is: interlace, filter, compress

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ExampleExample

NZ photo does not compress particularly well

– Uncompressed 12 MB

– RLE 9.5 MB– PNG 4 MB

Square compresses outstandingly well

– Uncompressed 193 kB

– RLE 13 kB– PNG 1 kB

2.24Si23_03

Image Formats - JPEG File Interchange Format (JFIF)Image Formats - JPEG File Interchange Format (JFIF)

Developed as an international standard interchange format using a compression method developed by Joint Photographic Experts Group (JPEG)

Supports 24 bit colour, or greyscale – but not palette

Copes with noisy data, so can handle scanned images

2.25Si23_03

JPEG CompressionJPEG Compression

Discrete Cosine Transform (DCT) applied to 8x8 subblocks of image– DCT transforms data values to a new

set of 64 data items (in a frequency space)

– ordered in importance (1st value represents average over subblock, etc, values at end represent high frequency, ie noise)

– ‘safe’ to discard these noise terms and store only the early terms from the DCT

2.26Si23_03

JPEG CompressionJPEG Compression

Resulting values are then compressed using Huffman encoding

Free of any legal problems Known as lossy compression,

because we cannot recreate exact original image (cf RLE and LZW which are lossless)

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JPEG Quality ControlJPEG Quality Control

JPEG allows user to tune the quality of encoding by a ‘quality setting’– corresponds to ignoring values in the

DCT– Q-factor from 0 to 100 (100 = high

quality) Suggested technique:

– encode using Q factor of 75%, decode and redisplay

– if defects seen, increase Q and re-encode

– if acceptable, decrease Q until just acceptable

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ExampleExample

For the NZ photograph, we have outstanding compression

– Uncompressed 12MB

– RLE 9.5MB– JPEG 339kB

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Image Transfer across WWW

- Summary

Image Transfer across WWW

- Summary

GIF format still predominates – lossless, 8-bit palette-based, LZW

compression, legal status in doubt, supported by all browsers

PNG– lossless, 8-bit palette or 24 bit colour, non-

patented compression, transparency, gamma correction, 2d interlacing, filtering

JPEG– lossy, 24-bit, non-patented compression

Choice:– PNG for synthetic images– JPEG for photographic images

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Reading MoreReading More

The source of PNG information– http://www.w3.org/Graphics/PNG/

Test your browser support for PNG transparency– http://trific.ath.cx/web/png/

What is difference between GIF87a and GIF89a?