Computing Theory

8/20/2019 Computing Theory

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GCIT 1007 - IT and Digital MediaLecture 2 – Image I


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To Print a Large Banner

You want to print a large banner of Hong Kong Baptist University

You download the logo from BU's website, is it OK?

The image turns out to be

very chunky when enlarged

• The vector version will

give you better quality


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Today’s Outline

Bitmap vs. Vector

Image Compression (run length)

Color models


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Representation of Bitmap images

Also called raster images

Made up of pixels (tiny dots of individual color) in a grid

a pixel

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::

What is inside the file?


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Representation of Vector images

Made up of a collection of lines, curves and shapes

And editable attributes such as color and outline

These information are written in graphics language like SVG

What is inside the file?


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• Zigzag edges becomenoticeable when an image isenlarged as pixels arerectangular shape

• Printing / displaying qualitydepends on resolution

- number of pixels in animage

- printing / screen resolution

• Not restricted to arectangular shape

• Resolution independent

Bitmap vs. Vector Scalability

Bitmap Vector


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Bitmap vs. Vector File Size

• File size is usually large

- Must record the value of everypixel, one by one

- Many repeated information ifpixels are having the samecolors

Bitmap

• File size is usually small- Information is more

compact

Vector

File size

= total no. of pixels

x no. of bytes used to

represent a color= 100 x 120 x 3

= 36000 bytes

100 pixels

120 pixels

File size = 86

bytes


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Bitmap vs. Vector Editable Components

• Not easy to edit

Bitmap

• Components can be editedindependently

Vector


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• DO NOT inherently supporttransparency

• Transparency depends onfile type

• Transparent, no background

Bitmap vs. Vector Transparency

Bitmap Vector


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• Mainly for Photo-realisticimages

• Images are usually made upof solid areas of color orgradients, but they CANNOTdepict the continuous subtle

tones of a photograph• Mainly for logo / cartoon-like

designs and drawing

Bitmap vs. Vector Usage

Bitmap Vector


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• All images from digitalcameras and all scannedimages are bitmaps

• Vectorization- convert bitmap to vector

format

- e.g. Trace Bitmap in Flash

Bitmap

Bitmap vs. Vector Conversion

• Fonts are vector objects

• Rasterizing

- Convert vector to bitmap

- e.g. Rasterize Type inPhotoshop

Vector


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• Microsoft Paint

• Adobe Photoshop

• Gimp

:

:

:

• Adobe Illustrator

• CorelDRAW

• Inkscape

:

:

:

Bitmap vs. Vector Editing Tools

Bitmap Vector


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Bitmap vs. Vector Summary

Bitmap Vector

Representation pixels geometry objects

Scalability

File Size

Editable Component

Transparency

Usage photologo / design /

drawing


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IMAGE COMPRESSION


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How to store an image?

Consider the images contain only black and white, not grayscale.

How to store thispicture in computer or

other electronic

devices?


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Digitization

Image is partitioned into small grids

We use 1 to represent a black grid and 0 to represent a whitegrid

The grid lines are just for reference, not

in the original image


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Compression?

Given this data, we can

save the data from left to

right, then from top to

bottom

It takes 10 x 10 = 100 bits

Can we compress it?

10

10


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Solution

COUNT the number of consecutive 0, then the number of consecutive 1…

54, 2, 7, 4,

5, 2, 2, 2,3, 8, 1, 10


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Run-length Encoding

If we use 6 bits to represent each number, it takes us

12 x 6 = 72 bits !!!

100 – 72 = 28 bits are saved!!!

54, 2, 7, 4,5, 2, 2, 2,

3, 8, 1, 10

54 110110

2 000010

7 000111

.. …


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Applications of Run-length Encoding

Fax machines

Since letter contains lots of white

space, run-length encoding is

very effective in Fax machine

Also used in

BMP, PCX, PICT files


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Decoding

110110

000010000111

…

54, 2, 7, 4,

5, 2, 2, 2,3, 8, 1, 10

Data

Information


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Further Improvement?

Can we? Yes!!!

Use 5 bits to present each number.

Because 6 bits are not necessary for most numbers

– E.g. 2 = 10(2) , 7 = 111(2)

Space can be saved with 5 bit representation


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Further Improvement?

But how about 54?

54 = 110110 (2) – 6 bits are needed to represent the whole number

The trick is:

54 31, 0, 23

11111, 00000, 10111

Then the sequence becomes

– 31, 0, 23, 2, 7, 4, 5, 2, 2, 2, 3, 8, 1, 10

So it takes 14 x 5 = 70 bits. < 72!!


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Other Compression Methods

JPEG is using Predictive DCT (Discrete Cosine Transform)

– A lossy compression method

GIF is using LZW

– Patented of Abraham Lempel, Jacob Ziv, and Terry Welch

– A lossless compression method

PNG is using Phil Katz Deflate compression method

– Phil Katz is famous for creating the PKZIP program

DCT, LZW and PK Deflate are too complex to be covered here


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COLORS


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RGB Color Model

Three primary colors, RGB (Red, Green, Blue), mixed up in different

proportions to produce all of the possible colors

Adding color light beams but NOT adding paint

An additive color model

– White is the additive combination of ALL primary colored light

– Black is the absence of light For Display Monitors


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RGB Color Model

How much Red, Green and Blue?

3 numbers (each range from 0 to 255)

for the amount of RGB

255, 0, 0 0, 255, 0 255, 255, 0

0, 0, 255

0, 255, 255255, 0, 255

0, 0, 0

255, 255, 255

For a RGB System, 8 bits

for each of the R, G an BTotally 8 + 8 + 8 = 24 bits of

different colors, known as

true colors ( 16,777,216

colors)

In Windows, true color is 32

bits as there is 8 bits

allocated for the alphachannel for “transparency”

value (RGBA)


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CMYK Color Model

Combining four colors: Cyan, Magenta, Yellow, and Key (blacK)

Each component measured in percent from 0 to 100

Possible to achieve a large range of colors

Adding paint

– White is the natural color of the paper / background

– Black results from a full combination of ALL colored inks

A subtractive color model

– Use reflected light to display color

For Color Printing


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HSB Color Model

Three fundamental properties of color

For old color TV

(non-HD TV)

Hue• Color

• Measured in degrees from

0 to 360• 0 is red, 60 is yellow,

120 is green, 180 is cyan,

240 is blue, 300 is magenta

Saturation• Purity: the amount of pure

color mixed with white color

• Measured in percent from

0 to 100

• The higher the percentage,the more pure the color

Brightness• Intensity: the amount of pure

color mixed with black color • Measured in percent from 0 to

100

• The higher the percentage,

the brighter the color


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Color Depth

Number of bits to describe the

color of one pixel in a bitmap file

More number of bits used

– more possible colors can be represented

– larger file size

n bits = 2n colors

24 bits – millions of colors

4 bits – 16 colors

1 bit – only black or white


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GCIT1007 IT and Digital Media, Spring 2015

All copyright and intellectual property rights, without limitation, are retained byDepartment of Computer Science, HKBU (http://www.comp.hkbu.edu.hk).

References

J. Straubhaar, R. LaRose, L. Davenport, Media

Now: Understanding Media, Culture, and

Technology, 7th Edition, Wadsworth Publishing,

2012. Nigel Chapman and Jenny Chapman, Digital

Multimedia, 3rd Edition, Wiley, 2011.

Documents

Computing Theory