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
255
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213212
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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.