Computer
Systems
Nat 4.5 Computing Science
Data Representation
Lesson 4:
Storing Graphics EXTENSION
Data Representation
Computing Science
Bitmap and Vector Graphics
National 5
Colour Bitmaps
 The
colour bitmap method is exactly the
same as for black and white with one
difference.
 Each
pixel is not black and white but can
represent a variety if colours.
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Colour Bitmaps
 Each
pixel has a binary value
representing the colour. The amount of
colours is known as the bit depth.
 So
an image with 8 bit colour depth could
have 256 Colours
 True Colour is defined as an image with 24bit
colour depth. 16,777,216 colours!
National 5
True Colour
 True
Colour is defined as an image with
24bit bit depth.

This means that 16,777,216 colours can be
represented.
 The
colour code for each pixel is constructed
of a single 8 bit number for each of the main 3
additive colours.

Red, green and blue
RGB Colour Codes
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Increasing Bit Depth
Higher colour
Depth = More
Colours
File Size
Increases
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Storage Space Example
A true colour image is 800 pixels by
900 pixels.
Calculate the storage requirements
and
express
the
answer
in
appropriate units
.
 Step
1: (Length x Breadth) * bit depth
 (800
 Step
* 900) * 24bits = 17,280,000 bits
2: Convert into appropriate units
 17,280,000/8
= 2,160,000 bytes
 2,160,000 bytes /1024 = 2,109.375 Kb
 2,109.375 Kb/1024 = 2.06 Mb
National 5
Alternate Storage Space Example
 Sometimes
you will be given the size and the
resolution of the image.
 One way in which this can be measured is dpi


Dots per inch is the amount of pixels in an inch.
A 16bit colour image is 4 inches by 6 inches with a
resolution of 300dpi.
 Step

((4*300) * (6*300))* 16 = 2,160,000 pixels
 Step


2: number of pixels * bit depth
2,160,000 * 16 = 34,560,000 bits
 Step

1: ((Length x dpi) x (Breadth x dpi)) * bit depth
2: Convert into appropriate units
34,560,000 /8 = 34,560,000 bytes
34,560,000 bytes /1024/1024 = 4.119 Mb
National 5
Bitmap Graphics – Pros and Cons
Advantages
 Can be manipulated
at pixel level
 Can create a wide
array of graphic effects
 Can represent photorealistic images
Disadvantages
 Requires large storage
space
 Image becomes
jagged when scaled
National 5
Vector Graphics – Pros and Cons
Advantages
 Do not lose quality
when scaled
 Require less storage
space
 Objects are easily
moved/manipulated
 Resolution
independent
Disadvantages
 Cannot be edited at
pixel level
 Cannot show photo
realistic scenes
 Will usually require
particular applications
to open