1. Computer Systems Nat 5 Computing Science Data Representation
Storing Graphics
Computer Systems
Nat 5 Computing Science
Data Representation
Lesson 4b:
Storing Graphics EXTENSION

2. Lesson Aims Pupils at National 5 level will be able to:
Describe how a vector image is stored
Calculate the storage requirements of a bitmap image with colours
Explain the difference between a vector and bitmap image
Explain the advantages and disadvantages of bitmap images compared to vector images

3. Vector Graphics This image is an example of a vector image.
Nat 5
Vector Graphics
This image is an example of a vector image.
A vector image is made up of lots of overlapping shapes
Each of the shapes can be edited separately from the other shapes.

4. Vector Graphics A vector image is stored using
Nat 5
Vector Graphics
A vector image is stored using
a different method than bitmapped images
The attributes (or qualities) of each shape are stored.
EG, the eyes(ellipses) may be stored as:
Centre X/Y-co-ordinates
Fill Colour
Border Style
Border Colour etc…

5. Storing Colour Bitmaps
National 5
Storing Colour Bitmaps
Storing a colour bitmap is exactly the same as for black and white with one difference - Each pixel is not just black and white but can represent a variety of colours.
This is done by allocating more than one bit for each pixel:
2 bits/pixel = 4 colours
4 bits/pixel = 16 colours
6 bits/pixel = 64 colours

6. National 5
Colour Bitmaps
The amount of bits allocated to each pixel is known as the bit depth.
So an image with 8 bit colour depth could have 256 colours [28 = 256]
True Colour is defined as an image with 24 bit colour depth. It has 16,777,216 colours! [224 = 16,777,216]

7. True Colour True Colour is defined as an image with 24 bit bit depth.
National 5
True Colour
True Colour is defined as an image with 24 bit bit depth.
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

8. File Size Increases Higher Bit Depth = More Colours
National 5
Increasing Bit Depth
Higher Bit Depth = More Colours
File Size Increases

9. Storage Space Example Step 1: (Length x Breadth) * bit depth
National 5
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 * 900) * 24 bits = 17,280,000 bits
Step 2: Convert into appropriate units
17,280,000/8 = 2,160,000 bytes
2,160,000 bytes /1024 = 2, KB
2, Kb/1024 = 2.06 MB

10. Storage Space: Example 2
National 5
Storage Space: Example 2
A 16 bit colour image is 4 inches by 6 inches with a resolution of 300dpi.
Step 1: (Length x dpi) x (Breadth x dpi)
(4*300) * (6*300)= 2,160,000 pixels
Step 2: number of pixels * bit depth
2,160,000 * 16 = 34,560,000 bits
Step 3: Convert into appropriate units
34,560,000 /8 = 4,320,000 bytes
4,320,000 bytes /1024 = 4,218.75KB
4,218.75KB /1024 = MB

11. Bitmap Graphics – Pros and Cons
National 5
Bitmap Graphics – Pros and Cons
Advantages
Disadvantages
Can be manipulated at pixel level
Can create a wide array of graphic effects
Can represent photo-realistic images
Requires large storage space
Image becomes jagged when scaled

12. Vector Graphics – Pros and Cons
National 5
Vector Graphics – Pros and Cons
Advantages
Disadvantages
Do not lose quality when scaled
Require less storage space
Objects are easily moved/manipulated
Resolution independent
Cannot be edited at pixel level
Cannot show photo realistic scenes
Will usually require particular applications to open

13. Tasks
National 4
National 5
Worksheet 4
Worksheet 4A and 4B