1. Computer Systems Nat 4/5 Data Representation Lesson 4:
Storing Graphics
Computer Systems
Nat 4/5
Data Representation
Lesson 4:
Representing/Storing Graphics

2. REVISION What does ASCII mean?
Nat 4/5
REVISION
What does ASCII mean?
Describe how text is stored by a computer system
What is a control character?
What is the character set?
What is UNICODE?

3. ANSWERS Revision: American Standard Code for Information Interchange
Nat 4/5
ANSWERS
Revision:
American Standard Code for Information Interchange
Each value has a unique ASCII value and this is stored in binary by the computer
A key that has an effect on screen
The character set is the name given to the complete set of characters that the computer can represent
UNICODE is a character set designed to represent all the world’s languages

4. Lesson Aims Pupils at National 5 level will be able to:
Describe how a computer system stores a black and white bitmap image
Calculate the storage requirements of a black and white bitmap image.
Explain the difference between a vector and bitmap image
Explain the terms pixel, dpi and resolution

5. Nat 4/5
Bitmap images
When you take a photo with your phone or digital camera it will be stored as a bitmap image

6. Lets look at a bitmap picture
Nat 4/5
Lets look at a bitmap picture
The whole image is made up of individual pixels.
Pixel stands for picture element

7. Bitmap Images A bitmap image stores each individual pixel.
Nat 4/5
Bitmap Images
A bitmap image stores each individual pixel.
First you need to understand black and white images. These are constructed as follows:
0 is stored for a white pixel, 1 is stored for a black pixel
Each black and white pixel takes up 1 bit of storage

8. Nat 4/5
Resolution
Resolution is the term given to the amount of pixels that the image is made of
Also used for the amount of pixels displayed on the screen
Higher resolution = clearer and more detailed picture
Pixel Pattern using 8x8 grid
Pixel Pattern using 16x16 grid

9. Bitmap images and resolution
Nat 4/5
Bitmap images and resolution
Bitmap images are resolution dependent
Resolution dependent mean the image is dependent on the amount of pixels that were used to originally capture the graphic
This means if you scale a bitmap image up enough then the picture will become blurry or jagged

10. Calculating Storage Requirements
Nat 4/5
Calculating Storage Requirements
You are often required to calculate the storage requirements of a black and white bitmap image
The steps are as follows:
Amount of pixels wide x Amount of pixels high
This answer is in bits
You then convert the number into appropriate units (Kilobytes/Megabytes)
Divide until its in an appropriate unit. Stop when the number is less than 1024, Gb, Mb or Kb.

11. Nat 4/5
Storage Space Example
A black and white image is 800 pixels by 900 pixels. Calculate the storage requirements and express the answer in appropriate units.
Step 1: Length x Breadth
800 * 900 = 720,000 bits
Step 2: Convert into appropriate units
720,000/8 = 90,000 bytes
90,000 bytes /1024 = Kb
If you already know the amount of pixels in the image then use this method

12. Alternate Storage Space Example
Sometimes you will be given the size of the image 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 black and white 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 bits
Step 2: Convert into appropriate units
2,160,000/8 = 270,000 bytes /1024 = = 264 Kb
If you are not given the amount of pixels in the image then use this method

13. Vector Graphics The image below is an example of a vector image.
Nat 4/5
Vector Graphics
The image below is an example of a vector image.
For example attributes for the eyes(ellipses) may be
Centre X/Y-co-ordinates
Fill Colour
Border Style
Border Colour etc…

14. Vector Graphics continued
Nat 4/5
Vector Graphics continued
Vector graphics are stored by their attributes (description), for example,
Square, starting X 390, Y 150, length 5
Line width 1.5, Line style solid, line colour Blue4
Fill solid, Fill colour Green1

15. Summary Bitmap images store every single pixel
Nat 4/5
Summary
Bitmap images store every single pixel
1bit for each pixel
0 for white pixel, 1 for black pixel
Resolution is the amount of pixels that make up an image
Higher resolution means you have a more detailed image.