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

2. REVISION What does ASCII mean?
Describe how text is stored by a computer system
What is a control character?

3. 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

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 terms pixel, dpi and resolution

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

6. Bitmap Images A bitmap image stores each individual pixel.
Nat 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

7. Nat 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 due to high number of pixels
Pixel Pattern using 8x8 grid
Pixel Pattern using 16x16 grid

8. Calculating Storage Requirements
Nat 5
Calculating Storage Requirements
You are often asked to calculate the storage requirements of a black and white bitmap image
The steps to do this 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.

9. Storage Space: Example 1
Nat 5
Storage Space: Example 1
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

10. Alternative Storage Space Question
Sometimes you will be given the size of the image and the resolution of the image.
Eg, an image is 3 inches by 4 inches with a resolution of 700dpi.
DPI = Dots per inch This is the amount of pixels across one inch of the image.
1 inch square
700 pixels (or dots)

11. Storage Space: Example 2
Nat 5
Storage Space: Example 2
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
270,000 bytes /1024 = KB
If you are not given the amount of pixels in the image then use this method

12. Summary Bitmap images store every single pixel
Nat 5
Summary
Bitmap images store every single pixel
using black and white pixels requires 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.

13. Pupil Task
National 4 - Complete Worksheet 4: Storing Graphics