Computer
Systems
Nat 4/5
Data Representation
Lesson 4:
Storing Graphics
1. What does ASCII mean?
2. Describe how text is stored by a computer
system
3. What is a control character?
 Int
2 Revision:
1. American Standard Code for Information
Interchange
2. Each value has a unique ASCII value and this is
stored in binary by the computer
3. A key that has an effect on screen
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
Nat 4/5
Lets look at a bitmap picture
The whole image is made up of
individual pixels.
Pixel stands for picture element
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
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
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:
1. Amount of pixels wide x Amount of pixels high

2.
This answer is in bits
You then convert the number into appropriate
units (Kilobytes/Megabytes)

Divide until its in an appropriate unit.
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 = 87.89 Kb
If you already know the amount of pixels in the
image then use this method
Nat 4/5
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

(4*300) * (6*300) = 2,160,000 bits
 Step


1: (Length x dpi) x (Breadth x dpi)
2: Convert into appropriate units
2,160,000/8 = 270,000 bytes
270,000 bytes /1024 = 263.67 Kb
If you are not given the amount of pixels in the image
then use this method
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…
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.