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?

REVISION

Int 2 Revision:1. American Standard Code for Information

Interchange2. Each value has a unique ASCII value and this

is stored in binary by the computer3. A key that has an effect on screen

ANSWERS

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

Lets look at a bitmap picture

Nat 4/5

The whole image is made up of individual pixels.

Pixel stands for picture element

Bitmap Images A bitmap image stores each individual

pixel. First you need to understand black and white

images. These are constructed as follows:

Nat 4/5

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

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

Nat 4/5

Pixel Pattern using 8x8 grid Pixel Pattern using 16x16 grid

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 This answer is in bits

2. 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

Nat 4/5

If you already know the amount of pixels in the image then use this method

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 270,000 bytes /1024 = 263.67 Kb

Nat 4/5

If you are not given the amount of pixels in the image then use this method

Vector GraphicsThe image below is an example of

a vector image.For example attributes for the

eyes(ellipses) may beCentre X/Y-co-ordinatesFill ColourBorder StyleBorder Colour etc…

Nat 4/5

SummaryBitmap 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.

Nat 4/5