1. BTEC NCF Dip in Comp - Unit 02 Fundamentals of Computer Systems Lesson 10 - Text & Image Representation
Mr C Johnston

2. Learning Outcome
Understand the purpose and implications of using codes to represent character sets
Know the features and uses of common character sets
Understand how bitmap/raster image data is stored and represented within a computer system
Understand the impact of image resolution on the way images are stored and represented
Understand the impact of sample /bit depth on the way the image data is stored and images are displayed
Understand the effects of compression on image data

3. Text Representation
Each character we enter into a computer needs to be stored as a binary number
To store character normally an 8bit byte (allows for 256 characters) is used
e.g. a = , A = , b = , B =
A character set is used to define which binary number is used for each character
A character set as a minimum will need to store 0-9, a-z, A- Z, but will also need to store punctuation and control characters
What is the rule to distinguish between lower case and UPPER case characters?

4. Text Representation – Character Sets
Two common characters sets are
ASCII (American Standard Code for Information Interchange)
UNICODE
ASCII Code
UNICODE
ASCII vs UNICODE
Activity
Carry out individual research into the purpose, features and implications of common character sets (including ASCII and UNICODE) producing notes which can be used for revision.
For a bit of fun complete this ASCII code worksheet

5. Image Representation – Storage
Images are stored within a computer system in either Bitmap/Raster or Vector format
Bitmap images are made from lots of dots called pixels
Each dot is assigned a colour and this colour is represented by a binary number
The number of colours the picture supports will depend on the size of the binary number
For black and white only a single bit is needed
For 256 colours 8 bits are required
Vector images are made up of shapes
Each shapes co-ordinates that define where objects are placed are stored along with properties including line thickness and fill type

6. Image Representation – Resolution
Bitmap/Raster images have a resolution which is the number of pixels that make up the image
Resolution is given as the number of pixels wide and the number of pixels high e.g x 768 = 768 rows of 1280 pixels
The higher the number of pixels the better quality of image as more detail is included
The higher the number of pixels the larger the file size as information about more pixels is stored
What is the difference in file size between an image that is 800x600 and one which is 1280x768 assuming 8 bit colour is being used in both?
How does DPI affect the quality of image?
Vector images tend to be a lot smaller why?

7. Image Representation – Colour Depth
Bitmap/Raster images also have a colour depth - this is the number of different colours which each pixel can be
Typical colour depths
Bits per Pixel
Colours Available 1 2 4 16 8
256
65536 24
16,777,216 32
16,777,216 + transparency
6 bit NES Graphics c1987
15 bit SNES Graphics c1992
What is the difference in file size of an 1280x768 that using 16-bit colour and one that uses 32-bit colour?

8. Image Representation – Size Change
When a Bitmap/Raster image is shrunk pixels get compressed and some will be removed = quality loss
When a Bitmap/Raster image is stretched the pixels will either be enlarged or software will “fill-in the gaps”
When a Vector image is stretched or shrunk the calculations are simply performed again proportionally
What is the result in terms of quality in changing a bitmap/raster and a vector image?

9. Image Representation – Compression
Data Compression [TechQuickie]
Lossy v Lossless Compression
Image Representation – Compression
The size of storage required by a Bitmap/Raster image can be decreased by the use of compression
Lossy Compression
Higher compression ratio but detail is lost from the picture
Lossless Compression
Lower compression ratio but detail loss is minimal
Briefly describe how the different compression techniques work

10. Activity
Bitmap vs Vector
Carry out independent research and produce written notes to revise from that cover
How bitmap/raster image data is stored and represented within a computer system – include the formula in words that can be used to calculate the storage requirements of a bitmap image
The factors which affect the choice of resolution used
The impact of image resolution on the way images are stored and represented – include an example storage requirement calculation
The factors which affect the choice of sample /bit depth used
The impact of sample / bit depth on the way the image data is stored and images are displayed – include an example storage requirement calculation
Different types of compression and example file types
The effects of compression on image data

11. Exam Questions
Q1 CyberCar Systems is a computing manufacturer that specialises in physical computing and robotics. It is currently developing a self-driving car.
The software for the user interface that the driver will see is encoded using the ASCII character set.
Explain why ASCII may not be a good character set to use. [2 marks]
Q2 A graphic designer uses computer systems to store and manipulate digital images for use in printed and digital products.
The images used in the designs are acquired from a range of sources, including mobile phones and the web. They are also created using graphics software. The graphic designer uses different bitmap file formats and vector images.
Analyse the impact that digital image representation has on the way images are viewed, stored and used. [12 marks]