1. Unit 2.6 Data Representation Lesson 3 ‒ Images

2. Objectives
Understand how a bitmap graphic is made up of individual pixels
Explain how each pixel is represented in binary
Understand that the number of bits per pixel determines the number of available colours for an image
Explain the need for image metadata
Explain the relationship between file size and image resolution

3. Type of Digital Image Two main types:
BITMAP - The page is divided into an invisible grid and each pixel is assigned a colour
VECTOR
Drawn by following a set of mathematical instructions
Draw a circle
radius: 6 pixels
centre: 10, 10
line thickness: 1 pixel

4. Bitmap (or Raster) images
Bitmap images are made up of PICture ELement or PIXELS
A pixel is the smallest identifiable area of an image
Each pixel is a single colour and is given a binary value which represents that colour e.g might equal Red
A pixel’s colour can be changed by changing this value

5. Image resolution
Resolution is the concentration of pixels within a specific area
The area is defined by the image width and height in pixels e.g. 3264x2448
72dpi = screen resolution
300 dpi = print quality resolution
1x x x x x x50 72x x300

6. Creating an Image Each pixel is given a binary value
Each value represents a different colour
Using one bit per pixel allows only 2 values, 0 and 1
1 = Black, 0 = White 1

7. Increasing the number of colours
More bits per pixel = more colour combinations
1 bit = 2 Colours
2 bits = 4 Colours
3 bits = 8 Colours
4 bits = 16 Colours
How many bits per pixel required for 256 colours? 10 11 01
01 =
10 =
00 =
11 =

8. Colours and resolution vs File Size
20 pixels
How does the number of colours affect file size?
How does the size of the image affect file size?
20 pixels
8 colours

9. Colour or bit depth 8 bits per pixel = 28 = 256 colours
Each pixel can represent a finite number of colours
A pixel is attributed a number of n bits
The number of combinations (2n) dictates the bit depth and therefore the number of colours that can be represented
A higher bit depth gives a greater range of colour and a better quality of image
8 bits per pixel = 28 = 256 colours
16 bits per pixel = 216 = 65,536 colours
24 bits per pixel = 224 = 16,777,216 colours

10. Variation in quality
Changing the colour depth of an image will affect the number of colours it can display, as shown below:
2 Colours Colours Colours Colours Colours m Colours

11. PBM monochrome images Store image dimensions
Change ‘colours’ by changing binary values

12. 181ml
126ml
61ml
Colour values

13. Looking at colour codes
Colour values of individual pixels are expressed as denary RGB values and in hexadecimal. Why not in binary in this instance?
RGB (Red, Green and Blue) values range between How many bits are required for 256 variations of each?
How many bits altogether?
In 32-bit colour what are the last 8 bits for?

14. Why file sizes don’t always add up
DucksBMP: 1000 x 750 pixels
24 bit colour depth = 16m colours
What is the file size in bytes and MB?
Why is there a difference of 2,800 bytes?

15. Image metadata Metadata is data about data
It is information other than image data that is stored with a file
This will include:
Colour depth in bits per pixel
Resolution (Height and width in pixels)
Date created
Author
How big is the DucksJPG file? Why is this different?

16. What have you learned?
A bitmap graphic is made up of __________________
Each _______ is represented in binary
The ________________________ determines the number of available colours for an image
Image _________ holds data about the image such as
___________________________________________
The greater the _________ of the image, the greater the file size

17. Plenary A bitmap graphic is made up of individual pixels
Each pixel is represented in binary
The number of bits per pixel determines the number of available colours for an image
Image metadata holds data about the image, such as colour depth, resolution, date created, author
The greater the resolution of the image, the greater the file size

18. Metadata
Certain information must be defined for the bitmap image, this is called METADATA (or data about data)
Size of grid
Width: 20 pixels Height: 20 pixels
A term for 'data about data'. In other words metadata describes the structure of the data file and is usually located right at the start of the file.
When a media player opens the file it looks for the metadata which describes the format of the file e.g. the file type, how many horizontal pixels are there, how many vertical pixels and the colour depth of the file.
The media player reads the file and correctly displays the image.

19. Direct Colour
By mixing the appropriate amount from each of the three colour channels you can get a variety of colours R G B α FF 00 80 96
There is a 4th channel, called the alpha channel which handles transparency What gets stored for each pixel is just a combination of each channel Eg FFFFFF00 means the pixel is white means the pixel is lilac 8 bit gives 256 colours Real life colour needs 15 or 16 bits 24-bit or “truecolor “gives over 16.7 million colours

20. Plenary
Complete the binary representation for the following:

21. Plenary - Answer Complete the binary representation for the following: