Image Representation

Recap Questions

  1. How many bits are in extended ASCII?
  2. What is the difference between ASCII and Unicode?
  3. What is 00110101 in Denary?
  4. What is 50 in Binary?
  5. Which of these units is most appropriate for a music track?

a. GB

b. KB

c. MB

d. B

Learning Objectives

  • To be able to calculate the file size of a BITMAP image
  • To be able to describe what a pixel is and how they relate to BITMAP images
  • To be able to describe colour depth and resolution

Digital Images

  • Digital images can be stored in a computer in many different file formats 

 

  • Can you name 5 different file formats used to store images?

Answer in your notebook 

Image File Types

Bitmap image file formats include:

  • BMP
  • JPG
  • GIF
  • PNG
  • TIFF

Vector image file formats include:

  • SVG

There are also editing formats such as Illustrator (AI) and Photoshop (PS)

Bitmap (or Raster) Images

Bitmap images are made up of PICture ELements 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., 111100000000 might equal red. 

 

A pixel's colour can be changed by changing this value

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., 1920x1080

  • 72 PPI (Pixels Per Inch) = typical screen resolution
  • 300 DPI (Dots Per Inch) = print quality resolution
  • Smartphones may have very high resolutions, 300+ PPI

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., 1920x1080

  • 72 PPI (Pixels Per Inch) = typical screen resolution
  • 300 DPI (Dots Per Inch) = print quality resolution
  • Smartphones may have very high resolutions, 300+ PPI

Task 

For the blank table, you can create your own design and then write out the binary code for it.

File Size of an Image

Height x Width x Colour Depth

 

 

Find the height of the image in pixels.

 

Find the width of the image in pixels.

File Size of an Image

To work out the colour depth, you need to look at how many bits are needed to represent the colours you have available in your image.

2. How many bits do we need to create a unique code for each colour?

1. How many individual colours do we have?

File Size of an Image

Height x Width x Colour Depth

46 x 29 x 3 = File size in bits

 

How large would this be in bytes?​

Task

What would be the file size of this image in:

a. bits

b. bytes