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Starter - Answers

Data Representation - Images

Bitmap Images

Pixels

•A bitmap graphic is composed of tiny squares, called pixels

•A pixel is a tiny dot of colour

​Pixels & File size

•The computer stores information about the colour value of every pixel in the image

•This can mean a large file size

Representing Images

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 = White, 0 = Black

Colour Depth – The number of
bits per pixel

• More bits per pixel = more

colour combinations

1 bit = 2 Colours

3 bits = 8 Colours

•
4 bits = 16 Colours

• How many bits per pixel

required for 256 colours?

•
8 bits

11 11 11 11 11 11 11 11 11

11 11 11 00 00 11 11 11 11

11 11 11 11 00 10 11 11

11 11 01 01 01 10 10

11 01 01 01

11 01

11

00 =

11 =

01 =

10 =

24 bit colour

• Most images have 24 bit colour which allows over 16

million colours.

Colours vs file size

• What if each pixel was

one of 256 colours?

• How does the number

of colours affect file
size?

1111111111

1111001111

1110000111

1100110011

1100110011

1100000011

1100000011

1100110011

1100110011

1111111111

Resolution

• The resolution describes how big the pixels are in

the image.

• The higher the resolution the better quality the image

is.

• Resolution is usually measures in pixels per inch.

Screen and print resolution

• Monitors display images at 72dpi

• Printers print images at over 300dpi

• Glossy magazine covers can print at 1200dpi

File size

• In order to work out the image size you would need to

know:

•
The size of the grid (width and depth)

•
The number of bits per pixel for the colour depth

• For instance an image which is 1000 x 800 pixels with a

16 bit colour depth would be:

•
(1000 x 800) x 16 = 12,800,000 bits

•
This is more commonly written as 12MB.