3_3_6 Representing images

You should be able to:

  • Understand what a pixel is and be able to describe how pixels relate to an image and the way images are displayed.
  • Describe the following for bitmaps:

•• size in pixels

•• colour depth.

  • Know that the size of a bitmap image in pixels (width x height) is known as the image resolution.
  • Describe how a bitmap represents an image using pixels and colour depth.
  • Describe using examples how the number of pixels and colour depth can affect the file size of a bitmap image.
  • Calculate bitmap image file sizes based on the number of pixels and colour depth.
  • Convert binary data into a black and white image.
  • Convert a black and white image into binary data.

REVISE:

Bitmap Image Basics

The basics can be found here:

Calculate File Sizes

To calculate the file size for an image you need to know three things:

  • Width in pixels (w)
  • Height in pixels (h)
  • Colour depth for each pixel (d)

The file size is w * h * d

This will give you the number of bits and can then be converted to bytes, kilobytes etc.

e.g.

An image has a resolution size of 1024 x 800

The colour depth is 4

So we do...

1024 x 800 x 4

This is...

3,276,800 bits or 409,600 bytes or 409 kilobytes or 0.4 megabytes.

Convert from Images to Binary and Binary to Images

The image in the example has a 1-bit colour depth. The white pixels are 0 and the black pixels are 1. To convert this image to binary we just write down the binary representation of each pixel. 

0000000000
0011001100
0011001100
0000000000
0100000010
0011111100
0000000000

This bitmap image has 3 colours. This means that we will need 2 bits for each pixel. Black is represented by 11, White is represented by 00 and grey is represented by 10.

To convert this bitmap image to binary we would use 2 digits for each pixel instead of 1. 

11 00 11 00
00 11 00 11
00 10 00 10
10 00 10 00

TEST:

  1. Download and print the test paper here: https://drive.google.com/open?id=0B5fLtQ0Xgr2Pc2xxYnpiTERXMDA
  2. Try the mock test yourself.
  3. Use the 3.3.6 Walking Talking Mock below to guide you through answering the questions.

SOURCE RECOGNITION - PLEASE NOTE: The examination examples used in these walking talking mocks are samples from AQA from their non-confidential section of the public site. They also contain questions designed by TeachIT for AQA as part of the publicly available lesson materials.