Bitmap images, colour depth and resolution

Overview

Pixels, colour depth and resolution, how each affects quality and file size, the image file size formula with worked calculations, and why metadata is stored.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   How is a bitmap image represented in a computer?

   Model answer

   A bitmap image is a grid of pixels, the smallest single points of colour in the image. Each pixel's colour is stored as a binary code.

2. 2

   What is colour depth?

   Model answer

   Colour depth is the number of bits used to store the colour of each pixel.

3. 3

   How many colours can be shown with a colour depth of 1 bit, 8 bits and 24 bits?

   Model answer

   A 1-bit colour depth gives 2 colours, and 8 bits give 256 colours. A 24-bit colour depth gives 16,777,216 colours.

4. 4

   An image uses 16 different colours. What is the smallest colour depth it needs?

   Model answer

   The image needs a colour depth of 4 bits. Four bits give 2⁴ = 16 different codes.

5. 5

   What is the resolution of an image?

   Model answer

   Resolution is the number of pixels in an image. Resolution is usually given as width × height, such as 1,920 × 1,080.

6. 6

   What happens to image quality and file size when the colour depth is increased?

   Model answer

   More colours can be shown, so the image looks more realistic. The file size also increases, because each pixel needs more bits.

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