Digital Images

The picture below looks as though it could have been taken with a normal camera. However, if we zoom in we notice that it is made up of many small picture elements or pixels. In the original picture they are too small for us to see.

The picture on the left is made up of 250 x 200 = 50,000 pixels. Its resolution is 33 pixels per cm ( 85 ppi). Any less than this and you would probably start seeing the actual pixels. It is a jpeg image (joint photographic experts group), the type most commonly used for pictures on the internet.

Now each pixel has a certain colour. On this image there are 256 different possible colours. We can give each colour an 8 bit code ( 8 bits gives us 256 possibilities) and so the colour information for the picture can be represented by 8 bits.

The total number of bits     =     number of pixels    x    bits per pixel

= 50,000 x 8 = 400,000 bits       or 50,000 bytes      or 50kb

The "file size" for the picture would be greater if:

• It was bigger in area

• The resolution was better (more ppi)

• More information was assigned to each pixel e.g. 16 bits. This would give 2¹⁶ = 64,500 possible colours

Notice; number of possibilities = 2ⁿ where n is the number of bits per pixel.