A level revision

Digital storage and processing

Digital data can only take set values - such as 0s and 1s. Images stored in digital form and therefore just an array of numbers which represent the image.

Digital storage

Pixels

Digital images are made up of pixels with each pixel representing a small square of the image and having a value assigned to it.

Bits

The value of each pixel is represented as a number of bits. A bit can take only two values: 0 and 1. Therefore the number of bits allocated to each pixel determines what values it can take. An image with only 1 bit per pixel means each pixel can either be assigned the value 1 or 0, the image will be black and white. The number of values a given number of bits can represent is given by the expression:

Where N is the number of values and b is the number of bits. So 3 bits will give 2³, 8, values - 8 different combinations of 0s and 1s

The total amount of information in an image is therefore equal to the number of pixels multiplied by the number of bits per pixel.

Bytes, Kilobytes, Megabytes...

A byte is just a group of 8 bits. A kilobyte is 1024 bytes and a megabyte is 1024 kilobytes and so on. This is important to remember as it is different from the standard units where a kilo is equal to 1000.

Digital processing

If an image is stored on a computer as an array of numbers it makes sense that changing the numbers will allow us to change the image.

We can improve the quality of images by taking the average of pixels surrounding each pixel, plus the pixel itself, to give an improved value for that pixel. We can take either the mean average or the median average. Generally the median average is better, however either way data will be lost and this is something to remembering when manipulating images like this.

Rates of information transmission

The rate of transmission of data is measured in bits per second. This is simply the amount of bits which are transmitted each second.