Bitmapped Graphics
Learn one method computers use to store graphics
Introduction
There are two main methods of storing graphics:
- Bitmap
- Vector
Computer must store all data as binary. This includes images!
We will focus on the Bitmapped method of storing images.
Pixels
If you zoom into a photo close enough you will see that it is made up of many little squares, these are called pixels (Picture Element).
A bitmapped graphic is a 2D grid of pixels.
Look at the black and white image below:
We can see in the image above that the computer stores the colour of each pixel in this grid by assigning it a binary value.
Since our image is black and white, we can store the colour of each pixel using either a 0 or a 1.
0 = black
1 = white
Colour Depth
If we add more colours to the image, we will need to use more bits for each pixel.
For example:
In the image above, each pixel requires 2 bits to store the colour. This is because we have 4 different colours in the image
00 = Blue
01 = Green
10 = Red
11 = Yellow
The number of bits used to store the colour of each pixel is known as the colour depth (or bit depth).
The colour depth tells us how many different colours an image can use.
The image above has a colour depth of 2. This means that we can store 4 different colours.
The black and white image in the previous example had a colour depth of 1. This means that the image can use 2 different colours.
The formula for working out the number of colours based on the colour depth is given below:
Number of colours = 2bits
Therefore:
| Colour Depth | Formula | Number of Colours |
|---|---|---|
| 1 | 21 | 2 |
| 2 | 22 | 4 |
| 8 | 28 | 256 |
| 16 | 216 | 65,536 |
| 24 | 224 | 16,777,216 |
The higher the colour depth the more colours the image will be able to use. However, this also means that we require more bits to store the colour of each pixel and therefore the file size will increase.
Resolution
Resolution refers to the number of pixels in an image.
The higher the resolution, the clearer the image will be! However, as we are then storing more pixels - the file size will increase.
One common way of measuring resolution is DPI (Dots Per Inch). This refers to how many pixels are contained within a square inch (approx. 2.5cm x 2.5cm).