3.1.10 Representing Images

1 Representing Images

When an image is displayed on a computer monitor, pixels (which are the smallest addressable point on a display) are illuminated in different colours.

In a standard bitmap image file (.bmp), each of these pixels is represented by three 8 bit binary numbers.
The first binary number represents the amount of red, the second the amount of green and the third the amount of blue.
So this colour here is:
- 1101 0101 Red (213)
- 1111 1111 Green (255)
- 1100 0000 Blue (192)
Strings of binary numbers are difficult to read however, so we use hex instead.

Bitmap images are made up of two parts.
The first part is the header. This contains information such as the size of the image and the type of file.
The second part is the image data itself.

Download this file to your computer - small.bmp
View it in an image viewer. You'll need to zoom in as it's a very small image.
Now visit this site - online hex editor
Now Open the bitmap file in the site.
The first few hex codes are just the header file.
Look for the codes 04 00 04 00. This is identifying the image as a 4 x 4 pixel file.
The long sting of FF codes is where each pixel colour is identified.

Try and edit the hex to produce an alternate pattern of red, blue and green pixels.
Export the image and open it in your viewer.
What other patterns can you make?

The bitmap image format is a simple one.
There is very little metadata in the file - that is data that isn't coding pixel colours.
Other image formats such as jpg, png, gif will contain far more meta data, such as dates, authors, even the type of camera that the image was taken with.
Bitmap images are also uncompressed. This means every pixel is individually coded.
In other image formats, compression means that groups of similar pixels can be encoded, which reduces the size of the image file.