1. Representation of Images You need to know: (k) explain the representation of an image as a series of pixels represented in binary (l) explain the need for metadata to be included in the file such as height, width and colour depth (m) discuss the effect of colour depth and resolution on the size of an image file.
N Martin Jan 2011

2. Representing pictures in binary
Bitmap images
The page is divided into an invisible grid and each pixel is assigned a colour
Vector images
Drawn by following a set of mathematical instructions
Draw a circle
radius: 6 pixels
centre: 10, 10
line thickness: 1 pixel

3. Bitmap images
Certain information must be defined for the bitmap image, this is called METADATA (or data about data)
Size of grid
Width: 20 pixels
Height: 20 pixels

4. Bitmap images Resolution depth
How many pixels will there be per unit of length
10 pixels per inch
72 pixels per inch
The greater the resolution depth:
The sharper the image
The more data needs to be stored and the larger the file size on disk

5. Bitmap images Colour depth
How many bits will be used to store the colour for each pixel in the grid
1 bit allows 2 different values
2 different colours
2 bit allows 4 different values
4 different colours
3 bit allows 8 different values
8 different colours
...
8 bit allows 256 different values
256 different colours
24 bit allows 16,777,216 different values
6,777,216 different colours
The greater the colour depth:
The more realistic colours
The more data needs to be stored and the larger the file size on disk

6. L- Meta Data
When an application such as a web browser or word processor wants to open a picture, it needs to know more than the information held about each pixel. For example, it needs to know:
the height and width of the image
how many pixels there should be in a square inch (the resolution)
how many bits are being used to define the colour of each pixel (colour depth).

7. Meta Data
The application needs to know this information so that it can display the picture properly. In addition, however, there may be other information held as part of the image file, other metadata. These might include:
who owns the copyright to the image so that someone knows who to contact if they want to use the image
when the image was created so that a user can see if the image is up-to-date
keywords (tags) so that the image can be easily searched for on the Internet
what software was used to create the image so someone can try the software for themselves if they were impressed with the image.

8. Bitmap Images

9. Bitmap Images
Low resolution so very ‘pixelated’

10. Bitmap Images

11. Indexed Colour
A colour palette is stored inside the image file as part of the meta data.
Advantages
Indexed colour saves a lot of memory, storage space, and transmission time
Disadvantages
Has a limited set of simultaneous colours per image, ok for 4 or 16 colour palettes but not for real life images.

12. Direct Colour
Every colour can be made up from the correct mix of RED, GREEN and BLUE

13. Direct Colour
By mixing the appropriate amount from each of the three colour channels you can get a variety of colours
There is a 4th channel, called the alpha channel which handles transparency R G B α FF 00 80 96
What gets stored for each pixel is just a combination of each channel Eg
FFFFFF00 means the pixel is white
means the pixel is lilac
8 bit gives 256 colours
Real life colour needs 15 or 16 bits
24-bit or “truecolor “gives over 16.7 million colours