YOU WILL NEED:

Fog.mfa
This mfa contains all the objects you will need for this tutorial. Apart from the objects, frame 1 is empty, while frame 2 includes the finished code. Download here.

Ellen Ripley (Active Object)
This is the player controlled by the user. Obviously, not required to create fog, but necessary to demonstrate a few key concepts. .

Fog 1, 2, 3 and 4 (Active Objects)
The fog will be created from 4 differernt active objects, each with a unique fog image.