The "active area" is just a name I came up with to describe the area of the game world that the camera views plus an area around it the size of the camera area:
Calculating if something is in the active area or not is explained in the
isOutsideActiveArea() function’s explanation later in this chapter. When we create new enemy squirrel or grass objects, we don’t want them to be created inside the view of the camera, since it’ll appear that they just pop out of nowhere.
But we also don’t want to create them too far away from the camera, because then they may never wander into the camera’s view. Inside the active area but outside the camera is where squirrel and grass objects can safely be created.
Also, when squirrel and grass objects are beyond the border of the active area then they are far away enough to delete so that they don’t take up memory any more. Objects that far away aren’t needed since it is much less likely that they’ll come back into view of the camera.
If you have ever played Super Mario World on the Super Nintendo, there is a good YouTube video explaining how Super Mario World’s camera system works. You can find this video at http://invpy.com/mariocamera.