Drawing box within a movie

Hey i’m new to the forums and fairly new to flash, currently using flash 8.

This code below is from a tutorial on this site, what i want to do is create a drawing box within my movie, rather than having the whole background available to draw on like in this code, I want the user only to be able to draw on a small area of the movie. I am making an interactive movie where the user is allowed to graffiti on a wall, but the wall does not take up the whole scene, and there are other interactive elements in the scene that i don’t want the user to be able to draw all over

Anyone got any tips on how I could go about doing this, i’ve had a few goes myself but can’t seem to work it out, thanks.

// 1. SETTING THINGS
_root.createEmptyMovieClip(“line”,1);
// 2. EVENTS IN _ROOT:
_root.onMouseDown = function(){
line.moveTo(_xmouse,_ymouse);
line.lineStyle(2,0x000000,100);
this.onMouseMove = function(){
line.lineTo(_xmouse,_ymouse);
updateAfterEvent();
}
}
_root.onMouseUp = function(){
this.onMouseMove = null;
}