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Tutorials » Actionscripting/Expert

Dynamic masking using the drawing API
Author: Patrick Mineault | Website: http://dakar.netqcca.com |

Movie 1: Draggable mask

Those of you that were around in the "good old days" of Flash 5 remember what a pain it was to create a draggable mask. For some reason the mask was unavailable for scripting and so the container movie clip and the maskee had to be moved simultaneously in a splendidly awkward workaround. Gone is this nightmare in MX. Let's start out by creating a new movie. Draw something on stage (anything, as long as it has colors). Now convert to movie clip using F8. We'll call this clip the maskee.

Select the movie clip and attach this script to it (not to the frame, the movie clip!):

onClipEvent(load)
{
  _root.createEmptyMovieClip('square',0);

 function drawSquare()
	 {
	   x = _root._xmouse;
    y = _root._ymouse;
    with(_root.square)
	   {
	     clear();
      moveTo(x-50,y-50);
	     beginFill(0x000088)
	     lineTo(x+50,y-50);
	     lineTo(x+50,y+50);
	     lineTo(x-50,y+50);
	     endFill();
	   }
  }
	 this.setMask (_root.square) 
}
onClipEvent(mouseMove)
{
	 drawSquare();
	 updateAfterEvent();
}

Test the movie (Ctrl+Enter): you now have a draggable mask! How does it work? Well, as with the first example an empty movie clip is created and assigned as the mask of the target movie clip. The trick here is that the mouseMove clip event is used to trigger the drawing of a square around the mouse. The mouseMove event is triggered every time the mouse is moved. When this is done the drawSquare function is called. Using the coordinates of the mouse it draws a square with the top left corner at (-50, -50) and (50,50) relative to the mouse cursor, hence drawing a square 100 pixels wide that follows the mouse.

Question: why bother using the drawing API every time the mouse is moved? Wouldn't it be better to draw the square once and then change the _x and _y of the mask when the cursor is moved, or even, gasp!, using startDrag()?

Answer: The advantage of using the method outlined above should become apparent with this simple modification: comment out the clear(); line. Test the movie. There you have it folks: a scratch and win game!

What just happened? By commenting out the clear(); line, the previously drawn squares are never erased. Therefore every time the mouse is moved a square is added to the clip, effectively creating a bigger and bigger mask. By adding a condition that checks for mouse down and using a smaller and different shape you could easily transform this into a "real" scratch and win game.

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