this.createEmptyMovieClip('mask_mc',0); 
bg_mc.setMask(mask_mc);
var contor:Number=0;

// function drawCircle draws a circle on mask_mc MovieClip of radius r and having center to mouse coordinates
function drawCircle(mask_mc:MovieClip):Void{
    var r:Number = 20;
    var xcenter:Number = _xmouse;
    var ycenter:Number = _ymouse;
    var A:Number = Math.tan(22.5 * Math.PI/180);
    var endx:Number;
    var endy:Number;
    var cx:Number;
    var cy:Number;

    mask_mc.beginFill(0x000000, 100);
    mask_mc.moveTo(xcenter+r, ycenter);
    for (var angle:Number = Math.PI/4; angle<=2*Math.PI; angle += Math.PI/4) {
       xend = r*Math.cos(angle);
       yend = r*Math.sin(angle);

       xbegin =xend + r* A *Math.cos((angle-Math.PI/2));
       ybegin =yend + r* A *Math.sin((angle-Math.PI/2));
       mask_mc.curveTo(xbegin+xcenter, ybegin+ycenter, xend+xcenter, yend+ycenter);
    }
    mask_mc.endFill();
}

// contor variable is used to hold if the mouse is pressed (contor is 1) or not (contor is 0)
this.onMouseDown=function(){
    drawCircle(mask_mc);
    contor=1;
}

// if the mouse is hold and moved then we draw a circle on the mask_mc
this.onMouseMove=this.onEnterFrame=function(){
    if (contor==1){
        drawCircle(mask_mc);
    }
}
this.onMouseUp=function(){
    contor=0;
}