Add blur to everything in an area AS3 - actionscript-3

Question

I'd like to know how can I add blur in a certain area for example behind a movieclip, maybe a mask that will blur everything in the area of that moviclip.
I don't want to blur everything, just what's behind a movieclip :)
Sort of like Apple is doing with their menus

Answer 1

The only way to do this in AS3 is draw whatever is behind the MovieClip to a Bitmap, then blur the bitmap.
For example, say your MovieClip is a modal dialog of some sort with a semi-transparent background, you could use the following script (inside the MovieClip) to blur whatever is on the stage behind it onto a bitmap surface:
// create a bitmap surface to use as a blurred background
var blurData:BitmapData = new BitmapData(width, height, false);
var blur:Bitmap = new Bitmap(blurData);
addChildAt(blur, 0); // put the blur surface behind everything in the movieclip
// this function draws the portion of the stage that's behind the movieclip
// onto the bitmap surface, then blurs it
function drawBlurBehind():void {
// fill the bitmap with the stage color first
blurData.fillRect(blurData.rect, stage.color);
// account for the coordinate offset of the stage and movieclip
var offset:Point = globalToLocal(new Point());
var matrix:Matrix = new Matrix();
matrix.tx = offset.x;
matrix.ty = offset.y;
// hide the movieclip so it doesn't show up in the blurred background
visible = false;
// draw the stage behind the movieclip onto the bitmap
blurData.draw(stage, matrix);
// blur the background bitmap
blurData.applyFilter(blurData, blurData.rect, new Point(), new BlurFilter(25, 25));
// show the movieclip after the background has been blurred
visible = true;
}
Note that this drawing is not "live" and any time anything changes (you move the movieclip or what's behind it changes) you have to redraw. You could use an ENTER_FRAME event handler to continually redraw every frame, which would basically make it live, but that will be relatively expensive so avoid it if you can.
Also note that this script draws and blurs the whole stage (excluding the movieclip), not what is strictly "behind" the movieclip. So if you want things to appear above the movieclip and not appear blurred behind the movieclip, you'll have to set those to visible=false while you draw the blurred background.