as3 draw line with LineStyle and then edit with transparent (invisible) pixels - actionscript-3

Question

I have MC with external loaded image (jpg).
Next layer is masked Sprite where I am drawing lines in different colors, opacity ,etc with graphics.lineStyle . Everything works fine, so I can see image and drawings over. Now I want eraser to make, and trying to make some kind of (transparent) line which will "erase" exising line.
If I for example draw (with mouse) red line, and then switch color to yellow and intersect on some point with previous line, it is ok, I see yellow color. Now I don't have idea how to make "transparent" color? So when mouse move over existing drawing, just see again part of background image. If I set one color background everything is OK, but I have image, end eraser leaves some color, and I need to be transparent. Tried blend modes, cached as bmp, everything I found but still no solution.

Answer 1

Seemingly you use Graphics objects that overlay the image. In order to erase something, you should give your sprite a topmost shape that will have BlendMode.ERASE as blend mode, and draw on it with alpha component of color being 0xFF. The sprite should have its blendMode to be BlendMode.LAYER. Here's a test to play with:
public class TestErase extends Sprite
{
public function TestErase() {
var b:Sprite = new Sprite();
var c:Shape = new Shape();
b.blendMode = BlendMode.LAYER;
c.blendMode = flash.display.BlendMode.ERASE;
c.graphics.beginFill(0, 1);
c.graphics.drawRect(50, 50, 50, 50);
c.graphics.endFill();
b.graphics.beginFill(0xff0000, 1);
b.graphics.drawRect(0, 0, 150, 150);
b.graphics.endFill();
b.addChild(c);
addChild(b);
this.graphics.beginFill(0x00c000, 1);
graphics.drawRect(75, 75, 100, 100);
graphics.endFill();
}
}
This makes a 50x50 hole, defined by "c" sprite in "b" sprite, that does not affect "this" sprite.
Okay you asked for an example. Here.
public class Main extends Sprite
{
public var sh:Shape;
public var drawing:Boolean;
public var sp:Sprite;
public var bd:BitmapData;
public var erasing:Boolean;
[Embed(source = '../lib/093.jpg')]
public static const Background:Class;
public function Main():void
{
if (stage) init();
else addEventListener(Event.ADDED_TO_STAGE, init);
}
private function init(e:Event = null):void
{
removeEventListener(Event.ADDED_TO_STAGE, init);
// entry point
var ba:Bitmap = new Background();
ba.scaleX = 0.5;
ba.scaleY = 0.5;
addChild(ba);
bd = new BitmapData(640, 400, true, 0);
var bm:Bitmap = new Bitmap(bd);
sp = new Sprite();
sp.addChild(bm);
sp.blendMode = BlendMode.LAYER;
var tf:TextField = new TextField();
tf.y = 401;
tf.background = true;
tf.backgroundColor = 0x0;
tf.defaultTextFormat = new TextFormat('Arial', 12, 0xe00000, true, false, false);
tf.selectable = false;
tf.text = 'Erase';
addChild(tf);
addChild(sp);
tf.addEventListener(MouseEvent.CLICK, toggleErasing);
sp.addEventListener(MouseEvent.MOUSE_DOWN, startDrawing);
sp.addEventListener(MouseEvent.ROLL_OUT, endDrawing);
sp.addEventListener(MouseEvent.MOUSE_UP, endDrawing);
}
private function toggleErasing(e:MouseEvent):void {
erasing = !erasing;
if (erasing) (e.currentTarget as TextField).text = 'Erasing...'; else (e.currentTarget as TextField).text = 'Erase';
e.stopPropagation();
}
private function startDrawing(e:MouseEvent):void {
if (drawing) return;
sh = new Shape();
sh.graphics.lineStyle(3, Math.floor(Math.random() * 0x100000), 1,true);
if (erasing) sh.blendMode = BlendMode.ERASE;
sh.graphics.moveTo(e.localX, e.localY);
addEventListener(MouseEvent.MOUSE_MOVE, trackMouse);
sp.addChild(sh);
drawing = true;
}
private function endDrawing(e:MouseEvent):void {
if (!drawing) return;
removeEventListener(MouseEvent.MOUSE_MOVE, trackMouse);
if (erasing) bd.draw(sh, null, null, BlendMode.ERASE);
else bd.draw(sh);
sp.removeChild(sh);
sh = null;
drawing = false;
}
private function trackMouse(e:MouseEvent):void {
sh.graphics.lineTo(e.localX, e.localY);
}
}
Tested by me, and should work as is, just change the background :)