As promised way to long ago (here), I finally manged to get the brush example done.
Not the best way, not the best code, but an example nonetheless.

(just press the left mousebutton and move the mouse ...)

And the zipped fla (F8): drawexample.zip (6,11 KB)

And for those hwo don't want to d/load the fla ... just the code. You just need this two images set for export in the first frame:
(Brush, with alpha)
(color / pattern)

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import flash.display.BitmapData;
import flash.geom.Point;
import flash.geom.Rectangle;
import flash.geom.ColorTransform;

var bmpColor:BitmapData = BitmapData.loadBitmap("color_00.png");                // the color to paint with
var bmpWall:BitmapData = new BitmapData(480, 200, false, 0xc0c0c0);                // a bmp to paint in

var bMouseDown:Boolean = false;
var iPaint:Number = 0;    // "color on brush"

this.onEnterFrame = function ():Void {
    
    var bmpAlpha:BitmapData = BitmapData.loadBitmap("brush_shape_and_alpha.png");     // the "brush"
    
    var iPosX:Number = this._xmouse - (bmpAlpha.width * 0.5);
    var iPosY:Number = this._ymouse - (bmpAlpha.height * 0.5);

    if (bMouseDown) {
        
        // modify alpha ...
        bmpAlpha.colorTransform(bmpAlpha.rectangle, new ColorTransform(1, 1, 1, 1, 0, 0, 0, iPaint));
            
        bmpWall.copyPixels(bmpColor, bmpAlpha.rectangle, new Point(iPosX, iPosY), bmpAlpha, new Point(0,0), true);
        this.attachBitmap(bmpWall, 0); // show ... (not the best way, imho ...)
        
        // Disolve paint ...
        iPaint -= 5;
        if (iPaint <= -250) {
            iPaint = 0; // just resti t
        }
    }
}

this.onMouseDown = function ():Void {
    bMouseDown = true;
}

this.onMouseUp = function ():Void {
    bMouseDown = false;
}

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nGFX