Masking Image

This Flex 4.6 project is demonstrating how to mask one BitmapData with another BitmapData.

Main explanation

What we have

We have two images:

1. Original image with alpha chanel
2. Mask image (White image with alpha chanel)

We need to apply mask to original image.

How we can do this

To get result in new Bitmap we do:

var originalBitmap:BitmapData = /* smth */;
var maskBitmap:BitmapData = /* smth */;

var resultBitmap:BitmapData = new BitmapData(size, size, true, 0x00000000);
resultBitmap.copyPixels(originalBitmap, rect, point, maskBitmap, point, true);

If we want to change originalBitmap we do this after previous step:

originalBitmap.copyChannel(resultBitmap, rect, point, BitmapDataChannel.ALPHA, BitmapDataChannel.ALPHA);

What is main moment

Main moment is we should to do copyPixels() to EMPTY and TRANSPARENT background. Why we can do this to original image? I'll tell you.

Our Exapmle:

targetBitmap.copyPixels(originalBitmap, rect, point, maskBitmap, point, true);

Let's split copyPixels() work to few steps:

1. It takes a originalBitmap and maskBitmap and generate a new image with merged alpha chanels. I call it 'mergeResult'.
2. But next step copyPixels() just draws a mergeResult over targetBitmap with NORMAL blend mode. It doesn't replace a target pixels with mergeResult pixels.

So, if targetBitmap has a image already, we have a result of drawing merged image to it. If targedBitmap is a originalBitmap we don't see any changes after copyPixels().

What doesn't work

I tryed a different ways to do this. We can't to do this with draw() function with any BlensMode parameter, I tryed all of them. I tryed a way with ERASE blend mode, it doesn't work becouse it make background black and not transparent.

Forgot about all another ways, this works fine.