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[web] Actionscript 3: Alpha blending using BitmapData.draw

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Recently, I've taken to playing around with ActionScript 3 development using Flex, and I've run across something that puzzles me. Perhaps I'm missing something completely obvious, but... Is there a way to perform (SourceColor*SourceAlpha)+(DestColor*(1-SourceAlpha)) style blending using a BimapData.draw() call without using a Pixel Bender shader? I've perused the possible parameters for BlendMode but none of them seem to offer this sort of blending. I currently use a pixel shader to perform the operation, but I was wondering if there is a more obvious solution, particularly one that doesn't force the requirement of Flash Player 10 as shaders do (not that that is a bad thing, imo). [Edited by - JTippetts on June 22, 2009 11:36:45 PM]

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You could use one ColorTransform each to transform your "source" and "destination" images, then draw them with blend mode set to ADD. I have no idea what the performance of that would be.

ie, something like:

var src:BitmapData = new BitmapData(100, 100, false, 0xFF000000);
var dst:BitmapData = new BitmapData(100, 100, false, 0xFFFFFFFF);

var srcXfrm:ColorTransform = new ColorTransform();
srcXfrm.alphaMultiplier = 1.0;
var dstXfrm:ColorTransform = new ColorTransform();
dstXfrm.alphaMultiplier = 0;

var rect:Rectangle = new Rectangle(0, 0, 100, 100);

while (xfrm.alphaMultiplier > 0) {
srcXfrm.alphaMultiplier -= 0.1;
dstXfrm.alphaMultiplier += 0.1;

dst.draw(..., dstXfrm, BlendMode.NORMAL, ...);
src.draw(..., srcXfrm, BlendMode.ADD, ...);

I didn't test this, so you'd have to give that a shot and let us know if it's tweakable or a waste of time.

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