Pre multiplied alpha problem

Question

Hello,
I am making a drawing app. In it I am using a feathered brush ( ie a gradient with 0 alpha on edges). I copypixel from this brush bitmap to my canvas on every mouse move. Problem with this is that color on the edges is different than that on the center. I read on internet that it is because flash uses pre-multiplied alpha in bitmapdata.
Is there a way to solve this problem. Can I turn off the pre-multiplexing of alpha with base color? Please help
Thanks

Answer 1

No that is not the problem. Problem is not about alpha getting darker, Its that when flash pre multiply alpha with RGB it changes the value of RGB, it changes the orignal color hence making a great data loss.
for example
var map:BitmapData = new BitmapData(1,1,true,0xffffffff);
map.setPixel32(0,0,0x00ffffff); // RGBA(255, 255, 255, 0);
trace( map.getPixel32(0,0).toString(16)) // traces 0x00000000
map.setPixel32(0,0,0x20fcfcfc);
trace( map.getPixel32(0,0).toString(16)) // traces 0x20ffffff
When I draw a yellow gradient circle on bitmapdata with 0 alpha outside and then copypixels of that bitmapdata to other bitmapdata the edges become green, because flash had pre multipled the yellow with alpha and change its orignal RGB
values.
you can see result here
Edit: Take color values like this:
1) A yellow that is slightly towards green like: 0xf5ff00 then it will round it to green
2) A yellow that is slightly towards red like: 0xffec00 then it will round it to red.
3) A perfect yellow 0xFFFF00 then it wont round it to anything and you will get the desired result