Pre-multiplied alpha compositing

Question

I am trying to implement pre-multiplied alpha blending. On this page : What Is Color Blending?, they do explain standard alpha blending but not for pre-multiplied values.

<

Answer 1

After many tries, here's what I came up with :

I pre-multiply the alpha channel as well and I keep my 2nd formula I posted first; it's the best result I got.

In the best docs I found about it talks of the ugly borders disappearing when pre-multiplying : http://www.td-grafik.de/ext/xfrog/alpha/index.html and http://blogs.msdn.com/b/shawnhar/archive/2010/04/08/premultiplied-alpha-in-xna-game-studio-4-0.aspx

Well, conventional alpha blending was not really a reference to compare with, I guess I am right now since it looks better than normal alpha-blending.

But to be honest, I don't really understand 100% of that, (looks like) it works; another mystery ...

Here's what makes me think it's okay;

Left : alpha blending; Right : pre-multiplied

Thank you all for your help !