RGB颜色空间alpha混合的方法

http://blog.csdn.net/xhhjin/article/details/6444782
http://blog.csdn.net/xhhjin/article/details/6445460
http://www.cnblogs.com/graphics/archive/2012/08/23/2643086.html
http://www.oschina.net/code/snippet_1425046_27446