textureMap blending for colour specific opacity


This should be a fairly simple effect to achieve - I am texture mapping a bitmap onto a GL_QUADS object. This texture map has two colours; white and blue. What i want to achieve is to have the blue completely opaque, and the white completely translucent.

I know you can use blending to change the opacity of the bitmap as a whole(which i can do) - however is there a method or function to allow a colour filter to this opacity?

I have searched the web and only come up with sites/forums talking about blending generally (which i can do) - but this is a fairly specific aspect of it…

Appreciate any suggestions,

regards, dani