Alpha/object fading


First excuse for my english, i’ve been googling a lot of hours now for a simple function.

I have a FBO with transparent background, i draw some object in, i want to fade the FBO without altering the transparent background.

For example :
RGBA Src = 0.5 1 0.5 0.7
I want RGBA dst = 0.5 1 0.5 0.6 (for example)

But if i have RGBA src = 0 0 0 0 i want RGBA dst = 0 0 0 0

I think i can do this with glBlendFuncSeparate but i don’t understand wich value i must put in the function

Thank you