Handling brightness & contrast in real-time

I need to interactively change brightness & contrast in my OpenGL app. Note that I need to change b&c of only one object (a full-screen textured quad). The other objects must remain the same.
My current solution uses ext_blend_subtract to handle negative brightness values, but does not handle scale factor that exceeds 1.0 (therefore contrast modification is very limited).
Another question.
My current graphics card is a Geforce2Pro. What type of imaging extensions are supported?
Thanks in advance
Nicola Mosca
somewhere in time