Transparencies when drawing an FBO in an FBO

I would take a look at ofClearAlpha() which clears the alpha channel. When you draw something with alpha, you not only (potentially) draw in the color channels you also alter the alpha channel, so alot of times with fbos you get results you don't expect (because you don't think of the alpha passing through). ofClearAlpha() is useful -- I sometimes do:

fbo1.begin()
fbo2.draw()
ofClearAlpha()
fbo1.end()
fbo1.draw()