I had the exact same problem when I wanted to render a set of overlapping framebuffer objects like layers. The problem is Open GL blending functions is linear and it works when destination background is opaque. But actual blending equation for blending two transparent layers is not a linear equation and contains a division part. I don't think it is possible to do this with current OpengGL implementations. So as a work around I had to user a pixel shader to manually calculate the output of each pixel.

I had the exact same problem when I wanted to render a set of overlapping framebuffer objects like layers. The problem is Open GL blending functions is linear and it works when destination background is opaque. But actual blending equation for blending two transparent layers is not a linear equation and contains a division part.

out_color = {src_color * src_alpha + dest_color * dest_alpha * (1-src_alpha)} / out_alpha

I don't think it is possible to do this with current OpengGL implementations. So as a work around I had to user a pixel shader to manually calculate the output of each pixel.