ofFbo with premultiplied alpha

alphafbo.cpp

// Kudos to armadilly

fbo.begin();
    ///pre-multiply background color of the fbo for correct blending!
    ofClear(ofColor(fboBgColor * (fboBgColor.a / 255.) , fboBgColor.a));
    //Set this blending mode for anything you draw INSIDE the fbo
    glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
    //your draw code here!
fbo.end();

//then draw the fbo to your screen buffer with this blending
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
fbo.draw(0,0);