Need help with blend function

This is probably a very simple question but i just can’t figure it out. For my application i need the blend function to be like the following one.

For some reason i find blending to be very confusing, how can i blend the color and alpha value separately like in the previous equation?

Just use a separate blend equation and blend factors. In your case, the color equation should be GL_ADD, with the parameters being GL_ONE and GL_ZERO. The alpha equation should be GL_FUNC_REVERSE_SUBTRACT, with the parameters GL_ONE for both source and destination.