What glBlendFunc should I use to ensure that the opacity of my drawing is always the same? When I use glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) and multiple images are drawn on top of each other, the result is more and more opaque until it's completely opaque after a certain number of imgaes.

The closest I have come is to use glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_ONE_MINUS_SRC_ALPHA) which maintains a constant opacity no matter how many images are on top of each other, although there is a slight variation in opacity if the images overlap each other.

Any other render states I should consider trying? Any other ideas? I am making a drawing app for my kid and I don't want the images (brush) they draw to cover up the background.

Heres the closest I've got:

I want to have it so that the overlap part of the circles is the same color and opacity as the center part of the circle.

I am using cocos2d iphone v. 0.99