I try to implement something called Opacity Shadow Mapping by which it is possible to calculate shadows of semi-transparent
objects. In order to do this, I have to render the scene several times with special clipping parameters. After rendering,
I will read back the resulting alpha values from the color buffer. But unfortunately, all alpha values are one. I am using
the blending equation GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA. My question: How can I render the scene and
get back different alpha values at different positions? To get more precisely: When I render a quad textured with a bitmap
that has an alpha value of 0.6, I want this 0.6 appear in the color buffer again. When I render this scene again with another
quad lying over the other, I want the two alpha values properly accumulated and stored again in the color buffer.
Any help will be appreciated,