I have an iphone app which uses OpenGL ES 1.x.  I detect hits on objects
by drawing vertex buffer objects into an offscreen hit buffer, where each
object's color is a different number.  Then I read the buffer location to
tell which object was hit.

I am adding the ability to have various 'layers' of objects, and each layer
has its own hit vertex buffer object with object 'colors' ranging from 1 to
the number of objects.

This works fine as long as I only detect hits on one layer at a time, but
there are occasions when I want to draw all the layers together and detect a
hit.  In that case, I would have an object with color '1' on each layer, and
I wouldn't be able to tell which was the chosen object.

I'm wondering if there is a way to do this without re-rendering all these
vertext buffer objects.  I was thinking maybe there was some way that I
could tell opengl to add a value to every color it drew.  Is that possible?

So then say layer 1 has 100 objects, I would draw it normally.  Then I would
draw layer two with a color offset of 100, where that value would be added
to every pixel.  Is there anything like that available?

I imagine not, because the color are probably treated by their rgb channels
anyway,so that addition wouldn't work.  Any other suggestions of a way to do
it without reconstructing my vbo's?

Thanks
B