How can I render reflections in OpenGL ES on the iPhone without a stencil buffer?

Question

I'm looking for an alternative technique for rendering reflections in OpenGL ES on the iPhone. Usually I would do this by using the stencil buffer to mark where the reflection can be seen (the reflective surface) and then render the reversed image only in those pixels. Thus when the reflected object moves off the surface its reflection is no longer seen. However, since the iPhone's implementation doesn't support the stencil buffer I can't determine how to hide the portions of the reflection that fall outside of the surface.

To clarify, the issue isn't rendering the reflections themselves, but hiding them when they wouldn't be visible.

Any ideas?

Answer 1

I don't have an answer for reflections, but here's how I'm doing shadows without the stencil buffer, perhaps it will give you an idea:

I perform basic front-face/back-face determination of the mesh from the point of view of the light source. I then get a list of all edges that connect a front triangle to a back triangle. I treat this edge list as a line "loop". I project the vertices of this loop along the object-light ray until it intersects the ground. These intersection points are then used to calculate a 2D polygon on the same plane as the ground. I then use a tesselation algorithm to turn that poly into triangles. (This works fine as long as your lights sources or objects don't move too often.)

Once I have the triangles, I render them with a slight offset such that the depth buffer will allow the shadow to pass. Alternatively you can use a decaling algorithm such as the one in the Red Book.

Answer 2

Render the reflected scene first; copy out to a texture using glCopyTexImage2D; clear the framebuffer; draw the scene proper, applying the copied texture to the reflective surface.