Planar Reflections using Texture Mapping

A technique similar to the stencil buffer technique uses texture mapping. The first pass is identical to the first pass of the previous technique: we draw the reflected scene. After drawing the scene, we copy the image into a texture (using the glCopyTexImage2D() command). During the second pass, this texture is mapped onto the reflective polygon. The sequence of steps for the second pass is as follows:

• Position the viewer at the ``real'' eyepoint.
• Draw the non-reflective objects in the scene.
• Bind the texture containing the reflected image.
• Draw the reflective object with the appropriate texture coordinates.

The texture coordinates at the vertices of the reflective object must be in the same location as the vertices of the reflective object in the texture. These coordinates may be computed by figuring the projection of the corners of the object into the viewing plane used to compute the reflection map (the command gluProject() may prove helpful). Alternately, the texture matrix can be loaded with the composite modelview and projection matrices and postmultiplied by a scale of 1 divided by the size in pixels of the region used to compute the texture. The texture coordinates would then be the model coordinates of the vertices.

The texture mapping technique may be more efficient on some systems. Also, we may be able to use a reflection texture during several frames (see below).