How to render vehicle window transparency in Blender for further compositing in Photoshop.
A tricky situation when compositing a vehicle rendered in Blender Cycles over a backplate is the question of window transparency.
The problem stems from the fact that it is not possible to convey glass diffraction information in an apha channel, since diffraction changes the light rays direction, represented in the render by the distortion visible when seeing objects through a glass material, and an alpha channel can only store transparency information. Cycles’ solution is simply to make glass materials non-transparent to environment HDRIs, probably the most sensible option as Brecht Van Lommel, Cycles’ architect, makes clear in this response to a non-bug report.
The solution to have glass material renders which can be composited in external applications is simple, but perhaps unsatisfactory depending on where your allegiance to realism stands: instead of a glass node, simply use a transparent one. Gone is the diffraction, but more importantly the transparency is back.
The implication of using the more realistic glass BSDF for automotive renderings as opposed to a transparent one is that the backplate needs to be integrated in the 3D scene, because it’s the only way that the rendering engine will be able to calculate the diffraction through the glass. If external compositing is required, this is currently the way to do it.