• POV-Ray 2
• GIMP

The lack of window caustics in this scene are due to a deliberate design decision in POV-Ray, to substantially improve performance POV-Ray uses backward ray tracing (rays shot from the camera) instead of forward ray tracing (rays shot from lights), this means that caustics are not simulated. A feature was added for this in version 3.5, known as "Photon Mapping", but of course we don't have access to that in the much older POV-Ray 2.