I know it is possible to create an invisible object that casts shadows but is it possible to keep an object invisible but still reflect light (like a backdrop)?
If I understand your question correctly, you can add a fade out animation to the geometry. This will make it invisible but still apply shadows.
If I misunderstood your question, please let me know.
Best Regards,
Erik Williams
Luxion Customer Support
If it is a still image you are creating the best way would be probably to render your reflection pass with the object you want to have invisible and then render without it. The reflection pass should give you all the information you want, you can then overlay it onto the second render and use the blending modes and selection tools to get the reflections how you want them. I hope this helps.