jamesetc

to do this you need to export a new parameter(float if you want it in the alpha)from a shadow matte node inside the shader for the specific object you want. then pick this up as a custom image plane in your rop node, and you could then also single lights out using the light mask option on the image plane.

ps. to make it easy, create a shadow matte shader, and copy the contents inside into your shader, then export your parameter.

i will do a example if you need.