![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgpTA_8q2_sV8af7N4BdpcI-Dc_uLk-pMUqHq6NIEPFS4g_FVTOBN03BJhNQl7gai7B-jZiWp6P7AlL8n8Pc91mSzIObFpjVb6xUi-f0uTn0a2dc-aXJek7f-1M4zd8CiPASh0giMteHrk/s400/compositing-base-vray.jpg)
This is something I couldn't figure out for a long time. Once I did, it opened up a lot of new techniques and ideas for me, and gave me a lot more creative control. I would recommend figuring this out, and then thinking of ways you can use it enhance your images in the post processing stage.
This tutorial lays out the technique very simply.