How to clear last rendered frame information??

[RSF]

Hello
How do I clear information from last rendered frame when rendering an animation?
I'm rendering with "region selected" but each frame is rendered on top of the other.
Here's a link to my post about rendering an animation with "region selected":
http://forums.cgsociety.org/showthread.php?t=394716

Any tips??

Thanks

[jacksc02]

in the system panel in vray there are options in the "previous render" pull down

[RSF]

Hi.
Can't seam to find that option.
I have "unchanged", "cross", "fields", "darken", "Blue"
I'm I missing something?
Thanks