Color Key filter not working in 10-bit

Hi All,

So here's a weird problem that's plagued me in the past. I'm color correcting an indie feature shot on the Varicam. The footage has been upres'd to 1080p23.98 DVCPRO-100.

I'm adding a vignette to a couple of shots and am seeing pretty noticeable evidence of a step-ladder effect in the vignette. Almost like it's quantized.

I can alleviate this by re-rendering in any format that supports 10-bit rendering... ie ProRes or 10-bit uncompressed. However...

There are a couple of greenscreen shots where the original editor used FCP's Color Key filter to remove the green. The key looks fine when rendered at 8-bit. But when I flip over to 10-bit the key stops working completely.

So, my question is two-fold:

1. does anyone know of a way to get rid of the step-laddering look of the vignette when working in 8-bit?

or...

2. does anyone know of a way to get FCP's Color Key to recognize the 8-bit footage when up-resing to 10-bit?

System specs:
Intel Dual 3Ghz
FCP 6.0.1
4GB RAM
Kona 3
SATA-Raid


Thanks!

-Michael Garber
5th Wall Entertainment
www.5thwall.tv