Chromakey and such

NOTE: GEEK ALERT :-)

So, I have a question for all you FCP pros out there. I'm using the chromakey filter on some green screen footage. Cool enough.

However, if I follow the chromakeyer with FCP's Gaussian Blur (in RGB+Alpha mode) with a large filter radius, then the green color, presumably in the areas that have been filtered out (by having the image areas set to zero or near-zero alpha), comes back into the blurred image.

Which leads me to believe that the chromakeyer FCP just sets the alpha channel of the image without actually converting the image to a premultiplied state, even though internally all filters are supposed to work with premultiplied images (like FCP's Gaussian Blur). THis causes the Gaussian Blur of FCP to not behave properly, because it appears the chromakeyed image creates a "straight" image instead of a premultiplied to be passed to the next filter, the Gaussian Blur, which is expecting a premultiplied image. Thus the green comes back in from the areas where the alpha was set to zero (or near zero) with out premultiplying the image.

Anyone else notice this? I presume it's a bug in FCP, yes? As a developer I'm curious as to what you guys do if you want to follow the chromakey process with a large blur (or other filter) that needs a premultiplied image as input. Do you use a premult filter, either within FCP or available from someone else?

Pete
RE:Vision Effects, Inc.