Where to start for YUV overlay

Hi

I'm just looking for a pointer to where I should begin -

We are changing the OS running on our hardware.  I am championing going  to
Linux with XFree86.  I have both are running fine.  (kernel 2.5.24, XFree86
4.2).

The platform has a Sigma EM8400 MPEG decoder chip, and a Chips & Tech 69000
graphics controller.  The Sigma YUV output is tied directly to the C&T YUV
input.

The Sigma is (according to it's status info) correctly decoding the input
stream I am passing it.  Not surprising as I ported the stream controller
from a working version.

So now I need to know how to enable the C&T YUV using X interfaces.  Also
are there any other requirements such as setting the pixel values in the
output area to a specific value?

Any pointers on where to begin my learning would be greatly appreciated!!

Thanks

Thomas Downing