Apply two 3x3 matrices to each pixel?

[MrPete]

What I want:
For each pixel of each frame
{
// Transform the (R, G, B) values by doing two matrix
// transformatoins:
A = (R * ConstRa) + (G * ConstGa) + (B * ConstBa)
B = (R * ConstRb) + (G * ConstGb) + (B * ConstBb)
C = (R * ConstRc) + (G * ConstGc) + (B * ConstBc)

pxR' = (A * ConstAr) + (B * ConstBr) + (C * ConstCr)
pxG' = (A * ConstAg) + (B * ConstBg) + (C * ConstCg)
pxB' = (A * ConstAb) + (B * ConstBb) + (C * ConstCb)

pxRGB = (pxR', pxG', pxB')
}

I have not found such a plugin/function... seems like it MUST already exist... Any ideas?


PS: Why... for arbitrary colorspace conversion using known (rgb)->xyz, xyz -> (r'g'b') matrices

In my case, I've captured video on my wive's Nikon DSLR in Adobe RGB, and want to convert back to normal sRGB...

Thanks!
Pete