Using custom inputExtent for Core Animation transition

I am trying to use core animation, in particular a CATransition with a  
CIFilter, to animate the transition of an image (layer contents) in a  
layer-hosting view. Problem is that the animation seems to override  
the inputExtent of the CIFilter to the full bounds of the layer. I  
really need it to be different from that. Is there any way this can be  
done with core animation transitions? Just making the layer smaller is  
not an option, because I do want the transition to play in the whole  
area I'm currently having.

Thanks,
Christiaan