Coordinates used by filter of CALayer

I am greatly confused by the different coordinate systems used by  
CALayer (there are at least three as I see it). In particular, I am  
trying to use a CATransition with a CIFilter to animate a transition  
in a layer. I would expect that the coordinate system relevant for the  
CIFilter would be the one for the layer, i.e. the one used by the  
layer's bounds. However, it seems to use the coordinate system for the  
superlayer (the one used by the layer's frame). Can anyone explain why  
this makes sense?

Christiaan