x and y in the turbulence C code

In the C code provided for the feTurbulence filter primitive, the
turbulence() function takes an x,y coord (in point[0] and point[1]).
 However it is perhaps not clear what coordinate space those coordinates
are defined in.

If it is supposed to be in the "coordinate system established by attribute
‘primitiveUnits’ on the <filter> element.", then should a note to that
effect be added to the description of feTurbulence?

>From my experiments, there does seem to be some inconsistency in how
renderers choose the origin and scale of x & y.

Paul