feTurbulence with filter primitive subregions

It's not clear to me from the spec what the 'point' parameter of the
'turbulence' function means.

Suppose I have
<filter primitiveUnits="userSpaceOnUse" x="0" y="0" width="20" height="20">
  <feTurbulence x="10" y="10" width="10" height="10"/>
</filter>

When we compute 'turbulence' for the top-left corner of the feTurbulence
filter primitive subregion, what is 'point'? I assume it's either (0,0) or
(0.5,0,5).

Also, I think one can deduce that 'fTileX', 'fTileY', 'fTileWidth' and
'fTileHeight' are the bounds of the filter primitive subregion, but it would
be helpful if that was documented.

Rob
-- 
"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
53:5-6]