[Inkscape-devel] How to access viewport dimensions?

I've fixed Inkscape to actually respect the filterUnits (and primitiveUnits) propertie(s), but now I need to be able to access the viewport dimensions for the filtered object... As far as I have been able to find this is only available reliably from SPItemCtx. But for a filter I need to access this not for the filter itself, but rather for the filtered object... Any help would be appreciated.

Getting this right would fix the following bug btw.: https://bugs.launchpad.net/inkscape/+bug/229246