cairo-device-to-user-distance (cr dx dy)

cr -- a cairo-t context

dx -- a double float x component of a distance vector (in/out parameter)
dy -- a double float y component of a distance vector (in/out parameter)

Transform a distance vector from device space to user space. This function is similar to the function cairo-device-to-user except that the translation components of the inverse CTM will be ignored when transforming (dx,dy).