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