One embodiment of the present invention sets forth a technique for generating a tube within a three-dimensional (3D) model. The technique involves receiving a first location on a surface of the 3D model. The technique further involves receiving a second location on the surface of the 3D model. The technique further involves receiving at least one constraint associated with a first tube to be generated within the 3D model. The technique further involves generating, based on the at least one constraint, the first tube, where the first tube connects a first opening at the first location with to a second opening at the second location.