A system and method for object modeling. A compositing module embeds geometric shapes to model varying densities of an object. The compositing module represents each volume domain of constant density within a segment with a separate geometric shape. A mass set properties module in the system calculates estimates of mass set properties such as center of mass by first subtracting integrals related, for example, to the lungs from an integral related to the total chest cavity. A deforming module in the system provides a geometric shape that is deformable to more closely match data points from the object. The deforming module adjusts a B-spline solid having control points to fit an irregularly shaped object. Movement and location of the control points locally adjust the B-spline solid's surface. These variation can be used to determine mass set properties including mass, volume, center of mass, inertia tensors, principal moments, and the like.