1. A method for constraining a region of a polygon mesh, the method comprising: receiving, from a user, a specifying a skeleton line over the polygon mesh at an eye point of the polygon mesh; casting a ray from the eye point through each measurement unit in the skeleton line for a predetermined length that passes through an entirety of the polygon mesh; calculating, by a computing device, a point at which each cast ray intersects the polygon mesh, the calculating resulting in two calculated points for each of the cast rays; creating a line segment at another point on each cast ray at a midpoint between the corresponding two calculated points; creating a first boundary plane that is perpendicular to a first end of the skeleton line; creating a second boundary plane that is perpendicular to a second end of the skeleton line; determining the region of the polygon mesh to be constrained by the first boundary plane and by the second boundary plane wherein the method is performed by a computer.