1. A method to build a 3D model for a physical object, comprising: converting depth information of a plurality of images into a plurality of 3D point clouds, wherein the images are gathered by a camera surrounding the physical object to sequentially photograph the physical object; respectively calculating motion parameters of the camera gathering each of the images relative to gathering a previous image according to the 3D point clouds; calculating a registration energy estimate value of the 3D point cloud for each of the images as being integrated into the 3D point cloud of the previous image according to a corresponding motion parameter; varying the motion parameters to minimize the registration energy estimate value; determining an optimal camera path according to the varied motion parameters; and building a 3D model of the physical object according the optimal camera path.