Because the coordinate systems of the measurements and the reference mesh may be different, the imported design mesh is repositioned before determining the coordinate deviation.
oSP3D supports the following algorithms:
- Coarse coordinate transformation (Auto translate)
Determines the center points of gravity. The design mesh is translated such that the center points are equal.
- Coarse coordinate transformation (Auto translate + rotate)
Determines the center points of gravity and the inertia tensors of both meshes. The design mesh is translated such that the center points are equal. The design mesh is then rotated such that the error between the two inertia tensors is minimized.
Note: The coarse rotation is not unique. For nearly symmetric bodies in particular, the result of this rotation may be wrong. You should visualize and check the results.
- Fine coordinate transformation (Auto translate + rotate with best fit)
Combines a pre-alignment adjustment as described for the previous algorithm and runs a modification of the Iterative Closest Point (ICP) projection algorithm for surface registration afterwards. The ICP projection algorithm also uses an iteration to minimize the actual "error" between both geometries. Each iteration step involves the computation of the geometric deviations between both meshes. For more information, refer to [10].
- Pre-aligned fine coordinate transformation (Auto translate + rotate with best fit)
Assumes already pre-aligned meshes and runs only the modified ICP projection algorithm as described for the previous algorithm.
The algorithms may also be instructed to work with a node subset only. Then the center of gravity and the inertia tensors are computed only for these nodes that are included by the subset. This option further limit iterative algorithms, particularly the rotation and the whole ICP algorithm, to build their optimization criteria only with nodes included in the subset given. It might further occur that the transformation reference node set is only defined in the reference mesh but is missing in the design mesh file. Such a case splits the actual coordinate transformation into three principle steps:
Perform a course alignment. Depending on the chosen transformation algorithm, a translation and rotation based on all nodes is applied.
Use the user-defined mesh mapper algorithm to project the reference node set onto the design mesh.
Note: The user-specified direction mesh mapper might be a poor choice for this task. Furthermore, be aware that oSP3D typically uses all nodes for the projection step and postprocesses this information only. The compatible mesh mapper is currently the only one that runs the projection step already on the reduced node set.
With node subsets defined for both meshes, the chosen transformation algorithm is applied.