Research On Modelling Technologies For Repairing Incomplete Geometric Features And Applications On Facial Prostheses

The model reconstruction technique for incomplete geometrical features is an important branch of three-dimensional reconstruction technique. The main task of traditional reverse engineering is reproduction and re-design for complete geometrical feature; while the characteristic of this technique is to recover the original defect information by analyzing the characteristics of known data. The modelling technique is widely used in medical engineering, product design and manufacturing, product testing and quality control, virtual reality, body measurement, heritage conservation and other fields. Especially in the facial plastic surgery field, more and more attention is paid. In the prosthetic treatment, in order to meet the repair requirements,such as, high precision, high efficiency and good aesthetic, the patient's facial models are analysed and feature-reconstructed. Our research will meet the goal of studying the characteristics of non-complete geometric features based on the analysis for restoring the defect data information,and developing the corresponding model construction and processing software tools based on the background of the facial plastic surgery and defect, and ultimately forming a highly integrated CAD system of the facial defect repair for clinical application.The accuracy of multi-view registration is essential to the 3D geometry modelling. In order to solve the bad pairwise registration and error propagation for the local details of the measurement data, a method for judging bad pairwise registration is proposed based on the computation of two views'overlapping percentage.As for the multi-view registration for incomplete geometrical features, a method combined measurement data for local details and minimizing error propagation is proposed. Experiments show that maximum ratio of the biggest gap between two views to the shortest side of the bounding box is only 0.0047, reaching the balance of getting whole 3D geometry model and the accumulated error diffusion successfully.For incomplete data with symmetrical features, the data in good condition will be mirrored into the defect region. However, since some points in good condition does not own the corresponding points in the defect area,and in order to ensure continuity and accuracy of convergence,the approach must be taken to judge the authenticity of the temporary corresponding points, getting rid of the point pairs with low credibility. We applied the M estimation ICP-based iterative algorithm, using the residual function instead of the residuals in process of minimizing for the outer points suppression, making the outer point of almost no effect on the minimum process. Experiments show that maximum ratio of the biggest gap to the shortest side of the bounding box is only 0.0046.Through qualitative and quantitative experimental verification, the algorithm can meet the clinical needs.For incomplete data with no self-information is available for the reconstruction, we firstly search the database for the template data with a most similar characteristic as the incomplete data, then use the constraint-based non-rigid alignment to map the template model to the defect region, ensuring the filled data can be transited smoothly with the surrounding issue data.After the construction for the defect area, another important issue is that how to attach the repaired incompleted feature to the surrunding mesh geometrical smoothly. First the amendment of the boundary conditions of the repaired incompleted feature can make it meet the the surrunding mesh with tangent continuity,then the iterations based on double umbrella operation can make the sum of the main curvature at the junction area arrive at the minimum, ensuring smooth transition in geometry at the boundary of the repaired incompleted feature to its surrounding mesh. Experiments show that the distance deviation in the boundary is -0.1034mm～0.1316mm, the curvature change is continuous.We used the above modelling techniques of incomplete data to the field of facial defects repair applications,developing a human facial defects repair CAD system. This software includes: the construction technique of incomplete data with symmetrical features can be used on the facial defects with symmetrical features (such as ocular defects, auditive defects, etc.); the construction technique of incomplete data with feature similarity can be used for asymmetric facial defects (such as nasal defects, etc.); and the technique of geometrical and color transition can be used for the fit of facial prosthesis and surrounding tissues.The system has been used in Shanghai Jiao Tong University Ninth People's Hospital (10 cases) ,the People's Liberation Army General Hospital (3 cases) and other hospitals. Clinical applications proved that the system can meet the requirements of digital prostheses modelling.Therefore, the facial defects repair CAD system will contribute to the treatment of facial defect, as well as costume design and other areas.