Study On Free-form Surface Reconstruction Based On The Gray Level In The Images

Shape from shading (SFS) is an important non-contact measurement method that can be applied in the fields of remote sensing technology, industrial measurement, pattern recognition, product monitoring and micrograph measurement, etc. Therefore, more and more researchers pay more attention to this field recently. However, there is some disadvantages existing in conventional SFS methods: the low accuracy of surface reconstructed, the divergence of the result obtained in iteration process and the complex process of the illuminant estimation. All these disadvantages show that SFS method is incomplete. In this dissertation, compared to other measurement methods, the advantages of SFS method are analyzed firstly. Then, theory and survey of SFS method is introduced. To overcome the disadvantages existing in conventional SFS methods, some improved algorithms are provided and proved by experiments. The contribution of this dissertation is to complete the theory system of SFS method and accelerate its conversion from theory to application.Most researchers pay more attention to the algorithms of reverse calculation in the image irradiance equation based on Lambert illuminant model. Actually, Lambert model is an empirical model, which lacks the consideration of influences for visual direction and roughness of the surface. Therefore, Lambert model can not represent the illuminant phenomenon objectively. Although some algorithms for non-linear partial differential equation (PDE) to resolve SFS problem have been provided, lacking consideration of the accuracy of the illuminant model, the increase of the surface accuracy is not obvious. In this dissertation, after analyzing the phenomenon of light reflection, the reflected light is divided into two components: the diffuse reflection term and the specular reflection term. Based on the features of SFS method, a diffuse reflection model is provided by simplifying the Oren-Nayar model, which is an accurate physical model derived from photometric theory and irradiant theory. Then, an improved illuminant model applicable to SFS method can be obtained by weighted combination of the diffuse reflection model provided with a specular reflection term. Compared to conventional illuminant model applied in SFS method, this model is more reasonable due to its advantages of high accuracy, little limitation to the roughness of the surface and simple mathematic expression.Moreover, the corresponding algorithm based on the improved illuminant model for minimization approach in SFS method is provided. Because the SFS method is to resolve an ill-conditioned PDE, some necessary constraint conditions, such as the brightness constraint, the smoothness constraint and the integrability constraint, etc., have to be added to convert this equation to a calculable one. Additionally, the boundary condition and convergence criterion is offered for iterative calculation. By applying the Lambert model and the improved model, respectively, into the algorithm of SFS method for surface reconstruction whose 3D dimension...