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Conclusion and Perspectives

在文檔中 基於影像之3D物體重建 (頁 116-123)

In this thesis, we proposed two new reflectance models for 3D surface reconstruction. First, a novel 3D image reconstruction model was proposed. This method considers the components of both diffusion and specular reflection in the reflectance model. We used two neural networks with symmetric structure to estimate these two reflection models separately and combined them with an adaptive ratio for each point on the object surface. The proposed network estimates the point-wise adaptive combination ratio of the diffusion and specular intensities such that the different reflecting properties of each point on the object surface can help to achieve better performance of surface reconstruction. The proposed symmetric neural network structure with adaptive learning procedure does not need any special parameter setting and the smoothing conditions. It is also easier to achieve the convergence condition and to make the system stable. The critical parameters, such as the light source and the viewing direction and so on, are also obtained from the learning process of the neural network. The obtained normal vectors of the surface can then be applied to 3D surface reconstruction by enforcing integrability approach.

Secondly, we further proposed another new nonlinear reflection model consisting of the diffusion and specular components. We do not need to separate the two components in the proposed nonlinear reflection model. Using the unsupervised non-linear ICA network for solving photometric stereo problems does not need any desired outputs and the smoothing conditions. It is easier to achieve the convergence

condition and make the system stable.

For 3D surface reconstruction, several conclusions are listed below. (a) When we estimate the surface shape, the success of the reflectance model depends on two major components, including the diffusion and specular components. (b) In our methods, we do not know the locations of light sources for solving the photometric stereo problems.

(c) The proposed symmetric neural network structure and the unsupervised post-nonlinear ICA network do not need any special parameter setting and the smoothing conditions.

On the other hand, we proposed a new approach in the surrounding illumination estimation of an image for color reconstruction. The proposed algorithm estimated the illuminant based on chromaticity histogram of the image. And a neural network back-propagation (BP) learning algorithm is used to estimate the spectral power distribution of the illuminant according to the center values of the chromaticity histogram. The proposed algorithm also eliminated the interference of the dominant colors and illumination estimation through low-pass filtering of the chromaticity histogram. The illumination estimation based on the chromaticity histogram can avoid unrealistic assumptions on the color images and provide the highly efficient and robust estimation. Compared with the methods based on neural networks which we proposed before, the size of their architectures is huge. Those methods need many connecting parameters and are not easy to implement on hardware. The size of our architecture is much smaller; hence our methods can estimate the relative parameters of outside sources effectively and rapidly. Thus, we could reconstruct the color of 3D objects by using these parameters. The comparisons in performance have demonstrated the superiority of the proposed algorithm both in estimation accuracy and robustness for color constancy.

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