Speaker:Simon Maximilian Fraiss
(193-02 Computer Graphics)
Point clouds are a common representation of three-dimensional shapes in computer graphics and 3D-data processing. However, in some applications, other representations are more useful. Gaussian Mixture Models (GMMs) can be used as such an alternative representation.
A GMM is a convex sum of normal distributions, which aims to describe a point cloud's density. In this thesis, we investigate both visualization and construction of GMMs. For visualization, we have implemented a tool that enables both isoellipsoid and density visualization of GMMs. We describe the mathematical backgrounds, the algorithms, and our implementation of this tool. Regarding GMM construction, we investigate several algorithms used in previous papers for constructing GMMs for 3D-data processing tasks. We present our implementations of the expectation-maximization (EM) algorithm and top-down HEM. Additionally, we have adapted the implementation of geometrically regularized bottom-up HEM to produce a fixed number of Gaussians. We evaluate these three algorithms in terms of the quality of their generated GMMs. In many cases, the statistical likelihood, which is maximized by the EM algorithm, is not a reliable indicator for a GMM's quality. Therefore, we instead rely on the reconstruction error of a reconstructed point cloud based on the Chamfer distance. Additionally, we provide metrics for measuring the reconstructed point cloud's uniformity and the GMM's variation of Gaussians. We demonstrate that EM provides the best results in terms of these metrics. Top-down HEM is a fast alternative, and can produce even better results when using fewer input points. The results of geometrically regularized bottom-up HEM are inferior for lower numbers of Gaussians but it can create good GMMs consisting of high numbers of Gaussians very efficiently.
Computationally demanding tasks are typically calculated in dedicated data centers, and real-time visualizations also follow this trend. Some rendering tasks, however, require the highest level of confidentiality so that no other party, besides the owner, can read or see the sensitive data. Here I present a direct volume rendering approach that performs volume rendering directly on encrypted volume data by using the homomorphic Paillier encryption algorithm. This approach ensures that the volume data and rendered image are uninterpretable to the rendering server. My volume rendering pipeline introduces novel approaches for encrypted-data compositing, interpolation, and opacity modulation, as well as simple transfer function design, where each of these routines maintains the highest level of privacy. The approach is open and secure by design, as opposed to secure through obscurity. Owners of the data only have to keep their secure key confidential to guarantee the privacy of their volume data and the rendered images. My work is, to my knowledge, the first privacy-preserving remote volume-rendering approach that does not require that any server involved be trustworthy; even in cases when the server is compromised, no sensitive data will be leaked to a foreign party.
15 + 20
Institute of Visual Computing & Human-Centered Technology
Favoritenstr. 9-11 / E193-02
Austria - Europe