三角網格模型的認證與壓縮技術研究; A Study on Authentication and Compression Techniques for Triangle Mesh Models

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题名:	三角網格模型的認證與壓縮技術研究;A Study on Authentication and Compression Techniques for Triangle Mesh Models
作者:	周昌民;Chang-Min Chou
贡献者:	資訊工程研究所
关键词:	3D認證;易碎型浮水印;漸近式網格;3D浮水印;網格壓縮;3D watermarking;3D authentication;fragile watermarking;progressive mesh;mesh compression
日期:	2007-03-13
上传时间:	2009-09-22 11:32:26 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	近年來，由於3D掃描裝置的改進，以及3D模型描述語言（VRML）的發展，對一般使用者而言，3D模型變得愈來愈容易接受及使用。而且，由於網際網路的成長，有關3D模型的許多議題；諸如3D模型視覺化、傳輸、壓縮、及資訊安全等問題也獲得愈來愈多的重視。在本論文中，我們將探討兩個與3D模型使用相關的主題：3D易碎型浮水印及3D漸進式壓縮傳輸。 3D易碎型浮水印的目的是在3D模型中置入浮水印，用來做為認證之用，使得將來對這一模型做任何變動都能被偵測出來。在3D易碎型浮水印的研究中，要在3D模型中置入浮水印時，經常會遇到兩個問題：ㄧ是前後次序的相關問題（causality problem），亦即後置入的浮水印會影響到之前所置入的浮水印。二是收斂問題（convergence problem），是指所置入的浮水印可能會對原來的3D模型造成太大的形變。在本論文中，我們提出可以克服這兩個問題的易碎浮水印方法；而且我們所提出的方法不需要原始模型和浮水印就可以認證；另外，認證所需的密鑰相對地小於前人所提的方法；而且，此一方法可由密鑰的設定來控制由於置入浮水印所產生形變的大小。基於此一技術，我們進一步發展出另ㄧ不受模型幾何轉換所影響的3D易碎型浮水印技術。此ㄧ技術的主要貢獻是它是不受到旋轉、平移、及放大縮小等幾何轉換的影響。我們認為這些幾何轉換並不會改變原始的模型，所以不應該被視為刻意偽造。這個不變性易碎浮水印方法維持原來方法的所有優點，再加上新的不變性特性。在技術層面上，兩個方法有顯著的不同。此外，我們也在論文中提出一個階層式的3D模型漸進式壓縮技術。此ㄧ壓縮技術可以有效地漸進壓縮並且以多重解析度方式傳輸3D模型。在處理3D模型漸進式壓縮問題時，通常是從最精細的3D模型依序每次消除一個頂點（vertex）的方式逐步化簡成一個最粗略的模型，然後將來可將此一最粗略的模型依化簡時的相反次序逐一加入頂點，而回復成原先最精細的3D模型。通常，在逐步加入頂點時，其所需編碼資訊會隨著模型的頂點數增多而增加，亦即壓縮率會隨著模型增大而降低。在本論文中，我們提出一個不因模型大小而影響壓縮率的階層式3D模型漸進式壓縮技術。大部份過去的3D模型壓縮研究都先編碼模型的連結資訊（connectivity information），再根據連結資訊來對模型的幾何資訊（geometry information）做預測及編碼。我們所提出的壓縮技術先編碼3D模型的幾何資訊，再根據幾何資訊來對模型的連結資訊做預測及編碼。我們的實驗證實此一編碼方式相當成功。 In recent years, 3D graphic models have become more accessible to general users due to the convenient use of advanced scanning devices and the virtual-reality modeling language (VRML) for graphic description. Moreover, due to the growth of Internet, many issues of 3D data processing such as 3D visualization, transmission, compression, and security problems gain more and more attention. In this dissertation, we propose the methods on two interesting 3D research issues: 3D fragile watermarking and 3D progressive compression. There are two major purposes for 3D fragile watermarking: integrity checking and changed region locating. Two problems frequently arise in the embedding stage: the causality problem and the convergence problem. The causality problem arises while the neighboring relationship of a former processed vertex is influenced by the perturbation of its latter processed neighboring vertices. The convergence problem means that the original model has been heavily distorted before some vertices reach the predefined relationship. In this dissertation, we propose a method to overcome these two problems. The proposed method is a public scheme which means that it does not need the original model and watermarks for authentication. The key for extracting watermarks is relatively smaller than that of the previous works. Our method can control the average distortion by the keys used in watermark embedding. Based on this technique, we also developed a transformation invariant fragile watermarking technique for 3D model authentication. The main contribution of the proposed scheme is that it is invariant to translation, rotation, and uniformly scaling operations. We think these operations do not change the integrity of the original models and should not be regarded as a specific forgery. The proposed technique holds all advantages that provided in the prior scheme and provides an extra transformation invariance characteristic. From the viewpoint of technique, these two schemes are quite different. Moreover, a geometry-driven hierarchical compression technique for triangle meshes is proposed such that the compressed 3D models can be efficiently transmitted in a multi-resolution manner. In 3D progressive compression, we usually simplify the finest 3D model to the coarsest mesh vertex by vertex and thus the original model can be reconstructed from the coarsest mesh by operating vertex-split operations in the inversed vertex simplification order. In general, the cost for the vertex-split operations will be increased as the mesh grows. In this dissertation, we propose a hierarchical compression scheme to keep the cost of the vertex-split operations being independent to the size of the mesh. Most previous 3D progression compression schemes first encode the connectivity information, and then predict and encode the geometry information based on the connectivity information. We propose the geometry-driven technique that predicts and encode the connectivity relationship of vertices based on their geometry information. Experimental results show the effectiveness of the proposed encoding scheme.
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