網格化簡與混合式顯像技術及其在即時顯像的應用

Abstract

即時顯像是互動應用，如：遊戲、虛擬實境以及虛擬模擬中核心的技術。即時顯像技術包含了幾何為基礎、影像為基礎以及混合式顯像技術。這本論文中我們首先提出兩個全新的幾何顯像式技術，而後提出一個混和式顯像系統架構。在幾個以幾何為基礎的即時顯像之關鍵技術中，多層次模型建構技術為其中極其重要而且相當廣為流傳的即時顯像技術。我們發展出一個全新、簡單且有效的貼圖適應性調整機制來消除在漸進式網格模型配合貼圖時產生的貼圖扭曲現象。以此機制為基礎，我們提出一個新的幾何化簡之曲面與曲面間誤差估測方式，能與現有的方法相競爭甚至更好。混合式顯像技術在遊戲及虛擬實境中已被證實為非常有用的幾何式顯像的輔助技術。我們提出一個混合式顯像機制，利用多層次模型與貼圖並探索可見性的區域連貫性，以額外的儲存空間及預先讀取來做影像品質與顯像效率的取捨。 為了支援漸進式網格模型的貼圖，我們常讓整個漸進式網格序列共享一個共有的貼圖。這個共有貼圖能用適當的幾何參數化方法考慮幾何與貼圖壓縮值，甚至考慮幾何邊折疊時的貼圖誤差得來。我們發現即使用很好的參數化貼圖，漸進式網格模型貼上貼圖時很容易產生明顯的貼圖扭曲現象，這主要是因幾何的變化與貼圖硬體的線性內差所致。在這論文中，我們提出一個全新、簡單且很有效率的方法針對每個幾何邊折疊來調整貼圖內容，以消除貼圖扭曲現象。貼圖之適應性調整機制及其反運算都是區域性及累進式運算且能以現有的圖學加速卡直接支援。我們另提出索引貼圖的機制來降低當調整貼圖時可能造成取樣不足進而產生的影像糊化現象。實驗結果顯示，在測試的例子中，貼圖扭曲現象幾乎都能被貼圖之適應性調整機制所消除。基於貼圖之適應性調整機制，我們提出一個新的以映像為基礎的誤差估測法，在不好的參數貼圖下，能夠比APS或QEM提供更精確的幾何誤差量測。從實驗結果得知，我們提出的方法比APS好上許多，而幾乎接近QEM的結果。 在我們所提出的混合式顯像架構下，場景先被分割成一些蜂巢式的空間，再對蜂巢內的物體用一般的顯像技術來顯像，而蜂巢外的物體則用多層次貼圖幾何配合投影式貼圖法來顯像。多層次貼圖幾何為一以物體為基礎，由原始幾何依據所得到的深度影像化簡而得來，而深度影像則是在蜂巢與其鄰近蜂巢中心點顯像取得。利用這個多層次貼圖幾何，許多在混合式顯像技術中常被發現的問題，如：因物體與物體間遮檔而產生的洞的問題，以及因解析度不一致所產生的裂縫問題都能被消除。而因為物體自身遮檔產生的洞的問題，也能被限制在使用者指定的範圍內。在我們的顯像架構下，數個由上百萬個三角片所組成的複雜場景，都可達到高於每秒鐘 600 個畫面的平均顯像速率，而且只有些微的影像失真。

Real time rendering has been a kernel technology for interactive applications such as game, virtual reality (VR), and visual simulation. Real time rendering technologies can be geometrybased, image-based, or hybrid. In this thesis, we ?rst present two novel techniques in geometrybased rendering and then propose a hybrid rendering framework. Among several key technologies in geometry-based real time rendering, level-of-detail (LOD) modeling has been a vital representation and a very popular technique in real-time applications. We develop a novel, simple, and effective texture adaptation scheme to eliminate the texture distortion commonly observed in mapping textures to progressive meshes. Based on this scheme, we propose a new surface-to-surface error metric, aiming to offer a simpli?cation error measurement that is competitive to or even better than the existing methods. Hybrid rendering has been proven to be a very useful supplement to geometry-based technologies used in game and VR. We present a hybrid rendering scheme that explores the locality of visibility at the cost of extra storage and prefetching, and makes a tradeoff between image quality and rendering ef?ciency by using textured LOD meshes. To support texture mapping progressive meshes (PM), we usually allow the whole PM sequence to share a common texture map. Such a common texture map can be derived by using appropriate mesh parameterizations that may consider the minimization of geometry stretch, texture stretch, or even the texture deviation introduced by edge collapses. We have found that even with a well parameterized texture map, the texture mapped PM still reveals apparent texture distortion due to geometry changes and the nature of linear interpolation used by texture mapping hardware. In this dissertation, we propose a novel, simple, and ef?cient approach that adapts texture content for each edge collapse, aiming to eliminate texture distortion. A texture adaptation and its reverse operation are local and incremental operations that can be fully supported by graphics hardware. We also propose the mechanism of indexing mapping to reduce blurred artifacts due to the under-sampling that might be introduced by the texture adaptation. Experimental results have demonstrated that texture distortion is almost eliminated in tested examples. Based on the texture adaptation scheme, we propose a new mapping-based error metric that is able to provide much more accurate measurement of simpli?cation error than APS or even QEM in the presence of badly parameterized texture maps. From the experimental results, we have observed that the proposed error metric outperforms the well-known error metric APS, and is better than or almost similar to QEM, depending on either maximum, mean, or RMS error used in measuring the approximation error. In the proposed hybrid rendering framework, the object space is ?rst subdivided into cells. For each cell, inside objects are rendered as normal while outside objects are rendered as textured LOD meshes using projective texture mapping. The textured LOD meshes is object based and derived from the original mesh based on the captured depth images viewed at the centers of the cell and its adjacent cells. With this textured LOD mesh, problems commonly found in hybrid rendering, such as hole problems due to occlusion among objects and the gap problems due to resolution mismatch, can be avoided. Moreover, the size of holes due to self-occlusion is constrained to be within a user-speci?ed tolerance. Several scenes with millions of polygons have been tested and higher than 600 FPS has been achieved with a little loss of image quality.