人造四邊形網格遊戲模型簡化

Abstract

四邊形網格(Quadrilateral mesh)因為適合於紋理的模擬以及更有利於對模型上材質的對應(texture mapping)等特性，在設計3維模型上使用的越來越多。同時一般遊戲因為效能上的要求，完成一個數量龐大的多邊形模型(high-polygon model)之後得再產生一個面數較低的模型再來做法向量貼圖，這之間的轉換基本上還是需要人工手動去重建或是做一些佈線規則的指定，這些動作不但耗時而且有些在一個模型相當複雜時並不是那麼的直覺。一般經由3維軟體建構出來的模型比起自動產生的模型會較符合在四邊形網格使用上的必要條件同時也不會有雜亂的網格分布；另一種情況是使用者已經依自己的使用需求手動建構出自己想要的架構或佈線之後需要做面數的調整，在這些情況下保持原本模型的網格分布以及結構是在簡化過程中最重要的，過去的一些四邊形網格的研究多要同時顧及處理結構混亂的模型，而增加一些對結構的最佳化處理或是為了增進局部的網格品質去做一些後製處理以改善簡化過程的損耗，這些舉動反而破壞了原本四邊形網格該有的性質並且經常不符合實際使用上的要求，而且在模型極大的前提下，這些動作反而會減慢簡化的速度，所以我們希望能在保持原本輸入的模型基礎結構的前提下去做簡化(mesh simplification)並更有效率的取得一個高質量的低多邊形模型(low-polygon model)。另一方面遊戲中還有細節層次(Level of detail)的使用，一般的遊戲模型因為有部件以及許多微小的細節，可能難以在細節層次中保留住重要的特徵，這邊我們使用了積元(voxel)的模型簡化法來建構一個適合用於細節層次中的模型，先以積元來過濾模型上的特徵並接合所有部件，轉換回網格模型之後再使用可保留特徵的平滑(smoothing)處理。

Quadrilateral mesh has been a popular choice in model design due to the fact that it makes better simulation of deformation, and is more intuitive for texture mapping. Because the performance requirement in games, after producing a high-polygon model, we will need to produce another low-polygon model to make a normal map. This process is either done all manually or a user needs to define several rules on the model to get the low-polygon model. These methods not only take much time and sometimes is not so intuitive when a model is very complex. When making a high-polygon model the common way is to start from a basic model to do deformation, add elements, or make more detailed pattern. So these models have better structure than automatically converted or constructed quadrilateral models. There is also another situation when a user already construct the low-polygon model with desired structure and topology but need to adjust the amount of polygons. In these situations we need a quadrilateral based simplification method that preserves the original mesh structure and topology. Previous works usually focus on certain unstructured models that introduce some structure optimization method or do certain geometry enhancement in order to get a better local quality. These operations often destroy the special attributes of quadrilateral model and the results are not conform to requirements of common use. When the model size is very big these operations also slow down the simplification seriously. Another issue in games is level of detail, models use in games usually have separate part and too detailed features that cause poor result in level of detail operation. We use a voxel-based method to reconstruct models to solved these problems.