應用 Stencil Buffer 的動態陰影計算

Abstract

在動態環境中，陰影的處理除了可以強化視覺效果外，最主要可以加強 整個環境的空間感。本論文希望針對物體和物體間的硬陰影(hard shadow)提出一個適合動態環境下的作法。首先，我們試圖去找出物體相 對於光源的輪廓，為有效陰影輪廓（effective shadowing contour)。利 用該輪廓建立陰影容積(shadow volume)。然後利用類似陰影鋪面法（ shadow tiling）和畫家演算法（painter's algorithm）的作法建立物體 間的遮檔關係。最後我們利用stencil buffer來做陰影處理。此外，我們 針對動態環境提出加速的方法，包括局部變動的遮檔關係建立、使用法向 量遮罩 (normal mask) 來加速多邊形面向╱背向光源的測試、利用frame coherence 來減少有效陰影輪廓計算以及使用occlusion culling的方法 來減少多邊形陰影處理的量。由實作後的結果我們可以得知，用stencil buffer來處理陰影比陰影容積二元空間切割樹 (SVBSP tree) 相關的作法 更有效率。另外，我們將比較三個應用stencil buffer處理陰影的方法， 並試著找出一個較好的架構。 In dynamic environments, shadows not only improve the image quality of the scene, but also enhance the perception of spatial relationships. In this thesis, we purpose an object-based hard shadow computation method for dynamic environments. First, we attempt to find the contour (silhouette), called effective shadowing contour (abbreviate ESC), for each object and each point light source. The shadow volumes are constructed by the effective shadowing contours. Next, we find occlusion relationships between objects in the scene by some methods similar to shadow tiling algorithm and painter's algorithm. Finally, we use stencil buffer to generate shadows. Moreover, we purpose speed-up methods for dynamic environments, such as local update of scene change, normal mask for light-face/back-face testing, frame coherence exploration for ESC computation, and occlusion culling for reducing the number of the polygons needed for shadow computation. According to our experiments, the stencil buffer based method is more efficient than shadow volume binary space partition tree (SVBSP tree) based algorithm. we also compare three stencil buffer based methods, and try to conclude with a suitable architecture for shadow computations.