Appearance

Shadow mapping (阴影贴图) ​

Shadows

  • How to draw shadows using rasterization? (如何使用光栅化绘制阴影?)

  • An Image-space Algorithm (一种图像空间算法)

    • no knowledge of scene’s geometry during shadow computation (在阴影计算过程中不了解场景的几何形状)
    • must deal with aliasing artifacts (必须处理混叠工件)

  • Key idea:

    • the points NOT in shadow must be seen both by the light and by the camera (不在阴影中的点必须同时被光线和相机看到)

基本流程 ​

  • 从光源看向场景并做深度测试
  • 从视锥体位置看向场景，从场景中看到的点投影回光源得到该点在深度图中的位置
    • 有些点可以被视锥体看到也可以被光源看到（视锥体投影到的深度图与光源中的深度图相同）
    • 有些点可以被视锥体看到但不能被光源看到（视锥体投影到的深度图与光源中的深度图不符）
  • 不能被光源看到的位置但可以被视锥体看到的位置就是阴影的位置

Pass 1: Render from Light

  • Depth image from light source

Pass 2A: Render from Eye

  • Standard image (with depth) from eye

Pass 2B: Project to light

  • Project visible points in eye view back to light source

Visualizing Shadow Mapping (可视化阴影映射) ​

  • A fairly complex scene with shadows
  • Compare with and without shadows
  • The scene from the light’s point-of-view
  • The depth buffer from the light’s point-of-view
  • Comparing Dist(light, shading point) with shadow map

绿色破碎的原因: 浮点数判断相等非常困难

Problems with shadow maps ​

  • Hard shadows (point lights only) (硬阴影)
  • Quality depends on shadow map resolution(general problem with image-based techniques) (质量取决于阴影贴图的分辨率(基于图像的技术的一般问题))
  • Involves equality comparison of floating point depth values means issues of scale, bias, tolerance (涉及相等比较浮点深度值意味着尺度，偏差，公差的问题)

Hard shadows vs. soft shadows