Hidden Surface Removal & Antialiasing¶

Hidden Surface Removal¶

glEnable/ glDisable(GL_CULL_FACE);
glCullFace(mode);
glutInitDisplayMode(...| GLUT_DEPTH);
glEnable(GL_DEPTH_TEST);

Visible Surface Determination¶

Goal
- Given: a set of 3D objects and view specification
- Determine: those parts of the objects that are visible when viewed along the direction of projection
Approaches: object space algorithm and image space algorithm
Object Precision Algorithm

for (each object in the world) {
    determine the parts of the object whose view is unobstructed by other parts or any other object;
    draw those parts;
}

Image Precision Algorithm

Back-face Culling: elimination of back-faces
- 在封闭多边形表面 like surface of a polyhedral volume or a solid polyhedron 很显然地我们看不到背面的部分
- back face: part of the object surface facing away from the eye
- Algorithm
  - 找到eye-vector/normal和面之间的角度
  - 如果这个角度在0°~90°，则为背面，剔除
- 处理不了的情况：
Painter's Algorithm: draw back to front
- Failure case: 排不出序
- Warnock's Area Subdivision
  - Start with whole image
  - If one of the easy cases is satisfied, draw what's in front
  - front polygon covers the whole window or
  - there is at most one polygon in the window.
  - Otherwise, subdivide region into 4 windows and repeat
  - If region is single pixel, choose surface with smallest depth
  - Advantages:
  - No over-rendering 不会重复绘制
  - Anti-aliases well. Go deeper to get sub-pixel information
  - Disadvantages: 不同情况运行效率差距大
  - Tests are quite complex and slow
  - Not amenable for hardware implementation 不适合做硬件实现

Z-buffer Algorithm

除了一个frame buffer存color值，还需要一个一样size的z-buffer存储depth(z) value

Code

for ( j=0; j<SCREEN HEIGHT; j++ )
for ( i=0; i<SCREEN WIDTH; i++ )
{
    WriteToFrameBuffer(i, j, BackgroundColor);
    WriteToZBuffer(i, j, MAX);
}
for ( each polygon )
for ( each pixel in polygon's projection )
{
    z = polygon's z value at (i, j) ;
    if( z< ReadFromZBuffer(i, j) ) {
        WriteToFrameBuffer(i, j, polygon's color at (i, j));
        WriteToZBuffer(i, j, z);
    }
}

Related OpenGL Functions

BSP Tree: 对画家算法很有效，帮助找到前后顺序。树可以预先计算
- Very efficient for a static group of 3D polygons as seen from an arbitrary viewpoint
- Correct order for Painter's algorithm is determined by a traversal of the binary tree of polygons
- Pros
  - simple, elegant scheme
  - only write to framebuffer
- Cons
  - 需要较为繁琐的预处理过程，适用于静态场景
  - splitting会增大场景中多边形数量
k-d Tree
Ray Casting

Aliasing¶

Aliasing是由于display设备的离散性，如用一些有限性采样连续信号，如果采样不充分，信息就会丢失
影响：jagged edges(锯齿), incorrectly rendered fine details, small objects might miss(有些细节没采样到)

Anti-aliasing¶

Super-sampling：增加采样率
低通滤波：滤去高频部分
Area sampling: Intensity of the boundary pixels is adjusted depending on the percent of the pixel area covered by the primitive.

Hidden Surface Removal & Antialiasing¶

Hidden Surface Removal¶

Related OpenGL Functions¶

Visible Surface Determination¶

Aliasing¶

Anti-aliasing¶

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