08 Shading 着色

Blinn-Phong Reflection Model

Specular highlights Term 高光
1. Intensity depends on view direction
  - Bright near mirror reflection direction
2. V close to mirror direction <–> half vector near normal(h)
  - Measure “near” by dot productof unit vectors
3. Formula: $L_s = k_s(I/r^2)max(0,<n,h>)^p$
  - What is the p in $max(0,<n,h>)^p$ for? Make it real:
    - If $\alpha $ is too big, $<n,h>^p$ is small.
Ambient Term 环境光照
- Assumption: Shading that does not depend on anything
  - Add constant color to account for disregarded
    illumination and fill in black shadows
  - This is approximate / fake !
- Formula: $L_a=k_aI_a$
Blinn-Phong Reflection Model:
- $L=L_a+L_d+L_s$

Shading Frequencies 着色频率

Shade each triangle (flat shading)
- Flat shading
  - Triangle face is flat — one normal vector
  - Not good for smooth surfaces
Shade each vertex (Gouraud shading)
- Gouraud shading
  - Interpolate colors from vertices across triangle
  - Each vertex has a normal vector (how?)
Shade each pixel (Phong shading)
- Phong shading
  - Interpolate normal vectors across each triangle
  - Compute full shading model at each pixel
  - Not the Blinn-Phong Reflectance Model

Defining Per-vertex Normal vectors 怎么定义逐顶点法线
- Simple scheme: average surrounding face normals
- Formula: 法向量加权平均 $ n_v=\frac{s_i\times \sum_iN_i}{||\sum_iN_i||}$
Defining Per-Pixel Normal vectors 怎么定义逐像素法线
- Barycentric interpolation (introducing soon) of vertex normals

Graphics (Real-time Rendering) Pipeline 实时渲染管线

What is it?

graph LR
A(Application) -.Vertex Stream.-> B(Vertex Processing)
    B -.Triangle Stream.-> C(Triangle Processing)
    C -.Fragment Stream.-> D(Rasterization)
    D -.Shaded Fragments.-> E(Fragment Processing)
    E --> F(Display)