Shading

这节课

Blinn-Phong reﬂectance model
- Specular and ambient terms (高光和环境光)
Shading frequencies (着色频率)
Graphics pipeline (渲染管线)
Texture mapping (纹理映射)

Blinn-Phong reﬂectance model

Specular Term (Blinn-Phong) (高光)

Intensity depends on view direction (强度取决于在视图方向上)

Bright near mirror reflection direction (明亮的近镜反射方向)

V close to mirror direction half vector near normal (V向量接近镜面方向一半的向量, 接近法向量)

Measure “near” by dot product of unit vectors (用单位向量的点积来度量“近”)

Cosine Power Plots (余弦功率图)

Increasing p narrows the reflection lobe

解释了为什么要加指数p

L_{s} = k_{s} (I / r^{2}) m a x (0, n \cdot h)^{p}

Ambient Term (环境光)

Shading that does not depend on anything (阴影不依赖于任何东西)

Add constant color to account for disregarded illumination and fill in black shadows (添加恒定的颜色来解释忽略的照明和填充黑色阴影)
This is approximate / fake!

Blinn-Phong Reflection Model

L = L_{a} + L_{d} + L_{s} = k_{a} I_{a} + k_{d} (I / r^{2}) m a x (0, n \cdot l) + k_{s} (I / r^{2}) m a x (0, n \cdot h)^{p}

Shading Frequencies (着色频率)

What caused the shading difference?

Shade each triangle (ﬂat shading) (为每个三角形着色(平面着色))

Flat shading

Triangle face is flat — one normal vector
Not good for smooth surfaces

Shade each vertex (Gouraud shading) (为每个顶点着色(顶点着色))

Gouraud shading

Interpolate colors from vertices across triangle
Each vertex has a normal vector (how?)

Shade each pixel (Phong shading) (为每个像素着色(像素着色))

Phong shading (注:此Phong非彼Phong)

Interpolate normal vectors across each triangle
Compute full shading model at each pixel

Face, Vertex or Pixel

如果面足够多, 最终效果相差不多, 但平面着色更高效.

Defining Per-Vertex Normal Vectors (定义每个顶点的法向量)

Best to get vertex normals from the underlying geometry (最好从底层几何中获得顶点法线)

e.g. consider a sphere

Otherwise have to infer vertex normals from triangle faces (否则必须从三角形面推断顶点法线)

Simple scheme: average surrounding face normals (简单的方案: 平均周围面法线) $N_{v} = \frac{Σ_{i} N_{i}}{| | Σ_{i} N_{i} | |}$

Barycentric interpolation (introducing soon) of vertex normals重心插值 (介绍) 顶点法线

逐像素着色通过顶点法线的重心插值来实现

Don’t forget to normalize the interpolated directions (单位化)

Graphics pipeline (渲染管线)

Graphics (Real-time Rendering) Pipeline

Shader Programs

Program vertex and fragment processing stages (程序顶点和片段处理阶段)
Describe operation on a single vertex (or fragment) (描述对单个顶点(或片段)的操作)

Example GLSL fragment shader program:

GLSL

uniform sampler2D myTexture; // program parameter 
uniform vec3 lightDir; // program parameter 
varying vec2 uv; // per fragment value (interp. by rasterizer)
varying vec3 norm; // per fragment value (interp. by rasterizer)
void diffuseShader()
{
    vec3 kd;
    kd = texture2d(myTexture, uv); // material color from texture
    kd *= clamp(dot(–lightDir, norm), 0.0, 1.0); // Lambertian shading model
    gl_FragColor = vec4(kd, 1.0); // output fragment color
}

Shader function executes once per fragment. (Shader函数每个片段执行一次。)
Outputs color of surface at the current fragment’s screen sample position. (输出当前片段屏幕样本位置表面的颜色。)
This shader performs a texture lookup to obtain the surface’s material color at this point, then performs a diffuse lighting calculation. (这个着色器执行纹理查找以获得表面的材质颜色，然后执行漫射照明计算。)