Lec 07 Shading 1 (Illumination, Shading and Graphics Pipelines)
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Z-Buffer
Painter's Algorithm: paint from back to front, overwrite in the framebuffer.
对三角形排序，在相互遮挡的情况下无法排序。因此对像素排序（每个像素记录最近的距离）。
Z-Buffer: store current min. z-value for each sample (pixel).
Needs an additional buffer for depth values. Frame buffer stores color values, depth buffer (z-buffer) stores depth.
NOTE: For simplicity we suppose z is always positive.
Shading
Definition
Shading: The darkening or coloring of an illustration or diagram with parallel lines or a block of color.
In this course: The process of applying a material to an object.
Blinn-Phong Reflectance Model
Shading: specular highlight + diffuse reflection + ambient lighting
Input:
- viewer direction: v
- surface normal: n
- light direction: l
- surface parameters
Diffuse Reflection
How much light (energy) is received?
Lambert’s cosine law: \(\cos\theta=\mathbb{i}\cdot\mathbb{n}\)
Light Falloff
点光源，能量集中在靠近中心的球壳，但是远近球壳能量守恒。
distance: r, intensity: I
平方反比，计算有多少光到达 shading point
**Lambertian (Diffuse) Shading. **
\(L_d\): diffusely reflected light
\(I/r^2\): energy arrived at the shading point
\(k_d\): diffuse coefficient
\(max(0, \mathbb{n}\cdot\mathbb{l})\): energy received by the shading point
kd 定义不同材质对光的吸收，kd 越大物体越亮
公式中不含 v：漫反射的反射光线均匀分布在各个方向上，和观察方向无关。 如果区分 rgb，则给不同区域赋予不同颜色。