Phong
1975 Illumination for Computer Generated Pictures
S_p = C_p(cos(i) * (1 - d) + d) + W(i) * pow(cos(s), n)
Blinn-Phong
1977 Models of Light Reflection for Computer Synthesized Pictures
i = P_a + d * P_d + s * P_s
d = max(0, dot(N, L))
s = pow(cos(N, H), c_1)
H = (L + V) / 2
Jim Blinn 对 Phong 模型的改进在于高光部分：cos(V, R) => cos((L+V)/2, N) 。
好处在于：
- angle(V, R) 的取值范围在 [0, 180°]，cos 值可能为负，此时用 max or clamp 直接截断会导致明显的明暗边缘。而 angle((L+V)/2, N) 的取值范围在 [0, 90°]，cos 值不可能为负，所以明暗过渡平滑。
- 半程向量比反射向量计算量小。
WebGL Phong Shading demo 测试一下效果：
明暗过渡问题：
【Jim Blinn 是个非常有趣的人，他的 Jim Blinns Corner 系列都是用自己的表情包作封面hhhh】
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