The lighting information in the light probes are encoded as Spherical Harmonics basis functions. We use third order polynomials, also known as L2 Spherical Harmonics. These are stored using 27 floating point values, 9 for each color channel.
The Enlighten Global Illumination implementation in Unity uses a different spherical harmonics approach than Geomerics, the feature’s original developer, namely the notation and reconstruction method from Peter-Pike Sloan’s paper, Stupid Spherical Harmonics (SH) Tricks. Geomerics’ original approach was based on the notation and reconstruction method from Ramamoorthi/Hanrahan’s paper, An Efﬁcient Representation for Irradiance Environment Maps.
The shader code for reconstruction is found in UnityCG.cginc and is using the method from Appendix A10 Shader/CPU code for Irradiance Environment Maps from Peter-Pikes paper.
[L00: DC] [L1-1: y] [L10: z] [L11: x] [L2-2: xy] [L2-1: yz] [L20: zz] [L21: xz] [L22: xx - yy]
R、G 和 B 的 9 个系数的排列顺序如下：
L00, L1-1, L10, L11, L2-2, L2-1, L20, L21, L22, // 红色通道 L00, L1-1, L10, L11, L2-2, L2-1, L20, L21, L22, // 蓝色通道 L00, L1-1, L10, L11, L2-2, L2-1, L20, L21, L22 // 绿色通道
有关 Unity 光照探针系统的更多背景信息，可阅读 Robert Cupisz 在 2012 年游戏开发者大会 (GDC 2012) 上的演讲报告“使用四面体曲面细分的光照探针插值 (Light Probe Interpolation Using Tetrahedral Tessellations)”- GDC 2012
在 5.6 版更新了光照探针