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. Even though Unity is using Geomerics’ Enlighten, we use a different SH basis than what you find on their blog (y and z axes are swapped). Unity is using the notation and reconstruction method from Peter-Pike Sloan’s paper, Stupid Spherical Harmonics (SH) Tricks, and Geomerics are using the notation and reconstruction method from Ramamoorthi/Hanrahan’s paper, An Efficient Representation for Irradiance Environment Maps.
The Unity 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.
The data is internally ordered like this:
[L00: DC]
[L1-1: y] [L10: z] [L11: x]
[L2-2: xy] [L2-1: yz] [L20: zz] [L21: xz] [L22: xx - yy]
The 9 coefficients for R, G and B are ordered like this:
L00, L1-1, L10, L11, L2-2, L2-1, L20, L21, L22, // red channel
L00, L1-1, L10, L11, L2-2, L2-1, L20, L21, L22, // blue channel
L00, L1-1, L10, L11, L2-2, L2-1, L20, L21, L22 // green channel
For more “under-the-hood” information about Unity’s light probe system, you can read Robert Cupisz’s talk from GDC 2012, Light Probe Interpolation Using Tetrahedral Tessellations", GDC 2012
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Light Probes updated in 5.6