Light Probe data format
Understand how Unity stores light as spherical harmonics data in Light Probes.
Read time 1 minuteLast updated 25 days ago
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 Realtime 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.
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 channelL00, L1-1, L10, L11, L2-2, L2-1, L20, L21, L22, // blue channelL00, 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