Light Probes: Technical information

The lighting information in the light probesLight probes store information about how light passes through space in your scene. A collection of light probes arranged within a given space can improve lighting on moving objects and static LOD scenery within that space. More info
See in Glossary 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’ EnlightenThe lighting system by Geomerics used in Unity for computing global illumination (GI). More info
See in Glossary, 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 ShaderA small script that contains the mathematical calculations and algorithms for calculating the Color of each pixel rendered, based on the lighting input and the Material configuration. More info
See in Glossary/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

• 2017–06–08 Page published with no editorial review

• Light Probes updated in 5.6