[crispyambulance]

Spherical harmonics are basically a fourier series. They're a complete orthonormal set of basis functions for functions for the unit sphere. Whereas the fourier series from calc 101 is a complete orthonormal set of basis functions on the unit interval (eg [0,1]).

In other words you can express any reasonable function on the unit sphere as a series of spherical harmonic terms. That makes them ideal for working with differential equations (eg schrodinger's equation for the hydrogen atom, or, emission from an arbitrary light source).

[groundzeros2015]

And the number of terms you need to get a good approximation is related to the frequency. Low frequency signals like lighting work well.

[lukko]

this is all so interesting.. Are there any particular functions / parameters that are typically used, that say replicates 3 point light setups?

I guess at a certain point the number of terms becomes so large that it makes sense to just use a cube map?

[groundzeros2015]

In the era im familiar with (ps3, 360) everyone used the first 9 coefficients. You can read the original Ramamoorthi paper for better theory applied to lighting.

But yes it’s an approximation. If you have a ton of terms it looks like a bitmap like you said.