访问各个 SH 系数。
This indexer accesses the spherical harmonics coefficients with two values. The first value is an RGB color channel index between 0 and 2. The second value is an index between 0 and 8 that combines the degree and order of the spherical harmonics in this order: the single 0th degree term, the three 1st degree terms, the five 2nd degree terms.