alt Hacker News> we write the color associated with a spectral distribution as C[S] where C is the function that takes a distribution and outputs the corresponding color.
Unrelated, but can anyone tell me the purpose of using the square bracket notation here, instead of the usual parentheses?
Square brackets are often used in contexts where the function is "higher-order" in some sense, with the type of the parameter itself being a function, distribution, or a similar object with richer structure. I think in this case C[S] is particularly supposed to evoke the expected-value notation E[X], because that's what C[S] essentially is – it gives the expected value of S if you interpret S as a random variable with that distribution.