Suggestion: clarify documentation on how to set Fbm parameters, or make fields private

[davechallis]

Since Fbm's fields are public, I assumed that the correct way of changing them was just to modify them.

This worked for many cases, but then I noticed that I sometimes started getting panics, which I tracked down to me setting octaves to greater than 6, e.g.:

use noise::{NoiseFn, Fbm};

fn main() {
    let mut fbm_noise = Fbm::new();
    fbm_noise.octaves = 7;
    println!("{}", fbm_noise.get([1.0, 1.0]));
}

which panics with

thread 'main' panicked at 'index out of bounds: the len is 6 but the index is 6', /rustc/4fb7144ed159f94491249e86d5bbd033b5d60550/src/libcore/slice/mod.rs:2842:10

After reading the source, I eventually found out that there was a MultiFractal trait which adds a set_octaves function to Fbm - using that avoids the panic.

It took me a while to disover that though, so adding some docs saying that Fbm's fields shouldn't be mutated directly (or maybe making them private), and adding a mention of the MultiFractal train in the Fbm docs could make things clearer.

[Razaekel]

Thank you for pointing this out. I'll have to go over Fbm and the other structs, and make sure that they can't end up in an invalid state during construction.

[tobiaskluth]

I have a question, which is kind of related:
Why is the field for octaves in Fbm pointer sized, when it is later clamped between 1 and MAX_OCTAVES(32)?
Forgive me, if this is a stupid question, i'm just starting with rust.