Even though I might not use this package as I don't have any Go project in my pipeline, reading the code was a nice learning experience and just demystified a few things about image processing for me.
Go might not be one of my favorite languages, but its readability is just great (or maybe this codebase isn't idiomatic Go?).
Part of that is because there aren't any 'higher level' functions so you are forced to write everything out as explicit loops. Often a bit tedious but it is definitely easier to see what exactly is happening when reading the code.
Side rant on Haskel: I really wanted to get into it, but when you see function signatures like this:
add :: Integer -> Integer -> Integer
add :: Integer, Integer -> Integer
Since you're touting the simplicity of Go, I can describe a reason why the function signatures is written like that: a,b,c->d would be a special case of a->b->c->d (multi-arity function without currying, I guess?), so it's not needed.
If it was written as
add :: Integer, Integer -> Integer
that would imply that it was a single function taking 2 arguments and that if you called it with only 1 argument it would fail instead of returning you a curried add function (a bit like partial application but not exactly).
I'm sure someone is going to tell me there is an obscure compsci reason why it couldn't have been this: [...]
add :: (Integer, Integer) -> Integer
add 1
So something like:
add 1 _
And then
add _ 1
add _ (minus 1 _)
let f = minus 1 _
in add _ f