GLSL and Rust modulo operator

2020-10-09

I recently was playing around with raymarching and SDFs, and I tried to implement something to have infinite repetition using the modulo operator.

As it turns out, the % operator in Rust is not actually the modulo, but the remainder. As shown in this question, the difference between the two appears with negative numbers:

-21 modulus 4 => 3
-21 remainder 4 => -1

Instead, GLSL uses the actual modulo, computed as x - y * floor(x/y) ¹.

The fix is simple, we just have to create a function that computes the modulo using the formula above. In my case, I implemented it like this in my small math library for vectors:

pub fn modulo(&self, other: Vec3) -> Vec3 {
    fn modulo(a: f32, b: f32) -> f32 {
        a - (b * (a / b).floor())
    }

    Vec3 {
        x: modulo(self.x, other.x),
        y: modulo(self.y, other.y),
        z: modulo(self.z, other.z),
    }
}

And here is the result with infinite white spheres:

Infinite repetition

Source