Pnt - Pnt -> Vec is not implemented?

[brendanzab]

I'm trying to calculate the normal of a triangle:

// A triangle
let x = Pnt3::new(0.0f32, 1.0, 0.0);
let y = Pnt3::new(0.0f32, 1.0, 1.0);
let z = Pnt3::new(1.0f32, 0.0, 1.0);

let v = y - x;       // first side
let w = z - x;       // second side
let n = cross(v, w); // the normal

I get this however:

/Users/brendan/dev/projects/voyager/experiments/village/src/main.rs:153:13: 153:18 error: the trait `nalgebra::structs::pnt::Pnt3SubRhs<f32, &_>` is not implemented for the type `nalgebra::structs::pnt::Pnt3<f32>`
/Users/brendan/dev/projects/voyager/experiments/village/src/main.rs:153     let v = y - x;       // first side
                                                                                    ^~~~~
/Users/brendan/dev/projects/voyager/experiments/village/src/main.rs:154:13: 154:18 error: the trait `nalgebra::structs::pnt::Pnt3SubRhs<f32, &_>` is not implemented for the type `nalgebra::structs::pnt::Pnt3<f32>`
/Users/brendan/dev/projects/voyager/experiments/village/src/main.rs:154     let w = z - x;       // second side
                                                                                    ^~~~~

[brendanzab]

Came across this a few time today, I've had to make some separate functions:

pub fn sub_pnt2<T: BaseFloat>(a: &Pnt2<T>, b: &Pnt2<T>) -> Vec2<T> {
    Vec2::new(a.x - b.x, a.y - b.y)
}

pub fn sub_pnt3<T: BaseFloat>(a: &Pnt3<T>, b: &Pnt3<T>) -> Vec3<T> {
    Vec3::new(a.x - b.x, a.y - b.y, a.z - b.z)
}

[sebcrozet]

The error message is very confusing here (perhaps because of the double dispatch trick). The real error is that you forgot to pass references to the cross product: let n = cross(&v, &w);. The following compiles:

extern crate "nalgebra" as na;

use na::Pnt3;

fn main() {
    // A triangle
    let x = Pnt3::new(0.0f32, 1.0, 0.0);
    let y = Pnt3::new(0.0f32, 1.0, 1.0);
    let z = Pnt3::new(1.0f32, 0.0, 1.0);

    let v = y - x; // first side
    let w = z - x; // second side
    let n = na::cross(&v, &w); // the normal
}

[brendanzab]

haha

[brendanzab]

oops.

[brendanzab]

By the way, we might be able to go without double dispatching now: servo/euclid#56

[sebcrozet]

Yes, we have to try this. There was a reason related to ncollide and the lack of proper where clauses that prevented me to change this sooner, though I don't remember exactly which one... We discussed this a bit on #38 btw.