Replace util ctor's with constants

I don't need functions to create new vectors, I just want a constant
that I can clone. The compiler probably does the same thing in both
cases, but this is more semantically accurate to that goal.

src/main.rs

@@ -30,8 +30,8 @@ fn main() {
     let scene = Scene {
         camera: Camera::new(
             Vec3::new(13.0, 2.0, 3.0),  // lookfrom
-            Vec3::zero(),              // lookat
-            Vec3::new(0.0, 1.0, 0.0),  // vup
+            Vec3::ZERO,                 // lookat
+            Vec3::UP,                   // vup
             20.0,
             aspect_ratio,
             0.1,  // aperture

src/primitives.rs

@@ -123,21 +123,24 @@ impl Vec3 {
         Vec3 { x, y, z }
     }
 
-    pub fn zero() -> Vec3 {
-        Vec3 {
+    pub const ZERO: Self = Vec3 {
         x: 0.0,
         y: 0.0,
         z: 0.0,
-        }
-    }
+    };
 
-    pub fn ones() -> Vec3 {
-        Vec3 {
+    pub const ONES: Self = Vec3 {
         x: 1.0,
         y: 1.0,
         z: 1.0,
-        }
-    }
+    };
+
+    /// "Up" is considered to be positive-y
+    pub const UP: Self = Vec3 {
+        x: 0.0,
+        y: 1.0,
+        z: 0.0,
+    };
 
     pub fn rand(srng: &mut SmallRng, distrib: Uniform<f32>) -> Vec3 {
         Vec3 {

src/renderer.rs

@@ -25,16 +25,16 @@ fn to_uv(coord: Vec2i, img_size: Vec2i) -> Vec2f {
 fn ray_color(r: Ray, surface: &Hittable, depth: u32, rng: &mut SmallRng) -> Vec3 {
     // recursion guard
     if depth == 0 {
-        return Vec3::zero();
+        return Vec3::ZERO;
     }
 
     // cast a ray, interrogate hit record
     if let Some(record) = surface.hit(r, 0.001, f32::INFINITY) {
         let mut scattered = Ray {
-            orig: Vec3::zero(),
-            dir: Vec3::zero(),
+            orig: Vec3::ZERO,
+            dir: Vec3::ZERO,
         };
-        let mut attenuation = Vec3::zero();
+        let mut attenuation = Vec3::ZERO;
         if record
             .material
             .scatter(r, &record, &mut attenuation, &mut scattered, rng)
@@ -49,7 +49,7 @@ fn ray_color(r: Ray, surface: &Hittable, depth: u32, rng: &mut SmallRng) -> Vec3
     // when nothing is struck, return sky color
     let unitdir = Vec3::as_unit(r.dir);
     let t = 0.5 * (unitdir.y + 1.0);
-    Vec3::ones() * (1.0 - t) + SKY_COLOR * t
+    Vec3::ONES * (1.0 - t) + SKY_COLOR * t
 }
 
 fn sample_pixel(
@@ -60,7 +60,7 @@ fn sample_pixel(
     // Supplied by the execution environment (the thread)
     rng: &mut SmallRng,
 ) -> Vec3 {
-    (0..render_props.samples).fold(Vec3::zero(), |color, _sample| -> Vec3 {
+    (0..render_props.samples).fold(Vec3::ZERO, |color, _sample| -> Vec3 {
         let uv = to_uv(coord, img_size);
         let ray = scene.camera.get_ray(uv.x, uv.y, rng);
         if ray.dir.x.is_nan() {

src/scene.rs

@@ -141,7 +141,7 @@ impl Material {
                 Vec3::dot(scattered.dir, rec.normal) > 0.0
             }
             Material::Dielectric { index_refraction } => {
-                *attenuation = Vec3::ones();
+                *attenuation = Vec3::ONES;
                 let refraction_ratio = if rec.front_face {
                     1.0 / index_refraction
                 } else {