Added some comments, Refactored a bit

ria-weekend/src/ray.rs

@@ -20,26 +20,34 @@ impl Ray {
     }
 }
 
-fn color(ray: Ray) -> Vec3 {
+// linear interpolation based on y coordinate
+// top to down
+fn linear_interpolate_y(ray: Ray) -> Vec3 {
     let unit_direction = ray.direction().unit_vector();
     let t = 0.5 * (unit_direction.y() + 1.0);
-    return Vec3::new(1.0, 1.0, 1.0) * (1.0 - t) + Vec3::new(0.5, 0.7, 1.0) * t;
+    // (1.0 - t) * start blend_color + t * end color
+    Vec3::new(1.0, 1.0, 1.0) * (1.0 - t) + Vec3::new(0.0, 0.0, 0.0) * t
 }
 
 pub fn create_ray_demo(buf: &mut String, w: u32, h: u32) {
-    let lower_left_corner = Vec3::new(-2.0, -1.0, -1.0);
-    let horizontal = Vec3::new(4.0, 0.0, 0.0);
+    // uses standard cg RHS notation
+    // y up, z pointing outwards and x to right
+    let lower_left_corner = Vec3::new(-1.0, -1.0, -1.0);
+    let horizontal = Vec3::new(2.0, 0.0, 0.0);
     let vertical = Vec3::new(0.0, 2.0, 0.0);
+    // observer
     let origin = Vec3::new(0.0, 0.0, 0.0);
 
     for j in (0..h).rev() {
         for i in 0..w {
+            // relative offsets
+            // current position to total width/length
             let u = i as f32 / w as f32;
             let v = j as f32 / h as f32;
 
             let ray = Ray::new(origin, lower_left_corner + horizontal * u + vertical * v);
 
-            let color = color(ray);
+            let color = linear_interpolate_y(ray);
             let ir = (255.99 * color[0]) as u32;
             let ig = (255.99 * color[1]) as u32;
             let ib = (255.99 * color[2]) as u32;