update pathtracer docs

docs/pathtracer/camera_rays.md

@@ -20,7 +20,9 @@ This tutorial from [Scratchapixel](https://www.scratchapixel.com/lessons/3d-basi
 
 **Step 2:** Implement `Camera::generate_ray`. This function should return a ray **in world space** that reaches the given sensor sample point. We recommend that you compute this ray in camera space (where the camera pinhole is at the origin, the camera is looking down the -Z axis, and +Y is at the top of the screen.). In `util/camera.h`, the `Camera` class stores `vert_fov` and `aspect_ratio` indicating the vertical field of view of the camera (in degrees, not radians) as well as the aspect ratio. Note that the camera maintains camera-space-to-world space transform matirx `iview` that will come fairly handy. 
 
-**Step 3:** Your implementation of `Pathtracer::trace_pixel` must support supersampling (more than one sample per pixel). The member `Pathtracer::ns_aa` in the raytracer class gives the number of samples of scene radiance your ray tracer should take per pixel (a.k.a. the number of camera rays per pixel. You should implement `Rect::Uniform::sample` (see `src/student/samplers.cpp`), such that it provides uniformly distributed random 2D points in the [0-1]^2 box. Supersampling should be implemented by calling get_sample() to obtain randomly chosen points within the pixel.
+**Step 3:** Your implementation of `Pathtracer::trace_pixel` must support super-sampling. The member `Pathtracer::n_samples` specifies the number of samples of scene radiance to evaluate per pixel. The starter code will hence call `Pathtracer::trace_pixel` one time for each sample, so your implementation of `Pathtracer::trace_pixel` should choose a new location within the pixel each time.
+
+To choose a sample within the pixel, you should implement `Rect::Uniform::sample` (see `src/student/samplers.cpp`), such that it provides (random) uniformly distributed 2D points within the rectangular region specified by the origin and the member `Rect::Uniform::size`. Then you may then create a `Rect::Uniform` sampler with a one-by-one region and call `sample()` to obtain randomly chosen offsets within the pixel.
 
 Once you have implemented `Pathtracer::trace_pixel`, `Rect::Uniform::sample` and `Camera::generate_ray`, you should have a working camera.
 

src/lib/ray.h

@@ -30,7 +30,7 @@ struct Ray {
 
     /// The origin or starting point of this ray
     Vec3 point;
-    /// The unit direction the ray travels in
+    /// The direction the ray travels in
     Vec3 dir;
     /// The minimum and maximum time/distance at which this ray should exist
     Vec2 time_bounds;