update bvh doc

docs/pathtracer/bounding_volume_hierarchy.md

@@ -6,7 +6,7 @@ permalink: /pathtracer/bounding_volume_hierarchy
 
 # (Task 3) Bounding Volume Hierarchy
 
-In this task you will implement a bounding volume hierarchy that accelerates ray-scene intersection. Most of this work will be in `student/bvh.cpp`.
+In this task you will implement a bounding volume hierarchy that accelerates ray-scene intersection. Most of this work will be in `student/bvh.inl`. Note that this file has an unusual extension (`.inl` = inline) because it is an implementation file for a template class. This means `bvh.h` must `#include` it, so all code that sees `bvh.h` will also see `bvh.inl`.
 
 First, take a look at the definition for our `BVH` in `rays/bvh.h`. We represent our BVH using a vector of `Node`s, `nodes`, as an implicit tree data structure in the same fashion as heaps that you probably have seen in some other courses. A `Node` has the following fields:
 
@@ -16,10 +16,12 @@ First, take a look at the definition for our `BVH` in `rays/bvh.h`. We represent
 * `size_t l`: the index of the left child node
 * `size_t r`: the index of the right child node
 
-The BVH class also maintains a vector of all primitives in the BVH. The fields start and range in the BVH `Node` refer the range of contained primitives in this array. The primitives in this array are not initially in any particular order, and you will need to _rearrange the order_ as you build the BVH so that your BVH can accurately represent the spacial hierarchy.
+The BVH class also maintains a vector of all primitives in the BVH. The fields start and size in the BVH `Node` refer the range of contained primitives in this array. The primitives in this array are not initially in any particular order, and you will need to _rearrange the order_ as you build the BVH so that your BVH can accurately represent the spacial hierarchy.
 
 The starter code constructs a valid BVH, but it is a trivial BVH with a single node containing all scene primitives. Once you are done with this task, you can check the box for BVH in the left bar under "Visualize" when you start render to visualize your BVH and see each levels.
 
+Finally, note that the BVH visualizer will start drawing from `BVH::root_idx`, so be sure to set this to the proper index (probably 0 or `nodes.size() - 1`, depending on your implementation) when you build the BVH.
+
 ![visualize](new_results/bvh_button.png)
 
 ![bvh](new_results/bvh.png)

docs/pathtracer/overview.md

@@ -28,7 +28,7 @@ We have also provided you very powerful debugging tool in `src/student/debug.h`
 | `student/pathtracer.cpp` |  This is the main workhorse class. Inside the ray tracer class everything begins with the method `Pathtracer::trace_pixel` in pathtracer.cpp. This method computes the value of the specified pixel in the output image. | Yes |
 | `student/camera.cpp` | You will need to modify `Camera::generate_ray` in Part 1 of the assignment to generate the camera rays that are sent out into the scene. |  Yes |
 | `student/tri_mesh.cpp`, `student/shapes.cpp` | Scene objects (e.g., triangles and spheres) are instances of the `Object` class interface defined in `rays/object.h`. You will need to implement the `bbox` and intersect routine `hit` for both triangles and spheres. |   Yes |
-|`student/bvh.cpp`|A major portion of the first half of the assignment concerns implementing a bounding volume hierarchy (BVH) that accelerates ray-scene intersection queries. Note that a BVH is also an instance of the Object interface (A BVH is a scene object that itself contains other primitives.)|Yes|
+|`student/bvh.inl`|A major portion of the first half of the assignment concerns implementing a bounding volume hierarchy (BVH) that accelerates ray-scene intersection queries. Note that a BVH is also an instance of the Object interface (A BVH is a scene object that itself contains other primitives.)|Yes|
 |`rays/light.h`|Describes lights in the scene. The initial starter code has working implementations of directional lights and constant hemispherical lights.|No|
 |`lib/spectrum.h`|Light energy is represented by instances of the Spectrum class. While it's tempting, we encourage you to avoid thinking of spectrums as colors -- think of them as a measurement of energy over many wavelengths. Although our current implementation only represents spectrums by red, green, and blue components (much like the RGB representations of color you've used previously in this class), this abstraction makes it possible to consider other implementations of spectrum in the future. Spectrums can be converted into a vector using the `Spectrum::to_vec` method.| No|
 |`student/bsdf.cpp`|Contains implementations of several BSDFs (diffuse, mirror, glass). For each, you will define the distribution of the BSDF and write a method to sample from that distribution.|Yes|