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layout: default
title: "(Task 2) Intersecting Objects"
permalink: /pathtracer/intersecting_objects
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# (Task 2) Intersecting Objects

Now that your ray tracer generates camera rays, we need to be able to answer the core query in ray tracing: "does this ray hit this object?" Here, you will start by implementing ray-object intersection routines against the two types of objects in the starter code: triangles and spheres. Later, we will use a BVH to accelerate these queries, but for now we consider an intersection test against a single object.

First, take a look at the `rays/object.h` for the interface of `Object` class. An `Object` can be **either** a `Tri_Mesh`, a `Shape`, a BVH(which you will implement in Task 3), or a list of `Objects`. Right now, you will be dealing with `Tri_Mesh`'s case and `Shape`'s case, and you can find the definition for `Triangle` and `Sphere` in  `rays/shapes.h` and `rays/tri_mesh.h`, respectively.

The `hit` routine that you need to implement (for both `Tri_Mesh` and `Sphere `) takes in a ray, and returns a `Trace` structure, which contains information on whether the ray hits the object and if hits, the information describing the surface at the point of the hit. See `rays/trace.h` for the definition of `Trace`.

In order to correctly implement `hit` you need to understand some of the fields in the Ray structure defined in `lib/ray.h`.

* `point`: represents the 3D point of origin of the ray
* `dir`: represents the 3D direction of the ray (this direction will be normalized)
* `time_bound`: correspond to the minimum and maximum points on the ray with its x-component as the lower bound and y-component as the upper bound. That is, intersections that lie outside the [`ray.time_bound.x`, `ray.time_bound.y`]  range should not be considered valid intersections with the primitive.

One important detail of the Ray structure is that `time_bound` is a mutable field of the Ray. This means that this fields can be modified by constant member functions such as `Triangle::hit`. When finding the first intersection of a ray and the scene, you almost certainly want to update the ray's `time_bound` value after finding each hit with scene geometry. By bounding the ray as tightly as possible, your ray tracer will be able to avoid unnecessary tests with scene geometry that is known to not be able to result in a closest hit, resulting in higher performance.

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### **Step 1: Intersecting Triangles**

The first intersect routine that the `hit` routines for the triangle mesh in `student/tri_mesh.cpp`.

While faster implementations are possible, we recommend you implement ray-triangle intersection using the method described in the [lecture slides](http://15462.courses.cs.cmu.edu/fall2017/lecture/acceleratingqueries). Further details of implementing this method efficiently are given in [these notes](ray_triangle_intersection.md).

There are two important details you should be aware of about intersection:

* When finding the first-hit intersection with a triangle, you need to fill in the `Trace` structure with details of the hit. The structure should be initialized with:
    
    * `hit`: a boolean representing if there is a hit or not
    * `time`: the ray's _t_-value of the hit point
    * `position`: the exact position of the hit point. This can be easily computed by the `time` above as with the ray's `point` and `dir`.
    * `normal`: the normal of the surface at the hit point. This normal should be the interpolated normal (obtained via interpolation of the per-vertex normals according to the barycentric coordinates of the hit point)

* When intersection occurs with the back-face of a triangle (the side of the triangle opposite the direction of the normal) you should flip the returned normal to point in that direction. That is, always return a normal pointing in the direction the ray came from!

Once you've successfully implemented triangle intersection, you will be able to render many of the scenes in the media directory. However, your ray tracer will be very slow!

While you are working with `student/tri_mesh.cpp`, you should implement `Triangle::bbox` as well, which are important for task 3.


### **Step 2: Intersecting Spheres**

You also need to implement the `hit` routines for the `Sphere` class in `student/sphapes.cpp`. Remember that your intersection tests should respect the ray's `time_bound`. Because spheres always represent closed surfaces, you should not flip back-facing normals you did with triangles.

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[Visualization of normals](visualization_of_normals.md) might be very helpful with debugging.