## Task 1: Generating Pixel Samples

Fill in PathTracer::raytrace_pixel(...) in src/pathtracer/pathtracer.cpp.

This function takes pixel coordinates $(x,y)$ as input and updates the corresponding pixel in sampleBuffer with a Spectrum representing the integral of radiance over this pixel. This integral will be estimated by averaging ns_aa samples.

The input pixel coordinates lie in the image space. As illustrated below, the image space has its own coordinate system, where $(0, 0)$ is positioned at the bottom left corner of the image. The input $(x, y)$ therefore corresponds to the bottom left corner of a pixel. Note this coordinate system is different from the one we used in Assignment 1, where $(0, 0)$ is at the top left corner of the image.

To estimate the integral of radiance over a pixel, you should generate ns_aa random rays through the pixel. For each ray, you should call PathTracer::est_radiance_global_illumination(...) to estimate the scene radiance along that ray and then incorporate it into the Monte Carlo estimate of the Spectrum value of the pixel.

Introduction to Samplers:

The concept of sampling, which has already appeared multiple times in this class, will be relied on heavily throughout this assignment. To that end, we introduce and provide a set of "Sampler" classes in src/pathtracer/sampler.h. You can use them to draw 2D or 3D random samples from a particular distribution. For example, PathTracer has a member variable gridSampler, which represents a uniform distribution on a unit square. You can call the get_sample() method of any "Sampler" to draw a random sample.

Implementation Notes:

• PathTracer has a member variable camera. You can generate a ray by calling camera->generate_ray(double x, double y), which you will actually implement in Task 2. By convention, the input coordinates $(x, y)$ to this method must be normalized by the width and height of the image, i.e., $(x, y) \in [0,1] \times [0,1]$. You can use member variable w and h of sampleBuffer to scale coordinates accordingly.
• In the starter code, PathTracer::est_radiance_global_illumination(...) simply returns a debugging color based on either (1) the direction of the camera ray or (2) the normal of some surface intersected by the camera ray. You will implement ray intersection with triangles and spheres in Task 3 and 4. As you proceed with the assignment, you will also update this radiance estimation function with more physically realistic lighting computations.
• You can use the update_pixel(...) method of sampleBuffer to update the Spectrum of a pixel.
• After completing Task 1 and 2, you will be able to generate images to debug your implementation.

## Task 2: Generating Camera Rays

Fill in Camera::generate_ray(...) in src/pathtracer/camera.cpp.

This function takes the normalized image coordinates $(x, y)$ that you calculated in Task 1 as input and outputs a Ray in the world space. By convention, you will first generate the ray in the camera space and then transform it into a ray in the world space.

Just like the image space, the camera space has its own coordiante system. As illustrated below, In the camera space, the camera is positioned at $(0, 0, 0)$ and looks along its $-Z$ axis, called the viewing direction.

Along this viewing direction, we define an axis-aligned rectangular virtual camera sensor that lies on the $Z=-1$ plane. The center of the sensor is at $(0, 0, -1)$. Its bottom left corner is at $(-tan(\frac{hFov}{2}), -tan(\frac{vFov}{2}), -1)$ and its top right corner is at $(tan(\frac{hFov}{2}), tan(\frac{vFov}{2}), -1)$, where $hFov$ and $vFov$ are field of view angles along $X$ and $Y$ axis.

This virtual camera sensor corresponds to an image. A point on the image can be mapped to the sensor, and vice versa. Specifically, normalized image coordinates $(0, 0)$ and $(1, 1)$ should map to the bottom left and top right corner of the sensor in camera space, respectively.

As shown in the figure, the ray in camera space should start at the camera and go through the point on the sensor that corresponds to the normalized image coordinates $(x, y)$ given as input. You want to then transform this ray into a ray in world space. A ray has an origin and a direction. You need to transform both correctly to properly convert a ray from camera space to world space. In addition, we require that the ray direction must be a normalized vector.

Implementation Notes:

Camera has the following member variables that you may use for implementation. This is not an exhaustive list.

• hFov and vFov define the sensor. Note they are in degress and not in radians.
• pos is the camera position in the world space and c2w is the camera-to-world rotation matrix.
• nclip and fclip are both greater than zero and represent the so-called near and far clipping planes. We consider everything that lies outside these two clipping planes invisible to the camera. You should initialize min_t and max_t of a Ray with nclip and fclip, respectively. We will explain this initialization in Task 3.

Sanity Check:

After completing Task 2, you should be able to run the following command to test your implementaions. Note you may need to adjust the path to the .dae file depending on your IDE!

./pathtracer -r 800 600 -f CBempty.png ../dae/sky/CBempty.dae


Your image, saved directly to CBempty.png, should look like the one below. The RGB values at each pixel are based on the direction of camera ray(s) through that pixel in the world space.

With a different command below, the image generated by your implementation should look like the following image.

./pathtracer -r 800 600 -f banana.png ../dae/keenan/banana.dae


Fill in Triangle::has_intersection(...) and Triangle::intersect(...) in src/scene/triangle.cpp.

Triangle::has_intersection(...) simply tests whether there is an intersection between a triangle and the input ray. Triangle::intersect(...) not only tests for intersection, but also reports the location of the nearest intersection point, along with other information detailed below. You are free to use any methods covered in lectures or discussions to test and compute the intersection point between a ray and a triangle.

Recall from lecture that a ray is parameterized by $t$ and only intersections with $t \ge 0$ are valid. The min_t and max_t of a Ray further restricts the range of $t$ where intersections are considered valid. Initially, we set them to nclip and fclip since everything outside this range along a camera ray is invisible to the camera. As we intersect a camera ray with triangles (and spheres), we always update its max_t to be the nearest intersection so that all future intersections that are farther away can be promptly ignored.

Therefore, for both functions, you should return true only if the intersection occurs at $t$ that lies within min_t and max_t of the input ray, in which case you should update max_t accordingly.

For Triangle::intersect(...), if there is a valid intersection, you should populate its input Intersection *isect structure with the following:

• t is the $t$-value of the input ray where the intersection occurs.
• n is the surface normal at the intersection. You should use barycentric coordinates to interpolate the three vertex normals of the triangle, n1, n2, and n3.
• primitive points to the primitive that was intersected (use the this pointer).
• bsdf points to the surface material (BSDF) at the hit point (use the get_bsdf() method). BSDF stands for Bidirectional Scattering Distribution Function, a generalization of BRDF that accounts for both reflection and transmission.

Sanity Check:

After completing Task 3, your rendered image from the following command should look like the one below.

./pathtracer -r 800 600 -f CBempty.png ../dae/sky/CBempty.dae


Fill in Sphere::has_intersection(...) and Sphere::intersect(...) in src/scene/sphere.cpp.
You may wish to implement ray-sphere intersection based on this slide. Just like in Task 3, for both functions, you should return true only if the intersection occurs at $t$ that lies within min_t and max_t of the input ray and you should update max_t accordingly.
For Sphere::intersect(), if there is a valid intersection, you should again populate its input Intersection *isect structure. Note unlike triangles, the surface normal on a sphere can be computed analytically: it is the normalized vector pointing from the sphere center to the intersection point.
./pathtracer -r 800 600 -f CBspheres.png ../dae/sky/CBspheres_lambertian.dae