A3T1Generating Camera Rays
"Camera rays" emanate from the camera and measure the amount of scene radiance that reaches a point on the camera's sensor plane. (Given a point on the virtual sensor plane, there is a corresponding camera ray that is traced into the scene.) Your job is to generate these rays, which is the first step in the raytracing procedure.
Take a look at
src/pathtracer/pathtracer.cpp. The job of this function is to estimate the amount of energy arriving at every pixel of a tile of the image. Conveniently, there is a function
Pathtracer::trace(rng, ray) that provides a measurement of incoming scene radiance along the direction given by ray
ray, split into emissive (direct) and reflected (indirect) components. See
src/lib/ray.h for the interface of
do_trace loops over all pixels in the image (and some number of samples in the pixel), selects a ray arriving at the pixel, calls
trace(), and accumulates the results.
This part has already been implemented for you, but be sure to understand what the code is doing.
do_trace to work, its camera rays must actually correspond to sensor pixels. Implement
src/scene/camera.h, which returns a random ray which passes through a given pixel. Notice that the
Camera has a
aspect_ratio indicating the vertical field of view of the camera (in degrees, not radians) as well as the aspect ratio; as well as
film.height parameters giving the camera's sensor plane size in pixels. Together, these fields will allow your code to transform from a pixel location into a location on the camera's sensor plane.
trace() multiple times for each pixel of the image, trusting
Camera::sample_ray to pick a new random location within the pixel. This, in turn, relies on
src/pathtracer/samplers.cpp), such that it provides a random uniformly distributed 2D point within the rectangular region specified by the origin and the member
Once you have implemented
Rect::sample, you will have a working camera (see Raytracing Visualization section below to confirm that your camera is indeed working).
Your code can also log the results of ray computations for visualization and debugging. To do so, simply call the function
Pathtracer::log_ray in your
Pathtracer::trace function. Function
Pathtracer::log_ray takes in 3 arguments: the ray that you want to log, a float that specifies the distance to log that ray up to, and a color for the ray. If you don't pass a color, it will default to white. We encourage you to make use of this feature for debugging both camera rays and those used for sampling direct & indirect lighting.
One thing to note is that you should only log only a small fraction of the generated rays. Otherwise, your result will contain too many generated rays, making the result hard to interpret. To do so, you can add
if(log_rng.coin_flip(0.0005f)) log_ray(out, 10.0f); to log 0.05% of camera rays.
Finally, you can visualize the logged rays by checking the box for Logged rays under Visualize and then starting the render (Open Render Window -> Start Render). After running the path tracer, rays will be shown as lines in visualizer. Be sure to wait for rendering to complete so you see all rays while visualizing.
We've already supplied some camera ray logging code, enabled by setting
LOG_CAMERA_RAYS=true at the top of
Camera and make them function properly. Aperture is the opening in the lens by which light enters the camera. Focal distance represents the distance between the camera aperture and the plane that is perfectly in focus. These parameters can be used to simulate the effects of de-focus blur and bokeh found in real cameras.
To use the focal distance parameter, you simply scale up the sensor position from step 1 (and hence ray direction) by
focal_dist instead of leaving it on the z = -1 plane. You might notice that this doesn't actually change anything about your result, since this is just scaling up a vector that is later normalized. However, now aperture comes in.
By default, all rays start a single point, representing a pinhole camera. But when
aperture > 0, we want to randomly choose the ray origin from an
aperture square centered at the origin and facing the camera direction (-Z). Note that typically aperture of a camera is roughly circular in shape, but a square suffices for our purposes.
Then, we can use this random point as the origin of the generated ray while keeping its sensor position fixed (consider how this changes the ray direction). Now it's as if the same image was taken from slightly off origin. This simulates real cameras with non-pinhole apertures: the final photo is equivalent to averaging images taken by pinhole cameras placed at every point in the aperture.
Finally, we can see that non-zero aperture makes focal distance matter: objects on the focal plane are unaffected, since where the ray hits on the sensor is the same regardless of the ray's origin. However, rays that hit objects closer or farther than the focal distance will be able to "see" slightly different parts of the object based on the ray origin. Averaging over many rays within a pixel, this results in collecting colors from a region larger slightly than that pixel would cover given zero aperture, causing the object to become blurry. We are using a square aperture, so bokeh effects will reflect this.
You can test aperture/focal distance by adjusting
focal_dist using the camera UI and examining logging rays. Once you have implemented primitive intersections and path tracing (tasks 2/4), you will be able to properly render
Write your own pixel sampler (replacing
Rect) that generates samples with a more advanced distribution. Refer to Physically Based Rendering chapter 7. Some examples include:
- Jittered Sampling (1 point)
- Multi-jittered sampling (1 point)
- N-Rooks (Latin Hypercube) sampling (2 points)
- Sobol sequence sampling (2 points)
- Halton sequence sampling (3 points)
- Hammersley sequence sampling (3 points)
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