Assignment 3-1: Pathtracer

Due Date

Tue March 12, 11:59pm

Overview

You will implement the core routines of a physically-based renderer using a pathtracing algorithm. This assignment reinforces many of the hefty ideas covered in class recently, including ray-scene intersection, acceleration structures, and physically based lighting and materials. By the time you are done, you'll be able to generate some stunning pictures (given enough patience and CPU time). You will also have the chance to extend the assignment in a plethora of technically challenging and intellectually stimulating directions.

This project is much longer than the other projects. Rendering images for your writeup will take several hours at the minimum, so start early! Make sure you read the "Experiments, Report, and Deliverables" article ahead of time.

Project Parts

This time, we've split off each part into its own article:

• Part 1: Ray Generation and Scene Intersection
• Part 2: Bounding Volume Hierarchy
• Part 3: Direct Illumination
• Part 4: Global Illumination
• Part 5: Adaptive Sampling

All parts are equally weighted at 20 points each, for a total of 100 points.

You'll also want to read these articles:

• Experiments, Report and Deliverables
• Optional Extra Credit Opportunities

In particular, please read the website writeup and deliverables page before beginning the project. Several parts of this assignment ask you to compare various methods/implementations, and we don't want you to be caught off guard!

Using the program

First, accept the assignment on Github Classroom. Then, clone your private repo. DO NOT clone the skeleton.

git clone <YOUR_PRIVATE_REPO>

As before, use cmake and make inside a build/ directory to create the executable (how to build and submit).

Command line options

Use these flags between the executable name and the dae file when you invoke the program. For example, to simply run the regular GUI with the CBspheres.dae file and 8 threads, you could type (note these commands will not ):

./pathtracer -t 8 ../dae/sky/CBspheres_lambertian.dae

If you wanted to save to the spheres_64_16_6.png file on the instructional machines with 64 samples per pixel, 16 samples per light, 6 bounce ray depth, and 480x360 resolution, you might rather use something like this:

./pathtracer -t 8 -s 64 -l 16 -m 6 -r 480 360 -f spheres_16_4_6.png ../dae/sky/CBspheres_lambertian.dae

For this assignment, we've provided a windowless run mode, which is triggered by providing a filename with the -f flag. The program will run in this mode when you are ssh-ed into the instructional machines.

This means that when trying to generate high quality results for your final writeup, you can use the windowless mode to farm out long render jobs to the s349 machines! You'll probably want to use screen to keep your jobs running after you logout of ssh. After the jobs complete, you can view them using the display command, assuming you've ssh-ed in with graphics forwarding enabled, or scp them back to your local machine.

Also, please take note of the -t flag! We recommend running with 4-8 threads almost always -- the exception is that you should use -t 1 when debugging with print statements, since printf and cout are not guaranteed to be thread safe. Furthermore, if you are using a virtual machine, make sure that you allocate multiple CPU cores to it. This is important in order for your -t flag to work. In practice, it's good to assign $N-2$ cores to the virtual machine, where $N$ is the number of your physical machine's number of CPU cores or hyperthreaded cores.

Moving the camera (in edit and BVH mode)

Keyboard commands

Cell render mode lets you use your mouse to highlight a region of interest so that you can see quick results in that area when fiddling with per pixel ray count, per light ray count, or ray depth.

Basic code pipeline

What happens when you invoke pathtracer in the starter code? Logistical details of setup and parallelization:

1. The main() function inside main.cpp parses the scene file using a ColladaParser from collada/collada.h.
2. A new Viewer and Application are created. Viewer manages the low-level OpenGL details of opening the window, and it passes most user input into Application. Application owns and sets up its own pathtracer with a camera and scene.
3. An infinite loop is started with viewer.start(). The GUI waits for various inputs, the most important of which launch calls to set_up_pathtracer() and PathTracer::start_raytracing().
4. set_up_pathtracer() sets up the camera and the scene, notably resulting in a call to PathTracer::build_accel() to set up the BVH.
5. Inside start_raytracing() (implemented in pathtracer.cpp), some machinery runs to divide up the scene into "tiles," which are put into a work queue that is processed by numWorkerThreads threads.
6. Until the queue is empty, each thread pulls tiles off the queue and runs raytrace_tile() to render them. raytrace_tile() calls raytrace_pixel() for each pixel inside its extent. The results are dumped into the pathtracer's sampleBuffer, an instance of an HDRImageBuffer (defined in image.h).

Most of the core rendering loop is left for you to implement.

1. Inside raytrace_pixel(), you will write a loop that calls camera->generate_ray(...) to get camera rays and est_radiance_global_illumination(...) to get the radiance along those rays.
2. Inside est_radiance_global_illumination, you will check for a scene intersection using bvh->intersect(...). If there is an intersection, you will accumulate the return value in Spectrum L_out,
   • adding the BSDF's emission with zero_bounce_radiance which uses bsdf->get_emission(),
   • adding global illumination with at_least_one_bounce_radiance, which calls one_bounce_radiance (which calls a direct illumination function), and recursively calls itself as necessary

You will also be implementing the functions to intersect with triangles, spheres, and bounding boxes, the functions to construct and traverse the BVH, and the functions to sample from various BSDFs.

Approximately in order, you will edit (at least) the files

• pathtracer.cpp (part 1)
• camera.cpp (part 1)
• static_scene/triangle.cpp (part 1)
• static_scene/sphere.cpp (part 1)
• bvh.cpp (part 2)
• bbox.cpp (part 2)
• bsdf.cpp (part 3)
• pathtracer.cpp (parts 3-5)

You will want to skim over the files

• ray.h
• intersection.h
• sampler.h/cpp
• random_util.h
• static_scene/light.h/cpp

since you will be using the classes and functions defined therein.

In addition, ray.h contains a defined variable PART. This is currently set to 5. You may use this variable if you'd like to test separate parts independently. For example, you can write things like if (PART != 4) { return false; } to easily "revert" parts of your code. In particular, you will need to set PART to 5 to get your rate sampling images in Part 5.

Speeding up rendering tests: setting field of view (cropping) and number of samples

High quality renders take a long time to complete! During development and testing, don't spend a long time waiting for full-sized, high quality images. Make sure you use the parameters specified in the writeup for your website, though!

Here's some ways to get quick test images:

• Render with fewer samples!
• Utilize the cell rendering feature - start a render with R, then hit C and click-drag on the viewer. The pathtracer will now only render pixels inside the rectangular region you selected.
• Set a smaller window size using the -r flag (example: ./pathtracer -t 8 -s 64 -r 120 90 ../dae/sky/CBspheres_lambertian.dae). Zoom out until the entire scene is visible in the window, then start a render with R.