A3T4 Path Tracing

Up to this point, your renderer has only computed object visibility using ray tracing. Now, we will simulate the complicated paths that light can take throughout the scene, bouncing off many surfaces before eventually reaching the camera. Simulating this multi-bounce light is referred to as global illumination, and it is critical for producing realistic images, especially when specular surfaces are present. Note that all functions in src/scene/material.cpp are in local space to the surface with respect to the ray intersection point, while functions in src/pathtracer/pathtracer.cpp are generally in world space.

Step 1: Pathtracer::trace

Pathtracer::trace is the function responsible for coordinating the path tracing procedure. We've given you code to intersect a ray with the scene and collect information about the surface intersection necessary for computing the lighting at that point. You should read this function and understand where/why functions of the bsdf are called.

Step 2: Lambertian

Implement Lambertian::scatter, Lambertian::evaluate, and Lambertian::pdf. Note that their interfaces are defined in src/scene/material.h. Task 5 will further discuss sampling BSDFs, so reading ahead may help your understanding.

Note: a variety of sampling functions are provided in src/pathtracer/samplers.h.

Note: for testing, notice that sample_direct_lighting_task4 already samples "delta lights" (i.e., non-area lights). So a scene with point or directional lights should show your material working without requiring Step 3 to be complete.

Step 3: Pathtracer::sample_indirect_lighting

In this function, you will estimate light that bounced off at least one other surface before reaching our shading point. This is called indirect lighting.

NOTE: you may wish to add some ray logging to help debug. See, for example, the code in sample_direct_lighting_task6 for and example of such code. Guarding it with a constant (in the example: LOG_AREA_LIGHT_RAYS) is useful so it is easy to turn off for increased performance.

Step 4: Pathtracer::sample_direct_lighting_task4

Finally, you will estimate light that hit our shading point after being emitted from a light source without any bounces in between. For now, you should use the same sampling procedure as Pathtracer::sample_indirect_lighting, except for using the direct component of incoming light. Note that since we are only interested in light emitted from the first intersection, we can trace a ray with depth = 0.

Note: separately sampling direct lighting might seem silly, as we could have just gotten both direct and indirect lighting from tracing a single BSDF sample. However, separating the components will allow us to improve our direct light sampling algorithm in task 6.

Reference Results

After correctly implementing task 4, your renderer should be able to make a beautifully lit picture of the Cornell Box with Lambertian spheres (A3-cbox-lambertian-spheres.s3d). Below is a render using 1024 samples per pixel (spp):

cbox lambertian

cbox lambertian

Note the time-quality tradeoff here. This image was rendered with a sample rate of 1024 camera rays per pixel and a max ray depth of 8. This will produce a relatively high quality result, but will take quite some time to render. Rendering a fully converged image may take a even longer, so start testing your path tracer early!

Thankfully, runtime will scale (roughly) linearly with the number of samples. Below are the results and runtime of rendering the Lambertian cornell box at 240x240 on an Intel Core i7-8086K (max ray depth 8):

cbox-lambertian timing

cbox-lambertian timing

Extra Credit

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