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(Task 4) Shadow Rays

In this task you will modify Pathtracer::trace_ray to implement accurate shadows.

Currently Pathtracer::trace_ray computes the following:

  • It computes the intersection of ray r with the scene.
  • It computes the amount of light arriving at the hit point hit.position (the irradiance at the hit point) by integrating radiance from all scene light sources.
  • It computes the radiance reflected from the hit point in the direction of -r. (The amount of reflected light is based on the BSDF of the surface at the hit point.)

Shadows occur when another scene object blocks light emitted from scene light sources towards the hit point. Fortunately, determining whether or not a ray of light from a light source to the hit point is occluded by another object is easy given a working ray tracer (which you have at this point!). You simply want to know whether a ray originating from the hit point (hit.position), and traveling towards the light source (direction to light) hits any scene geometry before reaching the light. (Note that you need to consider light’s distance from the hit point is given, more on this in the notes below.)

Your job is to implement the logic needed to compute whether hit point is in shadow with respect to the current light source sample. Below are a few notes:

  • In the starter code, when we call light.sample(hit.position), it returns us a Light_sample sample at the hit point . (You might want to take a look at rays/light.h for the definition of struct Light_sample and class light.) A Light_sample contains fields radiance, pdf, direction, and distance. In particular, sample.direction is the direction from the hit point to the light source, and sample.distance is the distance from the hit point to the light source.

  • A common ray tracing pitfall is for the “shadow ray” shot into the scene to accidentally hit the same objecr as r (the surface is erroneously determined to be occluded because the shadow ray is determined to hit the surface!). We recommend that you make sure the origin of the shadow ray is offset from the surface to avoid these erroneous “self-intersections”. For example, consider setting the origin of the shadow ray to be hit.position + epsilon * sample.direction instead of simply hit.position. EPS_F is defined in for this purpose(see lib/mathlib.h).

  • Another common pitfall is forgetting that it doesn’t matter if the shadow ray hits any scene geometry after reaching the light. Note that the light’s distance from the hit point is given by sample.distance. Also note that Ray has a member called time_bound…
  • You will find it useful to debug your shadow code using the DirectionalLight since it produces hard shadows that are easy to reason about.
  • You would want to comment out the line Spectrum radiance_out = Spectrum(0.5f); and initialize the radiance_out to a more reasonable value. Hint: is there supposed to have any amount of light before we even start considering each light sample?

At this point you should be able to render very striking images.

Sample results:

At this point, you can add all kinds of lights among the options you have when you create “New Light” in Layout mode, except for Sphere Light and Environment Map which you will implement in task 7 (Note that you can still fill in Sphere::Uniform::sample in Samplers.cpp now to view the result of a mesh under Sphere Light).

The head of Peter Schröder rendered with hemishphere lighting.

A sphere and a cube with hemishphere lighting

Hex and cube under directional lighting

Bunny on a plane under point light

Spot on a sphere under diretional lighting

Spot on a sphere under hemisphere lighting