- ② To further improve efficiency, Russian roulette can be applied to skip tracing many of the shadow rays that make a low contribution to the final image: to implement this approach, tentatively compute the potential contribution of each shadow ray to the final overall radiance value before tracing the ray. If the contribution is below some threshold, apply Russian roulette to possibly skip tracing the ray. Measure the effect your scheme has on Monte Carlo efficiency for a number of test scenes.
- ② Read Veach’s description of efficiency-optimized Russian roulette, which adaptively chooses a threshold for applying Russian roulette (Veach 1997; Section 10.4.1). Implement this algorithm in pbrt, and evaluate its effectiveness in comparison to manually setting these thresholds.
- ② If a scene has an object with a material that causes all but one of the wavelengths in SampledWavelengths to be terminated (e.g., due to dispersion), then rays may often undergo a number of scattering events before they hit such an object. In pbrt’s current implementation, the path’s radiance estimate is divided by the wavelength PDF values once, in the PixelSensor::ToSensorRGB() method. An implication of this design is that all the lighting calculations along the path are affected by the termination of wavelengths, and not just the ones after it happens. The result is an increase in color noise in such images. Modify one or more integrators to instead perform this division by the current set of wavelength PDFs each time the radiance estimate being calculated is updated and not in PixelSensor::ToSensorRGB(). Verify that the same image is computed for scenes without wavelength termination (other than minor differences due to round-off error). Is there any change in performance? Find a scene where this change improves the result and measure the reduction in MSE.
- ② Measure how much time is spent in Monte Carlo evaluation in the BSDF::rho() method when VisibleSurfaces are being initialized in the PathIntegrator. Do so for both simple and complex scenes that include a variety of BSDF models. Then, improve the BSDF interface so that each BxDF can provide its own rho() implementation, possibly returning either an approximation or the closed-form reflectance. How much does performance improve as a result of your changes?
- ③ Implement a technique for generating samples from the product of the light and BSDF distributions; see for example the papers by Burke et al. (2005), Cline et al. (2006), Clarberg et al. (2005), Rousselle et al. (2008), and Hart et al. (2020). Compare the effectiveness of the approach you implement to the direct lighting calculation currently implemented in pbrt. Investigate how scene complexity (and, thus, how expensive shadow rays are to trace) affects the Monte Carlo efficiency of the two techniques.
- ② Clarberg and Akenine-Möller (2008b) and Popov et al. (2013) both described algorithms that perform visibility caching—computing and interpolating information about light source visibility at points in the scene. Implement one of these methods and use it to improve the direct lighting calculation in pbrt. What sorts of scenes is it particularly effective for? Are there scenes for which it does not help?
- ③ Modify pbrt so that the user can flag certain objects in the scene as being important sources of indirect lighting, and modify the PathIntegrator to sample points on those surfaces according to to generate some of the vertices in the paths it generates. Use multiple importance sampling to compute weights for the path samples, incorporating the probability that they would have been sampled both with BSDF sampling and with this area sampling. How much can this approach reduce variance and improve efficiency for scenes with substantial indirect lighting? How much can it hurt if the user flags surfaces that make little or no contribution or if multiple importance sampling is not used? Investigate generalizations of this approach that learn which objects are important sources of indirect lighting as rendering progresses so that the user does not need to supply this information ahead of time.
- ③ Implement a path guiding algorithm such as the one developed by Müller and collaborators (Müller et al. 2017; Müller 2019) or Reibold et al. (2018). How much does your approach reduce error for scenes with highly varying indirect lighting? What is its effect on scenes with smoother lighting?