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**Chapter 10: Some Architectural Decisions** I won't write any code in this chapter. We’re at a crossroads where I need to make some architectural decisions. The mixture-density approach is to not have traditional shadow rays and is something I personally like, because in addition to lights you can sample windows or bright cracks under doors or whatever else you think might be bright. But most programs branch, and send one or more terminal rays to lights explicitly and one according to the reflective distribution of the surface. This could be a time you want faster convergence on more restricted scenes and add shadow rays; that’s a personal design preference. There are some other issues with the code. The pdf construction is hard coded in the color() function and we should clean that up. Probably we should pass something into color about the lights. Unlike bvh construction, we should be careful about memory leaks as there are an unbounded number of samples. The specular rays (glass and metal) are no longer supported. The math would work out if we just made their scattering function a delta function . But that would be floating point disaster. We could either separate out specular reflections, or have surface roughness never be zero and have almost-mirrors that look perfectly smooth but don’t generate NaNs. I don’t have an opinion on which way to do it -- I have tried both and they both have their advantages -- but we have smooth metal and glass code anyway, so I add perfect specular surfaces that do not do explicit f()/p() calculations. We also lack a real background function infrastructure in case we want to add an environment map or more interesting functional background. Some environment maps are HDR (the R/G/B components are floats rather than 0-255 bytes usually interpreted as 0-1). Our output has been HDR all along and we’ve just been truncating it. Finally, our renderer is RGB and a more physically-based one -- like an automobile manufacturer might use -- would probably need to use spectral colors and maybe even polarization. For a movie renderer, you would probably want RGB. You can make a hybrid renderer that has both modes, but that is of course harder. I’m going to stick to RGB for now, but I will revisit this near the end of the book.