> what's special about r,g,b Simply that our monitors reproduce images using red, green and blue light. A PBR engine models the entire process of the light fr

> what's special about r,g,b Simply that our monitors reproduce images using red, green and blue light. A PBR engine models the entire process of the light from a scene, falling into a lens, measuring the light that hits the imaginary rectangles that are the pixels of the output image. A PBR engine can model the light internally as a continuous spectrum using Monte Carlo, integrating paths of weighted-random wavelengths at each pixel. You could stop here if you just wanted physical accuracy, you got the spectral data. But if you want to see it, you'll have to get it onto a computer monitor somehow. So, you model a photo-sensitive medium using an exposure function, and apply colour-space transforms to turn the spectrum into just the three RGB wavelengths, in a way that has been measured empirically to be perceptually close to the original spectrum (look up how we got the CIE standards). This is all science so far. If you don't care about the full spectrum but only the RGB (or why not CMYK) result, there are some optimisations and approximations you can do because in most cases you don't need to simulate the full spectrum. Did you ever stop to think that RGB screens might look weird to dogs as well? However, if you somehow built a dog-monitor with only blue and yellow, you could optimize even further. It would be a lot harder to get accurate colour-space data like CIE from dogs, though. If you wanted to simulate a black-and-white photograph, it would be easier again. > and why do we multiply them together? I actually don't think we do, in modern PBR engines. Multiplying two colours together is done in simpler 3D engines because it models reflection; one RGB triplet represents an approximation of the light spectrum, and the other represents the reflective properties of the material. In every-day language we use the word "colour" to mean both. But they are actually of a different "type", if you will. We say "colour" to both the additive type (light) and subtractive type (material). What we model is the transformation of our photon path (additive) by the material of the surface it hits (subtractive). In the simpler model this happens to equate an element-wise multiplication (not "just" a multiplication). However, in modern PBR engines, the reflection operator is a lot more complex than that. The material is no longer simply represented by an RGB triplet, and the operation between the photon packet and the material isn't a multiplication.