Comparison of spectral vs rgb rendering?

(L) [2014/08/16] [ost by tomasdavid] [Comparison of spectral vs rgb rendering?] Wayback!

It really depends on your materials.
For anything refractive, the slight dispersion effect in the refractions obviously reads more natural.
For other stuff.. skin can be more realistic, you can probably do better hair (both have spectral models more or less readily available), but I don't think there is any rigorous perceptual study. Mostly because both still look different enough from the actual thing, that it would be tough to find the right question to ask.

As for "usual" materials (rocks, stones, wood), it might be tricky to get spectral data and the difference from "realism" probably won't be more than could be ascribed to natual variations in the given material. It could be interesting to try to match a larger object, but you'd actually get spectral textures for the object, as artists won't be painting spectral in photoshop anytime soon.

(L) [2014/08/16] [ost by Dade] [Comparison of spectral vs rgb rendering?] Wayback!

In my opinion, aside from considerations on the rendering quality where spectral rendering is likely to be always better, you have to factor also the impact on rendering times: not because it is computational more expansive than RGB/XYZ rendering but because it generates more noise. After all, you have yet another dimension to sample (i.e. wavelengths).

(L) [2014/08/16] [ost by andersll] [Comparison of spectral vs rgb rendering?] Wayback!

Thanks for all the replies guys. I had totally forgotten that PBRT could be compiled into either mode!

Certainly spectral rendering is going to be expensive which is why I'm interested in doing some comparisons to see if it actually makes a difference for "normal" scenes rather than specially constructed ones that are designed to show the difference, especially where the input is not spectral (i.e. normal painted textures, user-selected colours etc).

(L) [2014/08/19] [ost by Serendipity] [Comparison of spectral vs rgb rendering?] Wayback!

RGB vs. spectral rendering is basicly like comparing "wrong vs. right". RGB rendering is wrong as soon as you start to add color to your light.
While materials can be described quite decent with RGB colors since they usually have a rather smooth spectrum, RGB totally fails for most lightsources, especially LEDs with narrow peaks in the spectrum. In extreme cases the differences can be so huge that you basically have a yellow material in RGB that turns out to be pink in spectral rendering. So if you need to have reliable results, for example if you are a light designer for kitchens or something like that, you need to use spectral rendering.

Having said that, spectral rendering comes with its bunch of issues as well. First of all performance suffers. Depending on how accurate you want to go this might turn out to be not too bad if you decide to not consider dispersion and stuff like that. We are usually seeing a 2x-3x lower performance with spectral rendering of 40 wavelengths compared to RGB so it is not unusable but the performance impact is there. The bigger issue is: where do you get the spectral information from? Getting light spectral information is usually not too difficult, many manufacturers provide these. Material spectra are much harder to get so most of the time you end up using RGB for the anyways. There also is not a single spectral texture format publicly available.

So if you are interested in just creating pretty images, I would say just forget about spectral raytracing, there is not much to be gained from it. But if you need to have accurate results there is no way around it.

If you want to do some comparisons yourself, get a demo version of Autodesk VRED Pro, it allows you to switch between RGB and spectral raytracing instantly so you can compare it directly.

(L) [2014/08/20] [ost by joulsoun] [Comparison of spectral vs rgb rendering?] Wayback!

I thought that this paper was very informative as to when spectral rendering makes a difference:

A reflectometer setup for spectral BTF measurement - Lyssi [2009]
[LINK http://cg.cs.uni-bonn.de/en/publications/paper-details/lyssi-2009-btfspectral/]

It has very intuitive examples comparing RGB and spectral rendering.

In my opinion, spectral rendering is most apparent when you apply measured spectral data to both light sources and materials, it is in the product of those two that interesting things start to happen. In an artist-driven scene, the light source spectra are more usually derived from RGB sliders in one way or another, leading to very smooth spectra, where it is more difficult to spot actual visual differences between RGB and spectral rendering.

I've been doing quite a lot of experiments with Mitsuba, down to 10 nm and 1 nm precision just for fun, and RGB performs very good for typical non-measured scenes, where it's hard to pinpoint any difference. On the other hand, spectral rendering does not incur such a big cost on render times either, as intersection testing far outweighs the cost of the spectrum calculations.

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