It seems like OIT is kind of a misnomer when people are talking about deferred compositing. Storing data and sorting later isn’t exactly order independent, you still have to compute the color contributions in depth order, since transparency is fundamentally non-commutative, right?
The main benefit of spectral transparency is what happens with multiple different transparent colors… you can get out a different color than you get when using RGB or any 3 fixed primaries while computing the transmission color.
sRGB and P3, what most displays show, by definition use the D65 illuminant, which approximates "midday sunlight in northern europe." So, when you render something indoors, either you are changing the RGB of the materials or the emissive RGB of the light source, or tonemapping the result, all of which can approximate other light sources to some extent. Spectral rendering allows you to better approximate these other light sources.
> sRGB and P3, what most displays show, by definition use the D65 illuminant
I feel like that’s a potentially confusing statement in this context since it has no bearing on what kind of lights you use when rendering, nor on how well spectral rendering vs 3-channel rendering represents colors. D65 whitepoint is used for normalization/calibration of those color spaces, and doesn’t say anything about your scene light sources nor affect their spectra.
I’ve written a spectral path tracer and find it somewhat hard to justify the extra complexity and cost most of the time, but there are definitely cases where it matters and it’s useful. Also there’s probably more physically spectral data available now than when I was playing with it. I’m sure you’re aware and this is what you meant, but might be worth mentioning that it’s the interaction of multiple spectra that matters when doing spectral rendering. For example, it doesn’t do anything for the rendered color of a light source itself (when viewed directly), it only matters when the light is reflected or transmitted through spectra that are different from the light source, that’s where wavelength sampling will give you a different result than a 3-channel approximation.
Very helpful for things like skin, and light diffusion through skin with brute-force (i.e. Woodcock tracking) volume light transport.
If a ray carries full spectral information, then a transparent material can be described by its absorption spectrum — similar to how elements absorb specific wavelengths of light, as shown here: https://science.nasa.gov/asset/webb/types-of-spectra-continu...
In that view, transparency is just wavelength-by-wavelength attenuation. Each material applies its own absorption/transmission function to the incoming spectrum. Because this is done pointwise in the spectral domain, the order doesn’t matter:
OUT = IN × T₁ × T₂ (or in a subtractive representation: OUT = IN − ABS₁ − ABS₂).
So whether one material reduces 50% of the red first and another reduces 50% of the green second or vice verse doesn’t change the result. Each wavelength is handled independently, making the operation order-independent.