Spectral Rendering Wavelength Range

[Disgruntoad]

The default wavelength range is 360nm - 830nm. I was checking some of the materials https://github.com/mitsuba-renderer/mitsuba-data/tree/e451a4aa5b3d3a56e411f458ecf7733b9adfd4ec/ior and noticed their data largely supports ~300nm - 900nm.

If rendering outside the default range, for instance, over 300nm -- 1100nm, how does mitsuba extend these measurements?

Are there other rendering pitfalls when extending the rendering range beyond the default?

For instance, if I'm using an hdri, how does the RGB --> spectral conversion account for wavelengths outside the visible range?

[njroussel]

Hi @Disgruntoad

If rendering outside the default range, for instance, over 300nm -- 1100nm, how does mitsuba extend these measurements?

They will typically be 0. It's not explicitly documented, but typically spectral data is always represented by a Spectra plugin like regular or irregular across the entire codebase.

Are there other rendering pitfalls when extending the rendering range beyond the default?

I am sure there are some nasty pitfalls, but in general I would say that the rendering code (integrators) make no assumption about the actual wavelength values. The materials, their properties, the film, etc. might have some assumptions about spectral data being in the visible light range.

For instance, if I'm using an hdri, how does the RGB --> spectral conversion account for wavelengths outside the visible range?

Have a look at the last part of this paragraph which talks about spectral upsampling: https://mitsuba.readthedocs.io/en/stable/src/key_topics/variants.html#part-3-color-representation. In particular, you might want to have a deeper look at this paper to understand the limitations of the upsampling model: https://rgl.epfl.ch/publications/Jakob2019Spectral.