Full spectral rendering

[TzuChieh]

To support PLT, one of the first steps is to move to full spectral rendering. Some primary goals include:

• Able to simulate dispersion of light
• Use hero wavelength spectral sampling to reduce color noise
• Spectral sampling as a first-class citizen, but retain the ability to perform tristimulus color rendering with a compile time switch

Following design decisions must be made:

• [ ] What to store in math::Spectrum?
  • Spectral sample values, wavelengths and PDFs?
  • Should we rename it to indicate it is sampled?
• [ ] What type to store in SDL resources?
  • User may input scalar, tristimulus color, full/partial spectrum
• [ ] How to generate spectral samples (including wavelengths)?
  • Need to identify places where generating spectral samples/wavelengths is possible
  • We may need to extend TMIS to be able to handle N PDFs
• [ ] Do we need non-piecewise spectral curves?
  • Do we need Gaussian-based method for spectral resampling?

Things to do:

• [ ] We may not need CSpectralSampleProps, just use a struct for general SpectralSampleProps
• [ ] constexpr compatible SpectralSampleProps
• [ ] Compile time switch for spectral/tristimulus color mode; when in tristimulus color mode, able to choose a white point & color space for rendering
• [ ] Add constexpr color constants for convenience (math::constants::purple, math::constants::black, etc.)
• [ ] Handle #101