Clarify Belcour&Barla baseline for thin-film iridescence and allow first-principles alternative for spectral renderers

Author: peterkutz

index.html

@@ -898,9 +898,9 @@
 
 The thickness and IOR together affect the intensity, spacing, and hue of the color fringes. The coverage weight acts as a blend between the BSDF with and without the presence of the film, allowing the overall strength of the effect to be adjusted without altering its structure or color.
 
-The currently recommended thin-film model is that of Belcour and Barla [#Belcour2017], which pre-integrates interference effects using Fourier-domain convolutions and Gaussian filtering. This method efficiently produces high-quality fringe patterns in an RGB rendering context, but it can be challenging to implement and may introduce inaccuracies in some cases, as it assumes that Fresnel amplitude and phase coefficients remain constant across each spectral band, which limits the model's ability to capture wavelength-dependent dispersion effects.
+The currently recommended thin-film model for RGB rendering contexts is that of Belcour and Barla [#Belcour2017]. This model provides an efficient, high-quality approximation of thin-film interference suitable for typical RGB-based production rendering.
 
-A more direct alternative is a "locally spectral" approach that computes reflectance per light path by evaluating the full Fresnel and Airy interference stack -- including complex amplitudes, polarizations, and phase shifts -- at specific wavelengths sampled per path. This can begin with fixed red, green, and blue wavelengths, but better results are achieved by stochastically sampling wavelengths from approximate camera sensitivity curves. This enables convergence to neutral gray for very thick films and avoids the high-frequency color banding that fixed RGB wavelengths can produce. The same wavelengths can also be reused to model dispersion (as described in the Translucent base section), while all other BSDF components are free to ignore them and operate in RGB as usual. This approach uses only the Airy summation from Belcour and Barla (Equation 3 from [#Belcour2017]) but requires additional per-wavelength computations and assembling the necessary formulas from multiple sources rather than a single reference.
+In addition, implementations that operate in a spectral rendering context (or that otherwise wish to account for wavelength-dependent IOR and extinction) may compute the thin-film Fresnel effect directly from first principles by evaluating the Fresnel and thin-film interference equations at one or more wavelengths and integrating the result according to the renderer's spectral rendering pipeline. In such implementations, the thin-film effect is typically evaluated using the Airy-style multi-bounce formulation described by Belcour and Barla (e.g., Equation 3 in [#Belcour2017]) together with wavelength-dependent Fresnel amplitude and phase at the film interfaces. Implementations should verify that their thin-film Fresnel implementation agrees with the recommended Belcour and Barla model in the parameter regimes where that model applies.
 
 Regardless of which approach is chosen, several considerations apply to both: