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

Author: peterkutz

index.html

@@ -898,21 +898,21 @@
 
 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 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.
+Implementations that operate in a spectral rendering context (or that otherwise wish to account for wavelength-dependent IOR and extinction) may alternatively compute the thin-film Fresnel effect directly from first principles by evaluating Fresnel and thin-film interference at one or more wavelengths and integrating the result according to the renderer's spectral pipeline. In such implementations, the thin-film effect is typically evaluated using an Airy-style multi-bounce formulation (e.g., Equation 3 in [#Belcour2017]) together with wavelength-dependent Fresnel amplitude and phase at the film interfaces. Results should be validated against 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:
+Regardless of which approach is used, several considerations apply:
 
-   - The shape and color of the fringe patterns in the reflection from the film will be affected by the complex IOR of the adjacent media above and below the film, which in general are a statistical mix of metal and dielectric below, and of coat and ambient medium above (to which the fuzz is index-matched). Figure [ior_configs] illustrates the eight possible different structures depending on the presence of both the film and coat, each of which leads to different Fresnel effects due to the differing IORs at the interfaces. In principle the implementation should account for all of these configurations accurately, though the precise modeling of these effects is left to the implementation.
+   - The shape and color of the fringe patterns in the reflection from the film are affected by the complex IOR of the adjacent media above and below the film, which in general are a statistical mix of metal and dielectric below, and of coat and ambient medium above (to which the fuzz is index-matched). Figure [ior_configs] illustrates the eight possible different structures depending on the presence of both the film and coat, each of which leads to different Fresnel effects due to the differing IORs at the interfaces. In principle the implementation should account for all of these configurations accurately, though the precise modeling of these effects is implementation-dependent.
 
-   - The thin-film thickness is assumed to be smaller than the scale of the microfacets and assumed to be smooth. With this assumption, in practice the effect is most easily incorporated directly into the Fresnel factor of the microfacet BSDFs of both the metal and dielectric-base layers (thus it is *not* represented by incorporating an explicit thin-film Slab into the model).
+   - The thin-film thickness is assumed to be smaller than the scale of the microfacets and the film is assumed to be smooth. With this assumption, in practice the effect is most easily incorporated directly into the Fresnel factor of the microfacet BSDFs of both the metal and dielectric-base layers (thus it is *not* represented by incorporating an explicit thin-film Slab into the model).
 
-   - In the case of the dielectric base, the thin-film should also generate color fringes in the transmission lobe. This is important for example when rendering soap bubbles (see [#Belcour2017]).
+   - In the case of a dielectric base, the thin film should also generate color fringes in the transmission lobe. This is important, for example, when rendering soap bubbles (see [#Belcour2017]).
 
-   - In the case of the metallic base the physics is somewhat ambiguous since, as described in the Metal section, the Fresnel factor for metal is defined according to the Schlick-based "F82-tint" parametrization, which does not specify the underlying physical complex IOR. We suggest here that some reasonable approximation is employed to map the Fresnel factor to the best matching effective complex IOR, for example that described by [#Gulbrandsen2014].
+   - In the case of a metallic base the physics is somewhat ambiguous since, as described in the Metal section, the Fresnel factor for metal is defined according to the Schlick-based "F82-tint" parametrization, which does not specify the underlying physical complex IOR. We suggest that some reasonable approximation is employed to map the Fresnel factor to a best-matching effective complex IOR, for example that described by [#Gulbrandsen2014].
 
-   - Because the thin film is non-absorbing and interference-based, it only redistributes the probabilities of reflection and transmission; therefore, it should not violate energy conservation.
+   - Because the thin film is non-absorbing and interference-based, it only redistributes the probabilities of reflection and transmission; therefore, it should not violate energy conservation. 
 
 ![Figure [ior_configs]: Schematic of all 8 possible IOR configurations, including those involving the thin-film.](images/IOR_configs.svg width="95%" align="center")