AnisotropicWard BRDF (ward)
This plugin implements the anisotropic Ward reflectance model and several extensions. They are described in the papers
(i) “Measuring and Modeling Anisotropic Reflection” by GregWard 
(ii) “Notes on theWard BRDF” by BruceWalter 
(iii) “An Improved Normalization for theWard Reflectance Model” by Arne Dür 
(iv) “A NewWard BRDF Model with Bounded Albedo” by Geisler-Moroder et al. 
Like the Phong BRDF, the Ward model does not take the Fresnel reflectance of the material into account. In an experimental study by Ngan et al. , theWard model performed noticeably worse than models based on microfacet theory.
For this reason, it is usually preferable to switch to a microfacetmodel that incorporates knowledge about the material’s index of refraction. In Mitsuba, two such alternatives to ward are given by the plugins roughconductor and roughplastic (depending on the material type).
When using this plugin, note that the diffuse and specular reflectance components should add up to a value less than or equal to one (for each color channel). Otherwise, they will automatically be scaled appropriately to ensure energy conservation.
Parameter Type Description
- variant : string
- Determines the variant of theWard model to use:
- (i) ward:The originalmodel byWard —suffers from energy loss at grazing angles.
- (ii) ward-duer: Corrected Ward model with lower energy loss at grazing angles . Does not always conserve energy.
- (iii) balanced: Improved version of the ward-duer model with energy balance at all angles .
- Default: balanced
- alphaU, alphaV : float or texture
- Specifies the anisotropic roughness values along the tangent and bitangent directions. (Default: 0.1).
- specular Reflectance : spectrum or texture
- Specifies the weight of the specular reflectance component. (Default: 0.2)
- diffuse Reflectance : spectrum or texture
- Specifies the weight of the diffuse reflectance component (Default: 0.5)