Metal BSDF

asMetal

A metal shader, with anisotropy and a user-friendly [Gulbrandsen2014Fresnel] complex index of refraction Fresnel, with some presets for common metal types derived from real-world measurements [1].


Parameters


Fresnel Parameters

Face Reflectance
RGB reflectance at normal or facing incidence.
Edge Reflectance
RGB reflectance at edge or grazing incidence.

Specular Parameters

Roughness
The apparent surface roughness.
Energy Compensation

Microfacet models typically fail to take into account multiple scattering [Heitz:2016:MMB:2897824.2925943], and as such, with high roughness values there is a substancial energy loss. To what extent it depends mostly on the MDF used, with GGX exhibiting considerable energy loss.

In order to negate the impact of this energy loss a separate compensation term is applied. This parameter scales the contribution of this compensation term, with a value of 1.0 trying to compensate for all the energy lost, and a value of 0.0 essentially disabling any compensation.

See also

This is covered in SIGGRAPH 2017 physically based shading course Revisiting Physically Based Shading at Imageworks (Christopher Kulla and Alejandro Conty).

Anisotropy

Anisotropy Amount
Overall intensity of the anisotropy effect, with a value of 0.0 representing a isotropic specular highlight.
Anisotropy Angle
Rotation angle for the anisotropic highlight in [0,1], mapping a rotation from 0 to 360 degrees.
Anisotropy Mode

Toggles between accepting a direct texture map in the form of an anisotropy vector map, or between an explicit vector (or a connection to a node that generates such a vector [2] ). It can take the values

  • Anisotropy Map
  • Explicit Vector
Anisotropy Map
Also known as tangent field, encodes the anisotropy directions along X and Y in the Red and Green or Red and Blue channels of the image. Appleseed expects values encoded in the Red and Green channels. Valid when the Anisotropy Mode is set to Anisotropy Map only.
Anisotropy Direction
The explicit vector passed as the anisotropy direction. Valid when the Anisotropy Mode is set to Explicit Vector only.

Bump Parameters

Bump Normal
The unit length world space normal of the bumped surface.

Matte Opacity Parameters

Enable Matte Opacity
Parameter that toggles matte holdouts.
Matte Opacity
Matte opacity scaling factor.
Matte Opacity Color
Holdout color.

Advanced Parameters

Ray Depth
The maximum ray depth a ray is allowed to bounce before being terminated.

Outputs

Output Color
The metal BRDF output color.
Output Matte Opacity
The matte holdout.

Screenshots


Footnotes

[1]From .nk files, containing the data for several metals, alloys, semi-conductors in several wavelenght ranges (not exclusively in the visible light range). Maya attribute presets are provided for aluminium, brass, chromium, copper, gold, iron, lead, mercury, nickel, osmium, platinum, aluminium-gold intermetallic (purple plague), silver, titanium, titanium nitride, tungsten and zinc. See the LuxPop database and refractive index database for more measured complex ior data.
[2]Such as the anisotropy vector field node.

References

[Gul14]Ole Gulbrandsen. Artist friendly metallic fresnel. Journal of Computer Graphics Techniques (JCGT), 3(4):64–72, December 2014. URL: http://jcgt.org/published/0003/04/03/.
[HHdEonD16]Eric Heitz, Johannes Hanika, Eugene d’Eon, and Carsten Dachsbacher. Multiple-scattering microfacet bsdfs with the smith model. ACM Trans. Graph., 35(4):58:1–58:14, jul 2016. URL: http://doi.acm.org/10.1145/2897824.2925943, doi:10.1145/2897824.2925943.