Anisotropic BSDF Surface Overview

Note: For more information on the use of this model, you can view Parameters of Interpolation Enhancement.

With Anisotropic BSDF Surface, you can simulate both isotropic and anisotropic surfaces from goniometric measurement data.

Note: The model handles BSDF as well as BRDF only or BTDF only surfaces.
Warning: Anisotropic BSDF uses an approximation to deal with the spectrum.

With following diagram you can view angle definitions for incoming and outgoing directions according to the surface normal (Z) and the fixed anisotropic vector.



Note: The incoming photon is displayed in blue.

Z vector

Surface normal

X vector

Constructed for a given incoming photon as the projection of its direction on the surface plane

Y

Computed from X and Z

Anisotropic vector

The Anisotropic vector is the orientation vector of the BSDF on the geometry.

It defines an axis on the surface used as the origin when calculating the Anisotropic angle. It directs the surface when the vector given in the file is not in the tangent plane at the impact point, an orthogonal projection of it is used instead.

You can modify the anisotropic vector in the X, Y and Z boxes under Anisotropy vector.

The anisotropic vector is stored in .anisotropicBSDF files and is defined by the global coordinate system.

Anisotropic angle Phi_i

The anisotropic angle is the angle between the incidence plane and the anisotropic vector. When the vector is in the incidence plane, phi=0°. Angle between X and the anisotropic vector using trigonometrical convention (Phi_i is positive on the diagram)

Setting the Phi incidence angle value does not impact the incidence angle visualization in the XYZ axis system.

Theta_i

Incidence angle

Output direction (Theta_o, Phi_o)

Displayed in red and is given using standard spherical coordinates in the X,Y,Z axis system

Warning: Theoretical specular direction is always (Theta_o, Phi_o) = (Theta_i, 0) regardless what the Phi_i angle is.
Note: Intensity values corresponding to (Theta_o, Phi_o) is BRDF*cos(Theta_o).
Note: You must manually calculate the anisotropic vector each time you re-orientate the geometry on which the surface state is applied.

Angles ranges:

  • Theta_i must be in the [0° 90°] interval.

  • Theta_o must be in the [0° 90°] interval for reflection and in the [90° 180°] interval for transmission.

  • Phi_i and Phi_o must be in the [0° 360°] interval.