Houdini 16.5 Nodes VOP nodes

PBR Specular VOP node

Produce a normalized reflection bsdf.

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This node will produce a normalized (reflectivity of 1) reflection bsdf. Shaders using this bsdf should adjust the reflectivity by multiplying the component by an intensity parameter.

Parameters

Specular Model

The mathematical model used to simulate glossy reflections. For each viewing angle and surface normal, the model defines from which directions and at what intensity light is reflected. This is what shapes specular highlights and reflections in general.

The overall glossiness, and with it the size of highlights, is controlled by Roughness. The available models simulate the effects caused by Roughness with varying degrees of physical accuracy, with GGX currently being the most accurate.

The chosen model has no effect when Roughness is 0, since this causes light to be reflected from a single direction at full intensity, making the model irrelevant.

See Roughness for more information.

The figure below shows the result produced by various specular models across a range of roughness values.

Note how rough surfaces look more natural with the GGX model because the interaction of light with the rough surface is modeled more accurately.

Component Label

Specifies a label for the BSDF component. This can be used to export contributions from this component to a separate image plane.

Roughness

This is a measure of how bumpy a surface is at the microscopic level. The most obvious effect is that reflections become glossier as Roughness increases. At a value of 0, the surface is perfectly smooth and produces perfect mirror reflections. A value of 1 simulates a very rough surface, which results in very blurry reflections, similar to diffuse reflection.

In more accurate Specular Models like GGX, reflections on rough surfaces are also darkened at grazing angles. This is due to Masking-Shadowing effects, where parts of the surface are hidden from view and/or not reached by light due to microscopic grooves and spikes in the surface. Note that this is simulated using a simplified mathematical model, rather than using actual additional geometry.

The visual change when transitioning from 0 to 1 is close to linear.

Anisotropy

Causes reflections to be stretched in the direction defined by Anisotropy Direction.

This simulates microscopic bumps with a directional bias, causing light to be scattered more in the defined direction. This is typical of brushed metals.

The effect of this parameter increases with Roughness. It has no effect at all when Roughness is 0.0.

Anisotropy Direction

Controls the direction of Anisotropy relative to the UV coordinates of the surface. At 0.0, reflections are stretched in the U direction. At 0.5, the direction is rotated by 90 degrees to the V direction. 1.0 equals 180 degrees. Since the effect is symmetrical this produces the same result as 0.0.

The direction of rotation also depends on the UV layout. When the UVs are layed out such that textures appear on the surface without mirroring, higher values rotate counter-clockwise.

The effect of this parameter diminishes with decreasing Roughness and Anisotropy.

Masking

Enables the geometric masking term of the GGX Specular Model, see Roughness for a detailed explanation.

Reflectance

Controls the reflectance of surfaces facing the viewer. In other words, when a point on a surface has a normal pointing straight at the viewer, it has this reflectivity. This is often called F0, for reflectance at from the viewer.

Surfaces facing 90° degrees away from the viewer, as seen at the edges of rounded objects, are always 100% reflective.

At high Roughness values, the effect discribed above is diffused, because it is applied at the microfacet level within the GGX model. See Roughness for a more detailed explanation.

Inputs

nN

The normalized surface normal. When not connected, the global N is used instead.

nI

The normalized viewing direction. When not connected, the global I is used instead.

utan

The main surface tangent. This is used to compute the direction of anisotropy, anisodir is relative to the orientation of this vector.

Must be normalized and tangential to the shaded surface.

Tip

The Compute Tangents VOP offers various methods of computing suitable surface tangents.

vtan

The surface bitangent. Must be normalized and perpendicular to both baseN and utan.

Tip

The Compute Tangents VOP offers various methods of computing suitable surface tangents.

Outputs

f

The reflection BSDF.

See also

VOP nodes