While the most obvious use of Transparency BSDF is cutouts, i.e. creating big visible holes in the object without changing geometry, some actual physical materials have properties that can be represented with this node.
However, to achieve realistic appearance, the amount of transparency must change depending on the incoming ray direction, in order to account for effects occuring because in real life the material has thickness.
This file contains node groups that apply the Transparency BSDF node to emulate transparency properties of certain such materials:
The transparency in cloth occurs because it consists of interwoven threads that have gaps between them. In a way this is a case of cutout transparency, but the holes are so small that they have to be handled statistically, similar to roughness.
The transparency varies depending on the viewing angle because threads have thickness and occlude the openings at glancing angles. At a certain angle cloth becomes completely opaque.
While the actual threads in cloth have complicated interweaving shapes, the math in this node is based on an approximation: cloth is emulated as a perfect square grid of perfectly straight cylindrical threads that simply pass through each other at intersections.
Moreover, transparency actually depends not only on the normal but also the grid tangent. This difference is mostly pronounced around the transition to fully opaque. The diagonal direction is smoothest but slightly more complicated to compute, while the axis direction is faster but sharper. Two node groups implementing these extremes are provided.
Materials such as colored plastic and glass get their color because of volumetric absorption. A very thin film of such material won't change the direction of a ray of light in any noticeable way, so it can be modeled using the Transparency node.
However, for a realistic result it is necessary to account for the actual volumetric nature of light absorption, because the relative length of the path the light takes within the film changes depending on the angle.
This is most pronounced if the film does not have an index of refraction, as when the ray becomes parallel to the surface, the film becomes totally opaque, because the ratio of path length to thickness approaches infinity. This is however not very realistic and in the more natural case with internal refraction the effect is less severe.
These nodes only handle the transparency aspect of the relevant material, and require combining with other shaders like Glossy to create a complete realistic material. A demo scene provides simple examples of cloth and thin glass-like materials.
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