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:

* **Transparency-ThinGrid, Transparency-ThinGridFast**: cloth, emulated
as a perfect square grid of cylindrical threads.

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.

* **Transparency-ThinFilm, Transparency-ThinFilmIOR**: thin colored film.

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.