Unity surface shader for leaves/pine needles (on alpha cutout textures).
Lush greenery looks the most delicius when it feels like the light passes through the tree crowns a bit like it was a cloud.
The method is conceptually very simple:
In Unity terms, use alpha _Cutoff in combination with the built-in shadow map to control how much of the light is transmitted through the foliage (an additional alpha offset is added to control the rendered alpha cutoff):
To break up uniformity (specially needed for areas that are completely lit or completely shadowed), I used the classic dot(sunWorldDir, normal) and a bunch of tweaking parameters, for example Lambert90deg and Lambert180deg, which allows for control of the "shine though" lighting gain for 90 degrees and 180 degrees. (However, since we are working with larger sets of leaves/pine needles that are mapped on simpler geometry, I think of this as very rough approximation, I mostly just use the parameters to introduce variation that looks good)
These parameters are basically tools to counter issues like:
Code is small, just a few lines, as the Unity framework does most of the work. However, as someone who usually writes fragment shaders, using the surface shader API was not a super smooth experience...
The main drawback to this method is noise, even with large shadow maps and good PCF sampling, you might want to render larger and then filter down.
I implemented this as proof of concept, so use at your own risk. It does not do any fancy stuff like subsurface scattering or even normal/bump mapping (but the latter is certainly easy to add if you need it). It also has an issue with shadows further away bleeding through geometry that has high transmission (I think because the CollectShadows stage don't take into account the additional alpha offset)
Trees are CC0 and were created by ALTER'49, for the full set, and also with higher polycounts: PRO Forest Bundle