Transferring normals from a volume to soften the shading of foliage geometry.
The purpose of transferring normals with foliage is to turn the geometry from hard surfaces shaded individually to softer pieces of a whole volume. The lighting of foliage in real life is largely informed by the translucency and subsurface scattering of leaves. However, it is impossible to actually calculate all that light scattering, and much more efficient to simply achieve the effect.
How it Works
Attribute transferring is a function available in different 3D modeling apps such as Maya, and Houdini has a node dedicated to it. First you take the plant model, and create a much simpler piece of geometry to sample from - usually a sphere-like shape. Then, you transfer the normal attributes from the simpler geo to the foliage. The details depend on the software, and Houdini is described below.
Houdini makes things pretty easy. Take the input foliage geometry group, zero out the point normals, take the sphere geo, add point normals, wire them up and transfer the attribute N.