Can you modify Vertex Normal?

I have crossed some examples on how to improve a models shading by modifying their vertex normals. I want to see if I can create trees and grass similar to those in video games like Rime and The Witness.

So far the only thing in the API is this… start_vertex_normal

Here is an example of the look I am after…

Here are some videos on modifying vertex normals to get shading look…

If anyone finds more info on the subject let me know…Thanks!

It’s called smooth. You can also obtain vertex normals when exporting a mesh, but not set them. Most render engines respect it also and apply standard algorithms to interpolate shading between vertices.
There is no direct control however about the direction of normals at vertices.