This was something I needed for one of my prototypes, so it is not exactly biplanar mapping. I needed a separate surface texture for the top. Please consider this shader expensive to use.
Sorry about the dark textures. It is hard to find proper stylized textures, especially a good grass texture.
More Info: https://iquilezles.org/articles/biplanar/
Test it: https://selimanac.github.io/defold-biplanar-mapping/
