Alireza Khajehali: PBR Grass and Lighting

More tricks from the one behind Our Ghosts of War. Environment artist Alireza Khajehali talks about the challenges of mixing lighting and PBR grass materials. 

I looked around A LOT. Spent a lot of time trying to figure out a solution. This is just another flaw of the PBR in my opinion. We have translated the real world surface rules into game engines without having the power to have the same real world geometries with trillions of triangles to apply these rules to. Looking around I noticed the Cryengine community had the same problem but Crytek has already dealt with it. This slide gave me some clues.

Here’s a very very simple grass material:

And this is how it looks like on the landscape surface:

But as the sun starts to rotate, you start to have reflection problem. No where in the world a grassy ground looks like this:

So I reconstructed a Specular map using the Albedo, as I have explained in Tip 02 to prevent 100% of the surface from reflecting light equally. This is how it looks like in the material editor:

And this is how it looks like on the landscape, but it’s still way off…

To reduce the specular gain I went ahead and used the Fresnel node to mask out the area where the specular is gaining, and reduced the specular intensity for those areas from 1 to 0.33. This is how it looks like in the material editor:

And finally, this is how it looks like on the landscape surface:

But still, it does look off somehow. You notice in the middle the reflection is stronger. If we had grass meshes here, when they are culled it’d look too obvious that they are disappearing and breaks the immersion into bits. Next time I’ll cover how I went around this issue.

For demonstration I have used a grass material. But any material that’s rough and has a lot of volumes to it in real life like grass does, it can greatly benefits from this method.

The guide was originally published on Polycount.