Rendering massive amounts of animated grass

 I recently played Ghost of Tsushima and I was impressed by the amount of foilage that covers the world. Just as I was impressed when I played Horizon: Zero Dawn a few years back.

So my engine can already render a lot of instanced geometry, a lot of per-instance animations and so on, but for that much foilage that is needed for believable vegetation, this is too costly. The answer to the problem is pivot based animation and therefore some simple, stateless animation in the vertex shader.

In addition to that, the instances of for example grass, need to be clustered and culled efficiently. My two-phase occlusion and frustum culling pipeline is exhausted pretty fast, when we use it for 10000000 small instances without any hierarchical component. A cluster is best and easy a small local volume that covers enough instances to not mitigate the benefit of batching. For example it's not worth batching only 10 instances, only to be able to cull them better. 1000 instances seem to work well for me. I generate a cluster's instances randomly, so that I can just render the first n instances and scale n by distance between camera and cluster. This way, the cluster gets thinner and thinner, until completely culled. Hard culling results in pop-ins. For a smooth fadeout without alpha blending enabled - which would again kill the performance of foliage - screen door transparency can be used. This is again a simple few lines, now in the pixel shader, and culling is mostly hidden.

Three things that are for themselves very efficient team up for a nice solution for foliage: Pivot based animation, cluster culling and screen door transparency fading.

As stated under the first video, I don't have nicely authored pivot textures, so I created a veeeeery simple one that just goes from 0-1 from root to leaves of the grass.

Simple deferred translucent foliage rendering

Translucency seems to be one of the new illumination features that every graphics engine has to provide these days. The effect is most noticeable on organics, like skin (ears for example) and plant leafs. For the latter, there's a nice and easy way to implement it - this solution only works for very thin objects. This means double sided triangles, like paper, curtains, or leafs. Other cases would be more complicated.

You should have a mechanism to support multiple material types in your deferred renderer already. Then you need to disable backface culling, or the back side of the object won't be rendered at all...Let's talk about direct illumination first. Light traveled through the object and direct light is treated at the same time. Since we assume that our translucent objects are infinitely thin, we don't need to determine a thickness, like you would have to, regularly. An artificial thickness can be provided via per-object parameter or through a texture. One could assume, that texture coordinates are the same for the point on the front and the back side of the currently rendered fragment. For normals, another assumption is possible: The normal on the back side should be the current fragment's normal, but just negated. Using different normals or diffuse texture for the backside is not easy to add here, since you can't know if you are currently rendering the front or the back side ob the object.

After calculating the regular lighting, calculate the lighting with the artificial backface normal und multiply it with the object's thickness at this point. Add the two values and you're done. Even though the usage is limited to very thin and simple-colored objects, this can look very nice, for example with a curtain:

Look at the subtle shadow of the sphere above the curtain that you can now see from below the curtain.