GPU Grass - Landscape RVT -> questions

[Frenetic]

Out on the Roadmap for Unreal (https://portal.productboard.com/epicgames/1-unreal-engine-public-roadmap/c/1232-gpu-grass-and-micro-scattering), under PCG is this tidbit for GPU Grass And Microscattering:

“Added support to PCG GPU compute for sampling directly on GPU the Landscape RVT and Grass maps in order to build efficient and customizable runtime grass spawning. RVT Priming has been added with control over range and a specific mip level to use in order to run PCG GPU compute with the RVT rendered beforehand.”

I’ve only been able to just get into 561 stably and wanted to look at PCG replacements for landscape-grass. But given some familiarity with the methods of scatting meshes and whatnot, based on a few videos, I’ve not seen a ‘landscape RVT’; what is this?

Is it the thing the engine uses to hold painted-layer information? Is it something rendered discreetly to an RVT Output node in a material shader? Is it the physics/grass output node(s)? I get it’s some data-structure/texture-like think that maps to the landscape and can be sampled-from to drive whatever in PCG (it’s a data-in), but how is it populated to begin with?

[Frenetic]

I was able to figure this out.

You can use an RVT to store whatever maths you want for your landscape, including the physics/grass layers. Just be sure to rebuild as necessary if connections between things break.

What you would nominally pipe to a grass-output node, instead pipe to a new physics-layer node (shader cost ought to be the same -1/+1). In PCG you can then sample the physics layers and drive grass therein. Be sure to check the input for physics on the landscape data node in PCG.

I used this graph to generate concentric-rings of density. Each spawner culls in a decreasing ring to preserve screen-area/density and it overall only uses a single point-sampler.

Grass out to 1.75km takes ~0.01ms on the gpu…. (sometimes 0.00!!!)

Unreal.