Reducing geometric detail while keeping outlines intact is one of the major showstoppers that prevent current game engines from having realistic foliage. And that exact same problem is also why a Nerf with its near-infinite geometric detail is impractical to use for games. And this paper is yet another way to produce a Nerf.
SpeedTree already used billboard textures 10 years ago and that's still the way to go if you need a forest in UE5. Fortnite did slightly improve upon that by having multiple billboard textures that get swapped based on viewing angle, and they call that impostors. But the core issue of how to reduce overdraw and poly count when starting with a high detail object is still unsolved.
That's also the reason, BTW, why UE5's Nanite is used only for mostly solid objects like rocks and statues, but not for trees.
But until this is solved, you always need a technical artist to make a low poly mesh onto whose textures you can bake your high resolution mesh.
Nanite can actually do trees now, and Fortnite is using it in production, with fully modelled leaves rather than cutout textures because that turned out to be more efficient under Nanite. They talk about it here: https://www.unrealengine.com/en-US/tech-blog/bringing-nanite...
That's still ultimately triangle meshes though, not some other weird representation like NERF, or distance fields, or voxels, or any of the other supposed triangle-killers that didn't stick. Triangles are proving very difficult to kill.
My understanding is that while they allow masked textures, the geometry is still fully emitted by Nanite. That means you still need to start with a mesh that has multiple leaves baked into a single polygon plane, as opposed to starting with individual leaf geometry and then that is somehow baked automatically.
This illustration from the page you linked to shows that as well:
The alpha masked holes move around, but the polygons remain static. That means if you draw a tree with this, you still have the full overdraw of the highest-poly mesh.
Yeah alpha masking is inefficient under Nanite, but as they explain further down its handling of dense geometry is good enough that they were able to get away with not using masked materials for the foliage in Fortnite. The individual leaves are modelled as actual geometry and rendered with an opaque, non-masked material.
SpeedTree already used billboard textures 10 years ago and that's still the way to go if you need a forest in UE5. Fortnite did slightly improve upon that by having multiple billboard textures that get swapped based on viewing angle, and they call that impostors. But the core issue of how to reduce overdraw and poly count when starting with a high detail object is still unsolved.
That's also the reason, BTW, why UE5's Nanite is used only for mostly solid objects like rocks and statues, but not for trees.
But until this is solved, you always need a technical artist to make a low poly mesh onto whose textures you can bake your high resolution mesh.