Tessellation for Landscape

Tessellation is something of a holy grail for graphics: the ability to physically shape the surface based off of displacement maps and smooth out the surface with more polygons on the fly. But for the longest time it was seen as an impossibility that was extremely inefficient at best. Tessellation first appeared in UDK's DX11 renderer 5 years ago, but it wasn't until UE4 that the method was really perfected. The preferred method to tessellation was a pre-tessellated higher-poly base mesh, and for landscape some extremely inefficient parallax occlusion mapping. However, with UE 4.13, tessellation will only be applied to the lowest LOD. This allows the triangle explosion to only impact the closest meshes. And with DirectX 12 significantly reducing the impact of draw calls, I was able to achieve a better framerate using tessellation than POM on a landscape material with very few blends.

Pros to tessellation on landscape:

• Use artistic displacement maps to bump real geometry!
• Does not require premade geometry, only landscape material and displacement map!
• Proper sillhouettes! Proper ambient occlusion and shadows! Proper depth! Etc.
• Extremely and easily customizable! Since tessellation is realtime generated, surface values can change over time and are impacted immediately.
• Can be blended with other materials very easily.
• Tessellation can be baked into physical properties of the landscape heightfield.
• Cheaper than POM (in some cases)!

Limitations of tessellation on landscape:

• ...None!
• Some distant objects will render flat, but their depths would typically not be noticed anyways, and other methods would reveal more artifacts.
• In some extremely large landscapes with far less efficient LODs, POM or simple parallax is more performant than tessellation.

It is recommended that landscapes with tessellation have a very efficient LOD system. More sections and components at somewhat smaller sizes (31x31 quads or smaller) would render more efficiently than larger sizes and fewer components. This is because the triangle explosion result of tessellation would yield too many polygons for large sections and components to render efficiently. Also, DirectX 12 cuts draw call times by 1/8, so you can render more draw calls and components efficiently.

In order for tessellation to be enabled, go down the basic material settings under the Tessellation category and under D3D11 Tessellation Mode choose "Flat Tessellation." PN Triangles does smooth out the surface through splines, but the differences won't be noticeable under either method, and Flat Tessellation will be cheaper. I left adaptive tessellation checked on to lower the LOD in the distance, but it might be forcibly enabled just by being a landscape material, so your selection there may not matter.

The easiest setup for tessellated displacement is to multiply your heightmap by the vertex normal, then multiply that by a scalar parameter for your displacement. If you are blending heightmaps together, the final blend will be fine. I also recommend setting the tessellation multiplier down to 0.5 to save a bit on performance. The "reference plane" defaults to the ground, and all tessellation is lifted from the standard surface level. To change this to displace downwards (like POM default settings), subtract your heightmaps by 1. Subtract by 0.5 for an even 50:50 displacement above and below the surface (though the exact balance depends on the range of your heightmaps as well).

To make less triangles explode, under Project Settings > Rendering > Tessellation > Adaptive pixels per triangle, set this to a higher number (default is 48). For a game running in 4K using tessellation on landscape for medium-sized bumps, you might be able to get away with 200-600 pixels per triangle. Keep in mind the distance LOD is controlled by the Tessellation Multiplier while the number of triangles is controlled by the adaptive pixels-per-triangle value in the Project Settings when optimizing.

Also make sure to keep in mind that tessellation, while correctly calculating shadows, will not calculate the new surface normal. You will still need to provide a proper normal map with your displacement map to get accurate shading and lighting. Once you enable tessellation, you can even do things like animated displacement for lava flow. And since tessellation is compatible with texture maps, you are not limited to vertices for your shading. Your only limit is your imagination!