Would it be possible to either compute or approximate the fragment shader's vertex results on segments of the terrain that require collision detection? I bet it wouldn't be feasible to do to check collisions for gun projectiles in a video game, but it might be feasible for something like making sure the player's model doesn't clip beneath the tessellated terrain.

UE4 has adaptive tessellation anyway, and you can scale the tessellation multiplier based on distance with a material function so that the landscape has more triangles closer to the camera and less further away. Beyond a certain distance (where the player wouldn't notice the detail anyway, so there's no point drawing it) you can scale the multiplier to zero to improve performance.

I'm not an expert but honestly in a game engine like UE4 where collision is important, it seems like the adaptive geometry tessellation approach is the way to go, as least from my initial experiments.

I was thinking of a somewhat different use case. I was thinking of a flight simulator where tessellation only occurred close to the camera, to provide the appearance of a more detailed mesh when viewed up close.

The problem is, with a quad mesh where each vertex is 20 or 30 meters apart a plane flying nap of the earth could end up clipping through the tessellated mesh but not actually collide with the non-tessellated terrain quad mesh being used for collision detection. Or worse, the player could collide with the terrain quad mesh but the tessellation makes it visually appear to have not collided with the terrain.

To only two ways I see to solve this are to either not use tessellation up close, or to compute with the CPU the vertices produced by the tessellation and use that mesh for collision.