Terrain rendering
Drawing a landscape with more triangles near the camera and fewer far away, without the switch ever showing.
A heightmap can describe millions of square metres, but drawing every triangle at full resolution is waste — most of it is distant or off-screen. Terrain renderers answer with level of detail: ROAM (1997), geomipmapping, chunked LOD and CDLOD split the ground into tiles whose dense meshes are swapped for sparser ones the further off they sit.
The difficulty is the seams. When a tile drops resolution its edge mustn't crack against its neighbour, and the switch mustn't visibly pop — so the geometry is morphed smoothly between levels. On top of that sit frustum culling and often a quadtree that quickly discards everything the camera can't see.