Macro form and surface texture

Ymir shapes the large form of terrain: the mountains, valleys, ridgelines, coasts, and drainage that read at the scale of a landscape. It works the heightfield, the elevation across the world, and it is built to do that well.

The fine surface, the pebble-scale roughness and material grain you see from a metre away, is a different job, and one a dedicated texturing tool in a game engine or DCC package does better. Ymir hands off a heightmap (and, if you want them, the erosion byproduct layers) for that stage to work from.

Knowing which scale you are working at keeps a graph honest. Choose terrain that lives on macro form: an eroded range, a coastline, a fold of hills, a river basin. Judge the result at the scale of the whole world, not zoomed in on a single face.

The distinction is relative to the world, not absolute. A feature size that is coarse detail on a hundred-kilometre continent is the whole landform on a one-kilometre island. Because Ymir measures lengths against World extent, the same graph holds its proportions as the world grows or shrinks, and "macro" always means the same fraction of the terrain you are building.