Masks and selections

Ymir scopes an effect to part of the terrain with a grayscale field: 1 where the effect applies fully, 0 where it does not, and the soft values between for a gradual edge. The same values do two jobs, and keeping the two words apart keeps a graph readable.

A selection is what a selector produces. The Slope, Height, Curvature, and Aspect selectors each measure the terrain and turn it into a [0, 1] field: high on the steep ground, or the high ground, or the ridgelines, or the sun-facing slopes. You can also paint a selection by hand with the Paint node.

A mask is the input that a node reads to know where to act. Erosion, Blur, Levels, and the rest take a mask and confine themselves to it. A node that reads a mask and finds none applies everywhere, so leaving the input empty is the same as selecting the whole terrain.

The two connect in the obvious way: a selection feeds a mask. Measure the steep ground with a Slope selector, feed it into an erosion node's mask, and the erosion works only on the slopes. To combine conditions, blend two selections before the mask (steep and high, steep or near the coast). Reach for this whenever an effect should be placed rather than applied everywhere.