Tooling

Tools/Terrain/internal/detail/detail.go

@@ -0,0 +1,29 @@
+package detail
+
+import "salty/terrain/internal/manifest"
+
+// MarginCells is the overlap a tile must carry for the particle pass, in detail cells.
+//
+// Rule 2 of the tiling plan says to size a margin by how far the pass can move material, and for droplets
+// that is not simply the lifetime. Within one round a droplet travels at most its lifetime, plus one cell for
+// the cut brush. Across rounds the error compounds: a droplet in round two reads heights the round-one
+// droplets moved, so the cut edge's influence walks a lifetime further in with every round.
+//
+// Taking that literally would make the margin `rounds * lifetime`, which at the defaults is 640 cells against
+// a 2500-cell tile. Measured instead, at lifetime 12 and 8 rounds (TestHowFarTheCutEdgeReachesIn), the worst
+// difference between a tile and the same ground in one whole run falls off much faster than that:
+//
+//	cells in from the cut edge:   0     4     8    12    16    20    24    32    40
+//	worst difference, metres:   7.97  2.53  0.72  0.49  0.44  0.18  0.03  0.00  0.00
+//
+// It is the first lifetime that carries almost all of it, and by three the error is gone - a droplet has to be
+// unlucky in the same way several rounds running for it to keep propagating, and that stops happening. Three
+// lifetimes plus the brush is the margin, which at the default lifetime of 40 is 122 detail cells, 244 m, or
+// about five per cent of a 5 km tile on each side.
+func MarginCells(cfg manifest.Particle) int {
+	life := cfg.Lifetime
+	if life < 1 {
+		life = 1
+	}
+	return 3*life + 2
+}