terrain_erode_thermal

Talus-angle relaxation over one tile, one iteration per dispatch.

Loose material resting on a slope steeper than its angle of repose slides down. Repeat that and cliffs grow scree fans at their feet while the crests a droplet pass leaves knife-edged round off — which is why this is usually the finishing pass over hydraulic erosion rather than a substitute for it.

The rest angle is a world angle. A cell may stand tan(talus) * cellSize metres above a four-neighbour and still rest, so refining the grid resolves the same slope rather than changing which slopes are stable.

Each iteration is one dispatch: a cell above the rest angle sheds a fraction of its excess, split across the neighbours below the angle in proportion to how far below each one is. Half the excess is the ceiling per iteration, above which the two cells either side of a step trade places every step instead of settling.

Changes accumulate through the same fixed-point integer path droplet erosion uses, so the result is equally independent of the order threads land in.

One thread per INTERIOR cell. The apron is read and never written: an apron cell belongs to the neighbouring tile, which relaxes it itself, and relaxing it here as well would move its material twice. Material that slides out of the interior lands in the apron, and terrain_delta_gather carries it into the neighbour before either tile folds its accumulator into height.

Every op carries a mask, and for a relaxation the mask gates the cell that SHEDS. A cell the mask holds out keeps its own material, and what it still receives from an unmasked neighbour is the scree that slope throws past the mask's edge — material moving downhill, which is what this pass does. An op that names no mask relaxes at full rate everywhere.