A world that looks alive: climate, hydrology, biomes

Before this pass, the world looked like geology. After it, it looks like a place that something could live in.

The pain was straightforward. Tectonics produced ranges with realistic profiles, but the cliffs were everywhere, and the walkability test (no two neighbouring cells may differ in altitude by more than three units) failed across enormous fractions of the map. A deer could not cross the continent. A player eventually could not either. Three sub-phases shipped together to fix it.

The first added a synthetic climate model to the worldgen pipeline: per-cell mean temperature, mean rainfall, and snowpack arrays produced by latitude, lapse, and a small set of belts (intertropical convergence, the horse latitudes, the temperate west winds). Cheap integer math, on the order of ten milliseconds for the whole world. The original spec called for a full cellular automaton burn-in over ten thousand ticks; the synthetic model produces equivalently-shaped outputs in a thousandth of the time. A future polish phase can swap in the real burn-in without touching the consumers.

The second was an erosion overhaul. More hillslope passes, more incision passes, lower talus baselines, weaker resistance weights. Crucially, the new erosion takes the climate maps as input: rainfall-weighted incision means deserts carve at half rate and the ITCZ at two and a half times rate, so wet places get the river valleys and dry places keep their plateaus. A separate glaciation pass runs inside the same function, carving U-shaped valleys wherever snowpack accumulated, so the final hydrology rebuild sees the post-ice elevation. A new coastal-ramping pass softens the shelf-to-coast walls that used to read as vertical cliffs.

The third was Whittaker biomes: a nine-biome decision tree on the climate maps, persisted to the genesis blob. Polar desert, tundra, boreal forest, temperate forest, temperate rainforest, steppe, desert, savanna, tropical rainforest. A new biome-aware surface classifier replaces the bedrock-only one in the worldgen packing path: marine cells defer to the depth ladder, above-sea cells select a biome-specific soil, and very high elevation surfaces as exposed basalt regardless of biome.

The walkability gate is now a warn-only metric in the boot log, counting the fraction of land cells with any impassable neighbour. It will flip to a panic once tuning converges. That line has not been crossed yet, and there is no hurry to cross it. The work that gets the number under five percent is the same work that makes the world look like a place.