water-shore

What the water knows about the ground under it. A sea drawn as one flat sheet has no idea where the land is, so it cannot foam at a beach or freeze from the shore outward. This module gives each vertex of the water's mesh four numbers: the depth of the water there, how much ice covers it, how solid that ice is, and the distance to the waterline. b3d-water fills them in from the terrain (shore="on") and its shader draws from them. It needs a terrain in the scene to read depths from, and the ice also needs that terrain's biome="on", which is where the temperature comes from. It draws:

• a shoreline: wave fronts that rise about 16 m out and run in to the shore one after another, with a wet edge lapping at the waterline (drawn from the distance, so it is as wide under a cliff as on a flat), and paler water over the shallows (drawn from the depth);
• ice, in three states that run into each other: a solid sheet, then broken plates with water between them, then open water.

Where the ice is

Ice follows the cold and the shore. iceCover(temperature, depth) is the whole rule: nothing above the sea's freezing point (about -2 °C); below it the shallows freeze first and the cover spreads outward into deeper water as it gets colder. So one cold bay has a solid sheet along the beach, breaking into plates further out, and open sea beyond. The temperature is the terrain's climate at sea level INCLUDING the season, so the sea freezes in winter and breaks up in spring.

Colder still and the cracks close: iceSolid runs from that cracked sheet to one unbroken surface, shallows first.

Walking on it

Ice is water until it is solid. iceBears is true once the cracks have mostly closed, and there the water carries an invisible collision mesh: the same vertices, only the triangles whose ice bears. Anything that collides or picks meets it like ground, and it is in the ice collision group for anything that wants to tell it from land. Short of solid (plates, or a sheet still showing its cracks) there is nothing there and you fall through and swim. Waves stop under ice, so the sheet is flat where it collides.

A biped swimming at the surface climbs onto solid ice; one that dived first goes under it, with the ice as a ceiling. water.iceBearsAt(x, z) asks the question for a point in the scene.

A mesh that is fine where you are

Depth lives on vertices, so the mesh decides how sharp the shoreline is. shoreGrid is a square grid whose lines are close together in the middle (4 m by default) and spread geometrically toward the edge, so a sea 8 km across costs under ten thousand vertices and still has a vertex every few metres around the viewer. b3d-water's shoreFine asks for a finer one: a vertex every 2 m out to 64 m, about 16,600 in all.

Pure: no engine. The terrain and the climate arrive as numbers.