add fog/god rays to viridian forest

data/map_atmosphere.lua

@@ -0,0 +1,63 @@
+-- What hangs in the air of each map.
+--
+-- One entry per map that has an ATMOSPHERE: a ground fog the scene shader
+-- folds every surface into, and volumetric god rays -- light let down
+-- through an INVISIBLE canopy hanging above the map's real geometry, as
+-- if the trees drawn are only the understorey of something taller. The
+-- rays are not placed: a per-pixel march (see ForestAtmos) reads the
+-- frame's own depth and the sun's own shadow map, so the beams stand
+-- exactly where light really breaks between the tree hulls, trees and
+-- characters carve dark columns through them, and a wind-blown leaf
+-- field at the canopy plane opens and closes them like foliage moving
+-- overhead. The light leans along the mod's fixed noon shear (the one
+-- light a canopy map ever gets -- see DayNight.CANOPY); only its COLOUR
+-- and STRENGTH follow the clock: gold spears of sun by day, silver moon
+-- rays after dark, pollen adrift in the day's beams and fireflies once
+-- they cool.
+--
+-- A map with no entry here has no atmosphere at all: no fog uniform is
+-- raised, no march runs, nothing is spent. That is the contract a new
+-- map opts into by adding a line, and what a stale entry degrades to if
+-- its map id ever stops existing.
+--
+-- The knobs, in world pixels unless said otherwise (a map cell is 16):
+--
+--   canopyY   where the invisible canopy hangs. MUST clear the tallest
+--             real geometry under it (Viridian's carved tree hulls top
+--             at y = 32) -- a beam is alpha ZERO at this height and only
+--             fades in below it, so a canopy at or under the tree tops
+--             would cut every ray off before it cleared the leaves.
+--   fadeTo    the height by which a descending ray reaches full strength.
+--   fog       density  how fast distance dissolves into the haze
+--                      (1 - exp(-density * distance-past-start))
+--             start    how many pixels out the dissolve begins
+--             heightK  how quickly the fog thins with ALTITUDE
+--                      (exp(-y * heightK): 0.02 halves it by y = 35)
+--   rays      strength  overall in-scatter gain on the march
+--             reach     how far out the march walks, in world px
+--   motes     count of pollen/dust flecks adrift in the daylight beams
+--   fireflies count of the night shift
+--   seed      the xorshift seed the particle deal runs on
+--
+-- Two caveats for maps opting in later: the water pass has no fog term,
+-- so a lake under heavy haze stays clear-day sharp in its reflections;
+-- and the march runs after the water's mirror copy, so beams will not
+-- appear IN those reflections either. Neither can bite in a map without
+-- water.
+
+return {
+  ["VIRIDIAN_FOREST"] = {
+    canopyY = 56,
+    fadeTo = 28,
+    fog = { density = 0.0045, start = 64, heightK = 0.02 },
+    -- strength is calibrated against the march's real integral: the
+    -- under-canopy stretch of an orbit ray is short and thinly dense, so
+    -- the raw accumulation for a fully lit beam core is a few percent --
+    -- this gain lands it near +0.3 on screen. Halve it for a whisper,
+    -- double it for cathedral light.
+    rays = { strength = 16, reach = 380 },
+    motes = { count = 96 },
+    fireflies = { count = 48 },
+    seed = 0x51D,
+  },
+}