mapgen_rivers/polygons.lua

-- Fetch polygons from a given areas, and compute their properties
-- and find to which polygon every point belongs

local blocksize = mapgen_rivers.settings.blocksize
local X = math.floor(mapgen_rivers.settings.map_x_size / blocksize)
local Z = math.floor(mapgen_rivers.settings.map_z_size / blocksize)

local function index(x, z)
	return z*X+x+1
end

local map_offset = {x=0, z=0}
mapgen_rivers.register_on_grid_loaded(function(grid)
	X = grid.size.x
	Z = grid.size.y
	if mapgen_rivers.settings.center then
		map_offset.x = blocksize*X/2
		map_offset.z = blocksize*Z/2
	end
end)

-- Localize for performance
local floor, ceil, min, max, abs = math.floor, math.ceil, math.min, math.max, math.abs

local min_catchment = mapgen_rivers.settings.min_catchment / (blocksize*blocksize)
local wpower = mapgen_rivers.settings.river_widening_power
local wfactor = 1/(2*blocksize * min_catchment^wpower)
local function river_width(flow)
	flow = abs(flow)
	if flow < min_catchment then
		return 0
	end

	return min(wfactor * flow ^ wpower, 1)
end

local noise_heat -- Need a large-scale noise here so no heat blend
local elevation_chill = mapgen_rivers.settings.elevation_chill
local function get_temperature(x, y, z)
	local pos = {x=x, y=z}
	return noise_heat:get2d(pos) - y*elevation_chill
end

local glaciers = mapgen_rivers.settings.glaciers
local glacier_factor = mapgen_rivers.settings.glacier_factor

local init = false

-- On map generation, determine into which polygon every point (in 2D) will fall.
-- Also store polygon-specific data
function mapgen_rivers.make_polygons(minp, maxp)

	local grid = mapgen_rivers.grid
	local dem = grid.dem
	local lakes = grid.lakes
	local dirs = grid.dirs
	local rivers = grid.rivers

	local offset_x = grid.offset_x
	local offset_z = grid.offset_y

	if not init then
		if glaciers then
			noise_heat = minetest.get_perlin(mapgen_rivers.noise_params.heat)
		end
		init = true
	end

	local chulens = maxp.x - minp.x + 1

	local polygons = {}
	-- Determine the minimum and maximum coordinates of the polygons that could be on the chunk, knowing that they have an average size of 'blocksize' and a maximal offset of 0.5 blocksize.
	local xpmin, xpmax = max(floor((minp.x+map_offset.x)/blocksize - 0.5), 0), min(ceil((maxp.x+map_offset.x)/blocksize + 0.5), X-2)
	local zpmin, zpmax = max(floor((minp.z+map_offset.z)/blocksize - 0.5), 0), min(ceil((maxp.z+map_offset.z)/blocksize + 0.5), Z-2)

	-- Iterate over the polygons
	for xp = xpmin, xpmax do
		for zp=zpmin, zpmax do
			local iA = index(xp, zp)
			local iB = index(xp+1, zp)
			local iC = index(xp+1, zp+1)
			local iD = index(xp, zp+1)
			-- Extract the vertices of the polygon
			local poly_x = {
				(offset_x[iA]+xp)   * blocksize - map_offset.x,
				(offset_x[iB]+xp+1) * blocksize - map_offset.x,
				(offset_x[iC]+xp+1) * blocksize - map_offset.x,
				(offset_x[iD]+xp)   * blocksize - map_offset.x,
			}
			local poly_z = {
				(offset_z[iA]+zp)   * blocksize - map_offset.z,
				(offset_z[iB]+zp)   * blocksize - map_offset.z,
				(offset_z[iC]+zp+1) * blocksize - map_offset.z,
				(offset_z[iD]+zp+1) * blocksize - map_offset.z,
			}
			local polygon = {x=poly_x, z=poly_z, i={iA, iB, iC, iD}}

			local bounds = {} -- Will be a list of the intercepts of polygon edges for every Z position (scanline algorithm)
			-- Calculate the min and max Z positions 
			local zmin = max(floor(min(unpack(poly_z)))+1, minp.z)
			local zmax = min(floor(max(unpack(poly_z))), maxp.z)
			-- And initialize the arrays
			for z=zmin, zmax do
				bounds[z] = {}
			end

			local i1 = 4
			for i2=1, 4 do -- Loop on 4 edges
				local z1, z2 = poly_z[i1], poly_z[i2]
				-- Calculate the integer Z positions over which this edge spans
				local lzmin = floor(min(z1, z2))+1
				local lzmax = floor(max(z1, z2))
				if lzmin <= lzmax then -- If there is at least one position in it
					local x1, x2 = poly_x[i1], poly_x[i2]
					-- Calculate coefficient of the equation defining the edge: X=aZ+b
					local a = (x1-x2) / (z1-z2)
					local b = (x1 - a*z1)
					for z=max(lzmin, minp.z), min(lzmax, maxp.z) do
						-- For every Z position involved, add the intercepted X position in the table
						table.insert(bounds[z], a*z+b)
					end
				end
				i1 = i2
			end
			for z=zmin, zmax do
				-- Now sort the bounds list
				local zlist = bounds[z]
				table.sort(zlist)
				local c = floor(#zlist/2)
				for l=1, c do
					-- Take pairs of X coordinates: all positions between them belong to the polygon.
					local xmin = max(floor(zlist[l*2-1])+1, minp.x)
					local xmax = min(floor(zlist[l*2]), maxp.x)
					local i = (z-minp.z) * chulens + (xmin-minp.x) + 1
					for x=xmin, xmax do
						-- Fill the map at these places
						polygons[i] = polygon
						i = i + 1
					end
				end
			end

			local poly_dem = {dem[iA], dem[iB], dem[iC], dem[iD]}
			polygon.dem = poly_dem
			polygon.lake = {lakes[iA], lakes[iB], lakes[iC], lakes[iD]}

			-- Now, rivers.
			-- Load river flux values for the 4 corners
			local riverA = river_width(rivers[iA])
			local riverB = river_width(rivers[iB])
			local riverC = river_width(rivers[iC])
			local riverD = river_width(rivers[iD])
			if glaciers then -- Widen the river
				if get_temperature(poly_x[1], poly_dem[1], poly_z[1]) < 0 then
					riverA = min(riverA*glacier_factor, 1)
				end
				if get_temperature(poly_x[2], poly_dem[2], poly_z[2]) < 0 then
					riverB = min(riverB*glacier_factor, 1)
				end
				if get_temperature(poly_x[3], poly_dem[3], poly_z[3]) < 0 then
					riverC = min(riverC*glacier_factor, 1)
				end
				if get_temperature(poly_x[4], poly_dem[4], poly_z[4]) < 0 then
					riverD = min(riverD*glacier_factor, 1)
				end
			end

			polygon.river_corners = {riverA, riverB, riverC, riverD}

			-- Flow directions
			local dirA, dirB, dirC, dirD = dirs[iA], dirs[iB], dirs[iC], dirs[iD]
			-- Determine the river flux on the edges, by testing dirs values
			local river_west = (dirA==1 and riverA or 0) + (dirD==3 and riverD or 0)
			local river_north = (dirA==2 and riverA or 0) + (dirB==4 and riverB or 0)
			local river_east = (dirB==1 and riverB or 0) + (dirC==3 and riverC or 0)
			local river_south = (dirD==2 and riverD or 0) + (dirC==4 and riverC or 0)

			polygon.rivers = {river_west, river_north, river_east, river_south}
		end
	end

	return polygons
end