Added settings for margin, and documented in settingtypes.txt

Elevation gets closer to -50 when approaching the border

heightmap.lua

@@ -6,7 +6,7 @@ local transform_quadri = dofile(modpath .. 'geometry.lua')
 local sea_level = mapgen_rivers.settings.sea_level
 local riverbed_slope = mapgen_rivers.settings.riverbed_slope * mapgen_rivers.settings.blocksize
 
-local MAP_BOTTOM = -31000
+local out_elev = mapgen_rivers.settings.margin_elev
 
 -- Localize for performance
 local floor, min, max = math.floor, math.min, math.max
@@ -128,8 +128,8 @@ local function heightmaps(minp, maxp)
 				terrain_height_map[i] = terrain_height
 				lake_height_map[i] = lake_height
 			else
-				terrain_height_map[i] = MAP_BOTTOM
-				lake_height_map[i] = MAP_BOTTOM
+				terrain_height_map[i] = out_elev
+				lake_height_map[i] = out_elev
 			end
 			i = i + 1
 		end

pregenerate.lua

@@ -13,6 +13,38 @@ local time_step = mapgen_rivers.settings.evol_time_step
 local niter = math.ceil(time/time_step)
 time_step = time / niter
 
+local use_margin = mapgen_rivers.settings.margin
+local margin_width = mapgen_rivers.settings.margin_width / blocksize
+local margin_elev = mapgen_rivers.settings.margin_elev
+
+local function margin(dem, width, elev)
+	local X, Y = dem.X, dem.Y
+	for i=1, width do
+		local c1 = ((i-1)/width) ^ 0.5
+		local c2 = (1-c1) * elev
+		local index = (i-1)*X + 1
+		for x=1, X do
+			dem[index] = dem[index] * c1 + c2
+			index = index + 1
+		end
+		index = i
+		for y=1, Y do
+			dem[index] = dem[index] * c1 + c2
+			index = index + X
+		end
+		index = X*(Y-i) + 1
+		for x=1, X do
+			dem[index] = dem[index] * c1 + c2
+			index = index + 1
+		end
+		index = X-i + 1
+		for y=1, Y do
+			dem[index] = dem[index] * c1 + c2
+			index = index + X
+		end
+	end
+end
+
 local function pregenerate(keep_loaded)
 	local grid = mapgen_rivers.grid
 	local size = grid.size
@@ -26,6 +58,10 @@ local function pregenerate(keep_loaded)
 	dem.X = size.x
 	dem.Y = size.y
 
+	if use_margin then
+		margin(dem, margin_width, margin_elev)
+	end
+
 	local model = EvolutionModel(evol_params)
 	model.dem = dem
 	local ref_dem = model:define_isostasy(dem)
@@ -41,6 +77,9 @@ local function pregenerate(keep_loaded)
 		if i < niter then
 			if tectonic_step ~= 0 then
 				nobj_base:get_3d_map_flat({x=0, y=tectonic_step*i, z=0}, ref_dem)
+				if use_margin then
+					margin(ref_dem, margin_width, margin_elev)
+				end
 			end
 			model:isostasy()
 		end

settings.lua

@@ -74,6 +74,9 @@ mapgen_rivers.settings = {
 
 	grid_x_size = def_setting('grid_x_size', 'number', 1000),
 	grid_z_size = def_setting('grid_z_size', 'number', 1000),
+	margin = def_setting('margin', 'bool', true),
+	margin_width = def_setting('margin_width', 'number', 2000),
+	margin_elev = def_setting('margin_elev', 'number', -200),
 	evol_params = {
 		K = def_setting('river_erosion_coef', 'number', 0.5),
 		m = def_setting('river_erosion_power', 'number', 0.4),

settingtypes.txt

@@ -17,6 +17,16 @@ mapgen_rivers_grid_x_size (Grid X size) int 1000 50 5000
 #    Actual size of the map is grid_z_size * blocksize
 mapgen_rivers_grid_z_size (Grid Z size) int 1000 50 5000
 
+#    If margin is enabled, elevation becomes closer to a fixed value when approaching
+#    the edges of the map.
+mapgen_rivers_margin (Margin) bool true
+
+#    Width of the transition at map borders, in nodes
+mapgen_rivers_margin_width (Margin width) float 2000.0 0.0 15000.0
+
+#    Elevation toward which to converge at map borders
+mapgen_rivers_margin_elev (Margin elevation) float -200.0 -31000.0 31000.0
+
 #    Minimal catchment area for a river to be drawn, in square nodes
 #    Lower value means bigger river density
 mapgen_rivers_min_catchment (Minimal catchment area) float 3600.0 100.0 1000000.0