Feature: setting to indicate desert coverage for tropic climate

This is an indication value; the game tries to get as close as it
can, but due to the complex tropic rules, that is unlikely to be
exact.

In the end, it picks a height-level to base the desert/tropic
line on. This is strictly seen not needed, as we can convert any
tile to either. But it is the simplest way to get started with
this without redoing all related functions.

9 files changed, 194 insertions, 33 deletions

diff --git a/src/genworld_gui.cpp b/src/genworld_gui.cpp
index 94381fe89..20c12d8b4 100644
--- a/src/genworld_gui.cpp
+++ b/src/genworld_gui.cpp
@@ -111,7 +111,10 @@ static const NWidgetPart _nested_generate_landscape_widgets[] = {
 				NWidget(NWID_HORIZONTAL), SetPIP(0, 3, 0),
 					NWidget(NWID_VERTICAL, NC_EQUALSIZE), SetPIP(0, 4, 0),
 						NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_MAX_HEIGHTLEVEL, STR_NULL), SetFill(1, 1),
-						NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_SNOW_COVERAGE, STR_NULL), SetFill(1, 1),
+						NWidget(NWID_SELECTION, INVALID_COLOUR, WID_GL_CLIMATE_SEL_LABEL),
+							NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_SNOW_COVERAGE, STR_NULL), SetFill(1, 1),
+							NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_DESERT_COVERAGE, STR_NULL), SetFill(1, 1),
+						EndContainer(),
 						NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_DATE, STR_NULL), SetFill(1, 1),
 						NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_SMOOTHNESS, STR_NULL), SetFill(1, 1),
 						NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_QUANTITY_OF_RIVERS, STR_NULL), SetFill(1, 1),
@@ -124,11 +127,19 @@ static const NWidgetPart _nested_generate_landscape_widgets[] = {
 							NWidget(WWT_TEXTBTN, COLOUR_ORANGE, WID_GL_MAX_HEIGHTLEVEL_TEXT), SetDataTip(STR_BLACK_INT, STR_NULL), SetFill(1, 0),
 							NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_MAX_HEIGHTLEVEL_UP), SetDataTip(SPR_ARROW_UP, STR_MAPGEN_MAX_HEIGHTLEVEL_UP), SetFill(0, 1),
 						EndContainer(),
-						/* Snow line. */
-						NWidget(NWID_HORIZONTAL),
-							NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_DOWN), SetDataTip(SPR_ARROW_DOWN, STR_MAPGEN_SNOW_COVERAGE_DOWN), SetFill(0, 1),
-							NWidget(WWT_TEXTBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_TEXT), SetDataTip(STR_MAPGEN_SNOW_COVERAGE_TEXT, STR_NULL), SetFill(1, 0),
-							NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_UP), SetDataTip(SPR_ARROW_UP, STR_MAPGEN_SNOW_COVERAGE_UP), SetFill(0, 1),
+						NWidget(NWID_SELECTION, INVALID_COLOUR, WID_GL_CLIMATE_SEL_SELECTOR),
+							/* Snow coverage. */
+							NWidget(NWID_HORIZONTAL),
+								NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_DOWN), SetDataTip(SPR_ARROW_DOWN, STR_MAPGEN_SNOW_COVERAGE_DOWN), SetFill(0, 1),
+								NWidget(WWT_TEXTBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_TEXT), SetDataTip(STR_MAPGEN_SNOW_COVERAGE_TEXT, STR_NULL), SetFill(1, 0),
+								NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_UP), SetDataTip(SPR_ARROW_UP, STR_MAPGEN_SNOW_COVERAGE_UP), SetFill(0, 1),
+							EndContainer(),
+							/* Desert coverage. */
+							NWidget(NWID_HORIZONTAL),
+								NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_DESERT_COVERAGE_DOWN), SetDataTip(SPR_ARROW_DOWN, STR_MAPGEN_DESERT_COVERAGE_DOWN), SetFill(0, 1),
+								NWidget(WWT_TEXTBTN, COLOUR_ORANGE, WID_GL_DESERT_COVERAGE_TEXT), SetDataTip(STR_MAPGEN_DESERT_COVERAGE_TEXT, STR_NULL), SetFill(1, 0),
+								NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_DESERT_COVERAGE_UP), SetDataTip(SPR_ARROW_UP, STR_MAPGEN_DESERT_COVERAGE_UP), SetFill(0, 1),
+							EndContainer(),
 						EndContainer(),
 						/* Starting date. */
 						NWidget(NWID_HORIZONTAL),
@@ -228,7 +239,10 @@ static const NWidgetPart _nested_heightmap_load_widgets[] = {
 						NWidget(NWID_HORIZONTAL), SetPIP(0, 3, 0),
 							NWidget(NWID_VERTICAL, NC_EQUALSIZE), SetPIP(0, 4, 0),
 								NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_MAX_HEIGHTLEVEL, STR_NULL), SetFill(1, 1),
-								NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_SNOW_COVERAGE, STR_NULL), SetFill(1, 1),
+								NWidget(NWID_SELECTION, INVALID_COLOUR, WID_GL_CLIMATE_SEL_LABEL),
+									NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_SNOW_COVERAGE, STR_NULL), SetFill(1, 1),
+									NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_DESERT_COVERAGE, STR_NULL), SetFill(1, 1),
+								EndContainer(),
 								NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_MAPGEN_DATE, STR_NULL), SetFill(1, 1),
 								NWidget(WWT_TEXT, COLOUR_ORANGE), SetDataTip(STR_GAME_OPTIONS_TOWN_NAMES_FRAME, STR_NULL), SetFill(1, 1),
 							EndContainer(),
@@ -238,10 +252,17 @@ static const NWidgetPart _nested_heightmap_load_widgets[] = {
 									NWidget(WWT_TEXTBTN, COLOUR_ORANGE, WID_GL_MAX_HEIGHTLEVEL_TEXT), SetDataTip(STR_BLACK_INT, STR_NULL), SetFill(1, 0),
 									NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_MAX_HEIGHTLEVEL_UP), SetDataTip(SPR_ARROW_UP, STR_MAPGEN_MAX_HEIGHTLEVEL_UP), SetFill(0, 1),
 								EndContainer(),
-								NWidget(NWID_HORIZONTAL),
-									NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_DOWN), SetDataTip(SPR_ARROW_DOWN, STR_MAPGEN_SNOW_COVERAGE_DOWN), SetFill(0, 1),
-									NWidget(WWT_TEXTBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_TEXT), SetDataTip(STR_BLACK_INT, STR_NULL), SetFill(1, 0),
-									NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_UP), SetDataTip(SPR_ARROW_UP, STR_MAPGEN_SNOW_COVERAGE_UP), SetFill(0, 1),
+								NWidget(NWID_SELECTION, INVALID_COLOUR, WID_GL_CLIMATE_SEL_SELECTOR),
+									NWidget(NWID_HORIZONTAL),
+										NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_DOWN), SetDataTip(SPR_ARROW_DOWN, STR_MAPGEN_SNOW_COVERAGE_DOWN), SetFill(0, 1),
+										NWidget(WWT_TEXTBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_TEXT), SetDataTip(STR_MAPGEN_SNOW_COVERAGE_TEXT, STR_NULL), SetFill(1, 0),
+										NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_SNOW_COVERAGE_UP), SetDataTip(SPR_ARROW_UP, STR_MAPGEN_SNOW_COVERAGE_UP), SetFill(0, 1),
+									EndContainer(),
+									NWidget(NWID_HORIZONTAL),
+										NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_DESERT_COVERAGE_DOWN), SetDataTip(SPR_ARROW_DOWN, STR_MAPGEN_DESERT_COVERAGE_DOWN), SetFill(0, 1),
+										NWidget(WWT_TEXTBTN, COLOUR_ORANGE, WID_GL_DESERT_COVERAGE_TEXT), SetDataTip(STR_MAPGEN_DESERT_COVERAGE_TEXT, STR_NULL), SetFill(1, 0),
+										NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_DESERT_COVERAGE_UP), SetDataTip(SPR_ARROW_UP, STR_MAPGEN_DESERT_COVERAGE_UP), SetFill(0, 1),
+									EndContainer(),
 								EndContainer(),
 								NWidget(NWID_HORIZONTAL),
 									NWidget(WWT_IMGBTN, COLOUR_ORANGE, WID_GL_START_DATE_DOWN), SetDataTip(SPR_ARROW_DOWN, STR_SCENEDIT_TOOLBAR_TOOLTIP_MOVE_THE_STARTING_DATE_BACKWARD), SetFill(0, 1),
@@ -367,6 +388,7 @@ struct GenerateLandscapeWindow : public Window {
 			case WID_GL_MAPSIZE_Y_PULLDOWN:   SetDParam(0, 1LL << _settings_newgame.game_creation.map_y); break;
 			case WID_GL_MAX_HEIGHTLEVEL_TEXT: SetDParam(0, _settings_newgame.construction.max_heightlevel); break;
 			case WID_GL_SNOW_COVERAGE_TEXT:   SetDParam(0, _settings_newgame.game_creation.snow_coverage); break;
+			case WID_GL_DESERT_COVERAGE_TEXT: SetDParam(0, _settings_newgame.game_creation.desert_coverage); break;
 
 			case WID_GL_TOWN_PULLDOWN:
 				if (_game_mode == GM_EDITOR) {
@@ -458,6 +480,19 @@ struct GenerateLandscapeWindow : public Window {
 
 		/* Disable snowline if not arctic */
 		this->SetWidgetDisabledState(WID_GL_SNOW_COVERAGE_TEXT, _settings_newgame.game_creation.landscape != LT_ARCTIC);
+		/* Disable desert if not tropic */
+		this->SetWidgetDisabledState(WID_GL_DESERT_COVERAGE_TEXT, _settings_newgame.game_creation.landscape != LT_TROPIC);
+
+		/* Set snow/rainforest selections */
+		int climate_plane = 0;
+		switch (_settings_newgame.game_creation.landscape) {
+			case LT_TEMPERATE: climate_plane = SZSP_VERTICAL; break;
+			case LT_ARCTIC:    climate_plane = 0;             break;
+			case LT_TROPIC:    climate_plane = 1;             break;
+			case LT_TOYLAND:   climate_plane = SZSP_VERTICAL; break;
+		}
+		this->GetWidget<NWidgetStacked>(WID_GL_CLIMATE_SEL_LABEL)->SetDisplayedPlane(climate_plane);
+		this->GetWidget<NWidgetStacked>(WID_GL_CLIMATE_SEL_SELECTOR)->SetDisplayedPlane(climate_plane);
 
 		/* Update availability of decreasing / increasing start date and snow level */
 		this->SetWidgetDisabledState(WID_GL_MAX_HEIGHTLEVEL_DOWN, _settings_newgame.construction.max_heightlevel <= MIN_MAX_HEIGHTLEVEL);
@@ -466,6 +501,8 @@ struct GenerateLandscapeWindow : public Window {
 		this->SetWidgetDisabledState(WID_GL_START_DATE_UP,   _settings_newgame.game_creation.starting_year >= MAX_YEAR);
 		this->SetWidgetDisabledState(WID_GL_SNOW_COVERAGE_DOWN, _settings_newgame.game_creation.snow_coverage <= 0 || _settings_newgame.game_creation.landscape != LT_ARCTIC);
 		this->SetWidgetDisabledState(WID_GL_SNOW_COVERAGE_UP,   _settings_newgame.game_creation.snow_coverage >= 100 || _settings_newgame.game_creation.landscape != LT_ARCTIC);
+		this->SetWidgetDisabledState(WID_GL_DESERT_COVERAGE_DOWN, _settings_newgame.game_creation.desert_coverage <= 0 || _settings_newgame.game_creation.landscape != LT_TROPIC);
+		this->SetWidgetDisabledState(WID_GL_DESERT_COVERAGE_UP,   _settings_newgame.game_creation.desert_coverage >= 100 || _settings_newgame.game_creation.landscape != LT_TROPIC);
 
 		/* Do not allow a custom sea level with the original land generator. */
 		if (_settings_newgame.game_creation.land_generator == LG_ORIGINAL &&
@@ -500,6 +537,11 @@ struct GenerateLandscapeWindow : public Window {
 				*size = maxdim(*size, GetStringBoundingBox(STR_MAPGEN_SNOW_COVERAGE_TEXT));
 				break;
 
+			case WID_GL_DESERT_COVERAGE_TEXT:
+				SetDParamMaxValue(0, MAX_TILE_HEIGHT);
+				*size = maxdim(*size, GetStringBoundingBox(STR_MAPGEN_DESERT_COVERAGE_TEXT));
+				break;
+
 			case WID_GL_HEIGHTMAP_SIZE_TEXT:
 				SetDParam(0, this->x);
 				SetDParam(1, this->y);
@@ -674,6 +716,24 @@ struct GenerateLandscapeWindow : public Window {
 				ShowQueryString(STR_JUST_INT, STR_MAPGEN_SNOW_COVERAGE_QUERY_CAPT, 4, this, CS_NUMERAL, QSF_ENABLE_DEFAULT);
 				break;
 
+			case WID_GL_DESERT_COVERAGE_DOWN:
+			case WID_GL_DESERT_COVERAGE_UP: // Desert coverage buttons
+				/* Don't allow too fast scrolling */
+				if (!(this->flags & WF_TIMEOUT) || this->timeout_timer <= 1) {
+					this->HandleButtonClick(widget);
+
+					_settings_newgame.game_creation.desert_coverage = Clamp(_settings_newgame.game_creation.desert_coverage + (widget - WID_GL_DESERT_COVERAGE_TEXT) * 10, 0, 100);
+					this->InvalidateData();
+				}
+				_left_button_clicked = false;
+				break;
+
+			case WID_GL_DESERT_COVERAGE_TEXT: // Desert line text
+				this->widget_id = WID_GL_DESERT_COVERAGE_TEXT;
+				SetDParam(0, _settings_newgame.game_creation.desert_coverage);
+				ShowQueryString(STR_JUST_INT, STR_MAPGEN_DESERT_COVERAGE_QUERY_CAPT, 4, this, CS_NUMERAL, QSF_ENABLE_DEFAULT);
+				break;
+
 			case WID_GL_LANDSCAPE_PULLDOWN: // Landscape generator
 				ShowDropDownMenu(this, _landscape, _settings_newgame.game_creation.land_generator, WID_GL_LANDSCAPE_PULLDOWN, 0, 0);
 				break;
@@ -739,7 +799,7 @@ struct GenerateLandscapeWindow : public Window {
 
 	void OnTimeout() override
 	{
-		static const int raise_widgets[] = {WID_GL_MAX_HEIGHTLEVEL_DOWN, WID_GL_MAX_HEIGHTLEVEL_UP, WID_GL_START_DATE_DOWN, WID_GL_START_DATE_UP, WID_GL_SNOW_COVERAGE_UP, WID_GL_SNOW_COVERAGE_DOWN, WIDGET_LIST_END};
+		static const int raise_widgets[] = {WID_GL_MAX_HEIGHTLEVEL_DOWN, WID_GL_MAX_HEIGHTLEVEL_UP, WID_GL_START_DATE_DOWN, WID_GL_START_DATE_UP, WID_GL_SNOW_COVERAGE_UP, WID_GL_SNOW_COVERAGE_DOWN, WID_GL_DESERT_COVERAGE_UP, WID_GL_DESERT_COVERAGE_DOWN, WIDGET_LIST_END};
 		for (const int *widget = raise_widgets; *widget != WIDGET_LIST_END; widget++) {
 			if (this->IsWidgetLowered(*widget)) {
 				this->RaiseWidget(*widget);
@@ -812,6 +872,7 @@ struct GenerateLandscapeWindow : public Window {
 				case WID_GL_MAX_HEIGHTLEVEL_TEXT: value = DEF_MAX_HEIGHTLEVEL; break;
 				case WID_GL_START_DATE_TEXT: value = DEF_START_YEAR; break;
 				case WID_GL_SNOW_COVERAGE_TEXT: value = DEF_SNOW_COVERAGE; break;
+				case WID_GL_DESERT_COVERAGE_TEXT: value = DEF_DESERT_COVERAGE; break;
 				case WID_GL_TOWN_PULLDOWN: value = 1; break;
 				case WID_GL_WATER_PULLDOWN: value = CUSTOM_SEA_LEVEL_MIN_PERCENTAGE; break;
 				default: NOT_REACHED();
@@ -834,6 +895,11 @@ struct GenerateLandscapeWindow : public Window {
 				_settings_newgame.game_creation.snow_coverage = Clamp(value, 0, 100);
 				break;
 
+			case WID_GL_DESERT_COVERAGE_TEXT:
+				this->SetWidgetDirty(WID_GL_DESERT_COVERAGE_TEXT);
+				_settings_newgame.game_creation.desert_coverage = Clamp(value, 0, 100);
+				break;
+
 			case WID_GL_TOWN_PULLDOWN:
 				_settings_newgame.game_creation.custom_town_number = Clamp(value, 1, CUSTOM_TOWN_MAX_NUMBER);
 				break;
diff --git a/src/landscape.cpp b/src/landscape.cpp
index 98ff6fdbc..bdfcc51f1 100644
--- a/src/landscape.cpp
+++ b/src/landscape.cpp
@@ -968,11 +968,10 @@ static void GenerateTerrain(int type, uint flag)
 
 #include "table/genland.h"
 
-static void CreateDesertOrRainForest()
+static void CreateDesertOrRainForest(uint desert_tropic_line)
 {
 	TileIndex update_freq = MapSize() / 4;
 	const TileIndexDiffC *data;
-	uint max_desert_height = CeilDiv(_settings_game.construction.max_heightlevel, 4);
 
 	for (TileIndex tile = 0; tile != MapSize(); ++tile) {
 		if ((tile % update_freq) == 0) IncreaseGeneratingWorldProgress(GWP_LANDSCAPE);
@@ -982,7 +981,7 @@ static void CreateDesertOrRainForest()
 		for (data = _make_desert_or_rainforest_data;
 				data != endof(_make_desert_or_rainforest_data); ++data) {
 			TileIndex t = AddTileIndexDiffCWrap(tile, *data);
-			if (t != INVALID_TILE && (TileHeight(t) >= max_desert_height || IsTileType(t, MP_WATER))) break;
+			if (t != INVALID_TILE && (TileHeight(t) >= desert_tropic_line || IsTileType(t, MP_WATER))) break;
 		}
 		if (data == endof(_make_desert_or_rainforest_data)) {
 			SetTropicZone(tile, TROPICZONE_DESERT);
@@ -1296,15 +1295,50 @@ static void CreateRivers()
 }
 
 /**
- * Calculate the line from which snow begins.
+ * Calculate what height would be needed to cover N% of the landmass.
+ *
+ * The function allows both snow and desert/tropic line to be calculated. It
+ * tries to find the closests height which covers N% of the landmass; it can
+ * be below or above it.
+ *
+ * Tropic has a mechanism where water and tropic tiles in mountains grow
+ * inside the desert. To better approximate the requested coverage, this is
+ * taken into account via an edge histogram, which tells how many neighbouring
+ * tiles are lower than the tiles of that height. The multiplier indicates how
+ * severe this has to be taken into account.
+ *
+ * @param coverage A value between 0 and 100 indicating a percentage of landmass that should be covered.
+ * @param edge_multiplier How much effect neighbouring tiles that are of a lower height level have on the score.
+ * @return The estimated best height to use to cover N% of the landmass.
  */
-static void CalculateSnowLine()
+static uint CalculateCoverageLine(uint coverage, uint edge_multiplier)
 {
-	/* Build a histogram of the map height. */
+	const DiagDirection neighbour_dir[] = {
+		DIAGDIR_NE,
+		DIAGDIR_SE,
+		DIAGDIR_SW,
+		DIAGDIR_NW,
+	};
+
+	/* Histogram of how many tiles per height level exist. */
 	std::array<int, MAX_TILE_HEIGHT + 1> histogram = {};
+	/* Histogram of how many neighbour tiles are lower than the tiles of the height level. */
+	std::array<int, MAX_TILE_HEIGHT + 1> edge_histogram = {};
+
+	/* Build a histogram of the map height. */
 	for (TileIndex tile = 0; tile < MapSize(); tile++) {
 		uint h = TileHeight(tile);
 		histogram[h]++;
+
+		if (edge_multiplier != 0) {
+			/* Check if any of our neighbours is below us. */
+			for (auto dir : neighbour_dir) {
+				TileIndex neighbour_tile = AddTileIndexDiffCWrap(tile, TileIndexDiffCByDiagDir(dir));
+				if (IsValidTile(neighbour_tile) && TileHeight(neighbour_tile) < h) {
+					edge_histogram[h]++;
+				}
+			}
+		}
 	}
 
 	/* The amount of land we have is the map size minus the first (sea) layer. */
@@ -1312,7 +1346,7 @@ static void CalculateSnowLine()
 	int best_score = land_tiles;
 
 	/* Our goal is the coverage amount of the land-mass. */
-	int goal_tiles = land_tiles * _settings_game.game_creation.snow_coverage / 100;
+	int goal_tiles = land_tiles * coverage / 100;
 
 	/* We scan from top to bottom. */
 	uint h = MAX_TILE_HEIGHT;
@@ -1323,13 +1357,50 @@ static void CalculateSnowLine()
 		current_tiles += histogram[h];
 		int current_score = goal_tiles - current_tiles;
 
+		/* Tropic grows from water and mountains into the desert. This is a
+		 * great visual, but it also means we* need to take into account how
+		 * much less desert tiles are being created if we are on this
+		 * height-level. We estimate this based on how many neighbouring
+		 * tiles are below us for a given length, assuming that is where
+		 * tropic is growing from.
+		 */
+		if (edge_multiplier != 0 && h > 1) {
+			/* From water tropic tiles grow for a few tiles land inward. */
+			current_score -= edge_histogram[1] * edge_multiplier;
+			/* Tropic tiles grow into the desert for a few tiles. */
+			current_score -= edge_histogram[h] * edge_multiplier;
+		}
+
 		if (std::abs(current_score) < std::abs(best_score)) {
 			best_score = current_score;
 			best_h = h;
 		}
+
+		/* Always scan all height-levels, as h == 1 might give a better
+		 * score than any before. This is true for example with 0% desert
+		 * coverage. */
 	}
 
-	_settings_game.game_creation.snow_line_height = std::max(best_h, 2u);
+	return best_h;
+}
+
+/**
+ * Calculate the line from which snow begins.
+ */
+static void CalculateSnowLine()
+{
+	/* We do not have snow sprites on coastal tiles, so never allow "1" as height. */
+	_settings_game.game_creation.snow_line_height = std::max(CalculateCoverageLine(_settings_game.game_creation.snow_coverage, 0), 2u);
+}
+
+/**
+ * Calculate the line (in height) between desert and tropic.
+ * @return The height of the line between desert and tropic.
+ */
+static uint8 CalculateDesertLine()
+{
+	/* CalculateCoverageLine() runs from top to bottom, so we need to invert the coverage. */
+	return _settings_game.game_creation.snow_line_height = CalculateCoverageLine(100 - _settings_game.game_creation.desert_coverage, 4);
 }
 
 void GenerateLandscape(byte mode)
@@ -1421,9 +1492,11 @@ void GenerateLandscape(byte mode)
 			CalculateSnowLine();
 			break;
 
-		case LT_TROPIC:
-			CreateDesertOrRainForest();
+		case LT_TROPIC: {
+			uint desert_tropic_line = CalculateDesertLine();
+			CreateDesertOrRainForest(desert_tropic_line);
 			break;
+		}
 
 		default:
 			break;
diff --git a/src/lang/english.txt b/src/lang/english.txt
index be1d50427..6c01914c0 100644
--- a/src/lang/english.txt
+++ b/src/lang/english.txt
@@ -1355,6 +1355,9 @@ STR_CONFIG_SETTING_SNOWLINE_HEIGHT_HELPTEXT                     :Controls at wha
 STR_CONFIG_SETTING_SNOW_COVERAGE                                :Snow coverage: {STRING2}
 STR_CONFIG_SETTING_SNOW_COVERAGE_HELPTEXT                       :Controls the approximate amount of snow on the sub-arctic landscape. Snow also affects industry generation and town growth requirements. Only used during map generation. Land just above sea level is always without snow
 STR_CONFIG_SETTING_SNOW_COVERAGE_VALUE                          :{NUM}%
+STR_CONFIG_SETTING_DESERT_COVERAGE                              :Desert coverage: {STRING2}
+STR_CONFIG_SETTING_DESERT_COVERAGE_HELPTEXT                     :Control the approximate amount of desert on the tropical landscape. Desert also affects industry generation. Only used during map generation
+STR_CONFIG_SETTING_DESERT_COVERAGE_VALUE                        :{NUM}%
 STR_CONFIG_SETTING_ROUGHNESS_OF_TERRAIN                         :Roughness of terrain: {STRING2}
 STR_CONFIG_SETTING_ROUGHNESS_OF_TERRAIN_HELPTEXT                :(TerraGenesis only) Choose the frequency of hills: Smooth landscapes have fewer, more wide-spread hills. Rough landscapes have many hills, which may look repetitive
 STR_CONFIG_SETTING_ROUGHNESS_OF_TERRAIN_VERY_SMOOTH             :Very Smooth
@@ -2902,6 +2905,10 @@ STR_MAPGEN_SNOW_COVERAGE                                        :{BLACK}Snow cov
 STR_MAPGEN_SNOW_COVERAGE_UP                                     :{BLACK}Increase snow coverage by ten percent
 STR_MAPGEN_SNOW_COVERAGE_DOWN                                   :{BLACK}Decrease snow coverage by ten percent
 STR_MAPGEN_SNOW_COVERAGE_TEXT                                   :{BLACK}{NUM}%
+STR_MAPGEN_DESERT_COVERAGE                                      :{BLACK}Desert coverage:
+STR_MAPGEN_DESERT_COVERAGE_UP                                   :{BLACK}Increase desert coverage by ten percent
+STR_MAPGEN_DESERT_COVERAGE_DOWN                                 :{BLACK}Decrease desert coverage by ten percent
+STR_MAPGEN_DESERT_COVERAGE_TEXT                                 :{BLACK}{NUM}%
 STR_MAPGEN_LAND_GENERATOR                                       :{BLACK}Land generator:
 STR_MAPGEN_TERRAIN_TYPE                                         :{BLACK}Terrain type:
 STR_MAPGEN_QUANTITY_OF_SEA_LAKES                                :{BLACK}Sea level:
@@ -2929,6 +2936,7 @@ STR_MAPGEN_HEIGHTMAP_SIZE                                       :{ORANGE}{NUM} x
 
 STR_MAPGEN_MAX_HEIGHTLEVEL_QUERY_CAPT                           :{WHITE}Change maximum map height
 STR_MAPGEN_SNOW_COVERAGE_QUERY_CAPT                             :{WHITE}Snow coverage (in %)
+STR_MAPGEN_DESERT_COVERAGE_QUERY_CAPT                           :{WHITE}Desert coverage (in %)
 STR_MAPGEN_START_DATE_QUERY_CAPT                                :{WHITE}Change starting year
 
 # SE Map generation
diff --git a/src/settings_gui.cpp b/src/settings_gui.cpp
index 884f7cb98..69a5bcf20 100644
--- a/src/settings_gui.cpp
+++ b/src/settings_gui.cpp
@@ -1691,6 +1691,7 @@ static SettingsContainer &GetSettingsTree()
 			genworld->Add(new SettingEntry("game_creation.variety"));
 			genworld->Add(new SettingEntry("game_creation.snow_coverage"));
 			genworld->Add(new SettingEntry("game_creation.snow_line_height"));
+			genworld->Add(new SettingEntry("game_creation.desert_coverage"));
 			genworld->Add(new SettingEntry("game_creation.amount_of_rivers"));
 			genworld->Add(new SettingEntry("game_creation.tree_placer"));
 			genworld->Add(new SettingEntry("vehicle.road_side"));
diff --git a/src/settings_type.h b/src/settings_type.h
index d99201b25..b774ff718 100644
--- a/src/settings_type.h
+++ b/src/settings_type.h
@@ -298,6 +298,7 @@ struct GameCreationSettings {
 	byte   oil_refinery_limit;               ///< distance oil refineries allowed from map edge
 	byte   snow_line_height;                 ///< the configured snow line height (deduced from "snow_coverage")
 	byte   snow_coverage;                    ///< the amount of snow coverage on the map
+	byte   desert_coverage;                  ///< the amount of desert coverage on the map
 	byte   tgen_smoothness;                  ///< how rough is the terrain from 0-3
 	byte   tree_placer;                      ///< the tree placer algorithm
 	byte   heightmap_rotation;               ///< rotation director for the heightmap
diff --git a/src/table/settings.ini b/src/table/settings.ini
index 1a7eb12e3..55518ef76 100644
--- a/src/table/settings.ini
+++ b/src/table/settings.ini
@@ -1433,6 +1433,21 @@ strhelp  = STR_CONFIG_SETTING_SNOW_COVERAGE_HELPTEXT
 strval   = STR_CONFIG_SETTING_SNOW_COVERAGE_VALUE
 cat      = SC_BASIC
 
+[SDT_VAR]
+base     = GameSettings
+var      = game_creation.desert_coverage
+type     = SLE_UINT8
+flags    = SLF_NOT_IN_SAVE | SLF_NO_NETWORK_SYNC
+guiflags = SGF_NEWGAME_ONLY
+def      = DEF_DESERT_COVERAGE
+min      = 0
+max      = 100
+interval = 10
+str      = STR_CONFIG_SETTING_DESERT_COVERAGE
+strhelp  = STR_CONFIG_SETTING_DESERT_COVERAGE_HELPTEXT
+strval   = STR_CONFIG_SETTING_DESERT_COVERAGE_VALUE
+cat      = SC_BASIC
+
 [SDT_NULL]
 length   = 4
 to       = SLV_144
diff --git a/src/tgp.cpp b/src/tgp.cpp
index d47cdecac..fabc91e2a 100644
--- a/src/tgp.cpp
+++ b/src/tgp.cpp
@@ -236,17 +236,6 @@ static height_t TGPGetMaxHeight()
 
 	int map_size_bucket = std::min(MapLogX(), MapLogY()) - MIN_MAP_SIZE_BITS;
 	int max_height_from_table = max_height[_settings_game.difficulty.terrain_type][map_size_bucket];
-
-	/* Tropic needs tropical forest to have all industries, so make sure we allow TGP to generate this high.
-	 * Tropic forest always starts at 1/4th of the max height. */
-	if (_settings_game.game_creation.landscape == LT_TROPIC) {
-		max_height_from_table += CeilDiv(_settings_game.construction.max_heightlevel, 4);
-		/* Make flat a bit more flat by removing "very flat" from it, to somewhat compensate for the increase we just did. */
-		if (_settings_game.difficulty.terrain_type > 0) {
-			max_height_from_table -= max_height[_settings_game.difficulty.terrain_type - 1][map_size_bucket];
-		}
-	}
-
 	return I2H(std::min<uint>(max_height_from_table, _settings_game.construction.max_heightlevel));
 }
 
diff --git a/src/tile_type.h b/src/tile_type.h
index e81d2799a..e9fc272a5 100644
--- a/src/tile_type.h
+++ b/src/tile_type.h
@@ -30,6 +30,7 @@ static const uint DEF_SNOWLINE_HEIGHT = 10;                    ///< Default snow
 static const uint MAX_SNOWLINE_HEIGHT = (MAX_TILE_HEIGHT - 2); ///< Maximum allowed snowline height
 
 static const uint DEF_SNOW_COVERAGE = 40;                      ///< Default snow coverage.
+static const uint DEF_DESERT_COVERAGE = 50;                    ///< Default desert coverage.
 
 
 /**
diff --git a/src/widgets/genworld_widget.h b/src/widgets/genworld_widget.h
index 28e2a17a5..dfc7b1a78 100644
--- a/src/widgets/genworld_widget.h
+++ b/src/widgets/genworld_widget.h
@@ -38,6 +38,10 @@ enum GenerateLandscapeWidgets {
 	WID_GL_SNOW_COVERAGE_TEXT,          ///< Snow coverage.
 	WID_GL_SNOW_COVERAGE_UP,            ///< Increase snow coverage.
 
+	WID_GL_DESERT_COVERAGE_DOWN,        ///< Decrease desert coverage.
+	WID_GL_DESERT_COVERAGE_TEXT,        ///< Desert coverage.
+	WID_GL_DESERT_COVERAGE_UP,          ///< Increase desert coverage.
+
 	WID_GL_LANDSCAPE_PULLDOWN,          ///< Dropdown 'Land generator'.
 
 	WID_GL_HEIGHTMAP_NAME_TEXT,         ///< Heightmap name.
@@ -55,6 +59,9 @@ enum GenerateLandscapeWidgets {
 	WID_GL_WATER_NE,                    ///< NE 'Water'/'Freeform'.
 	WID_GL_WATER_SE,                    ///< SE 'Water'/'Freeform'.
 	WID_GL_WATER_SW,                    ///< SW 'Water'/'Freeform'.
+
+	WID_GL_CLIMATE_SEL_LABEL,           ///< NWID_SELECTION for snow or desert coverage label
+	WID_GL_CLIMATE_SEL_SELECTOR,        ///< NWID_SELECTION for snow or desert coverage selector
 };
 
 /** Widgets of the #CreateScenarioWindow class. */