diff --git a/OpenRA.Game/Graphics/TerrainRenderer.cs b/OpenRA.Game/Graphics/TerrainRenderer.cs
index fe6cb84410..f8562ed359 100644
--- a/OpenRA.Game/Graphics/TerrainRenderer.cs
+++ b/OpenRA.Game/Graphics/TerrainRenderer.cs
@@ -46,9 +46,9 @@ namespace OpenRA.Graphics
 		public void Draw(WorldRenderer wr, Viewport viewport)
 		{
 			var verticesPerRow = 4*map.Bounds.Width;
-			var bounds = viewport.CellBounds;
-			var firstRow = bounds.Top - map.Bounds.Top;
-			var lastRow = bounds.Bottom - map.Bounds.Top;
+			var cells = viewport.VisibleCells;
+			var firstRow = cells.TopLeft.Y - map.Bounds.Top;
+			var lastRow = cells.BottomRight.Y - map.Bounds.Top + 1;
 
 			if (lastRow < 0 || firstRow > map.Bounds.Height)
 				return;
diff --git a/OpenRA.Game/Graphics/Viewport.cs b/OpenRA.Game/Graphics/Viewport.cs
index 4c61e2e18c..540e97f379 100755
--- a/OpenRA.Game/Graphics/Viewport.cs
+++ b/OpenRA.Game/Graphics/Viewport.cs
@@ -9,6 +9,7 @@
 #endregion
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Drawing;
 using System.Linq;
@@ -48,6 +49,9 @@ namespace OpenRA.Graphics
 		int2 viewportSize;
 		bool cellBoundsDirty = true;
 
+		CellRegion cells;
+		bool cellsDirty = true;
+
 		float zoom = 1f;
 		public float Zoom
 		{
@@ -61,6 +65,7 @@ namespace OpenRA.Graphics
 				zoom = value;
 				viewportSize = (1f / zoom * new float2(Game.Renderer.Resolution)).ToInt2();
 				cellBoundsDirty = true;
+				cellsDirty = true;
 			}
 		}
 
@@ -111,6 +116,7 @@ namespace OpenRA.Graphics
 		{
 			CenterLocation = worldRenderer.ScreenPxPosition(pos).Clamp(mapBounds);
 			cellBoundsDirty = true;
+			cellsDirty = true;
 		}
 
 		public void Scroll(float2 delta, bool ignoreBorders)
@@ -118,6 +124,7 @@ namespace OpenRA.Graphics
 			// Convert scroll delta from world-px to viewport-px
 			CenterLocation += (1f / Zoom * delta).ToInt2();
 			cellBoundsDirty = true;
+			cellsDirty = true;
 
 			if (!ignoreBorders)
 				CenterLocation = CenterLocation.Clamp(mapBounds);
@@ -158,5 +165,24 @@ namespace OpenRA.Graphics
 				return b.HasValue ? Rectangle.Intersect(cachedRect, b.Value) : cachedRect;
 			}
 		}
+
+		public CellRegion VisibleCells
+		{
+			get
+			{
+				if (cellsDirty)
+				{
+					// Calculate the intersection of the visible rectangle and the map.
+					var map = worldRenderer.world.Map;
+					var tl = map.Clamp(worldRenderer.Position(TopLeft).ToCPos() - new CVec(1, 1));
+					var br = map.Clamp(worldRenderer.Position(BottomRight).ToCPos());
+
+					cells = new CellRegion(tl, br);
+					cellsDirty = false;
+				}
+
+				return cells;
+			}
+		}
 	}
 }
\ No newline at end of file
diff --git a/OpenRA.Game/Map/Map.cs b/OpenRA.Game/Map/Map.cs
index 01f5685963..4fc1858558 100644
--- a/OpenRA.Game/Map/Map.cs
+++ b/OpenRA.Game/Map/Map.cs
@@ -9,6 +9,7 @@
 #endregion
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Drawing;
 using System.IO;
@@ -118,6 +119,8 @@ namespace OpenRA
 		public Ruleset Rules { get { return rules != null ? rules.Value : null; } }
 		public SequenceProvider SequenceProvider { get { return Rules.Sequences[Tileset]; } }
 
+		[FieldLoader.Ignore] public CellRegion Cells;
+
 		public static Map FromTileset(TileSet tileset)
 		{
 			var tile = tileset.Templates.First();
@@ -229,11 +232,6 @@ namespace OpenRA
 			NotificationDefinitions = MiniYaml.NodesOrEmpty(yaml, "Notifications");
 			TranslationDefinitions = MiniYaml.NodesOrEmpty(yaml, "Translations");
 
-			CustomTerrain = new int[MapSize.X, MapSize.Y];
-			for (var x = 0; x < MapSize.X; x++)
-				for (var y = 0; y < MapSize.Y; y++)
-					CustomTerrain[x, y] = -1;
-
 			MapTiles = Exts.Lazy(() => LoadMapTiles());
 			MapResources = Exts.Lazy(() => LoadResourceTiles());
 
@@ -254,6 +252,14 @@ namespace OpenRA
 		void PostInit()
 		{
 			rules = Exts.Lazy(() => Game.modData.RulesetCache.LoadMapRules(this));
+
+			var tl = new CPos(Bounds.Left, Bounds.Top);
+			var br = new CPos(Bounds.Right - 1, Bounds.Bottom - 1);
+			Cells = new CellRegion(tl, br);
+
+			CustomTerrain = new int[MapSize.X, MapSize.Y];
+			foreach (var cell in Cells)
+				CustomTerrain[cell.X, cell.Y] = -1;
 		}
 
 		public Ruleset PreloadRules()
@@ -455,6 +461,7 @@ namespace OpenRA
 		public void ResizeCordon(int left, int top, int right, int bottom)
 		{
 			Bounds = Rectangle.FromLTRB(left, top, right, bottom);
+			Cells = new CellRegion(new CPos(Bounds.Left, Bounds.Top), new CPos(Bounds.Right - 1, Bounds.Bottom - 1));
 		}
 
 		string ComputeHash()