diff --git a/OpenRA.Game/Graphics/Minimap.cs b/OpenRA.Game/Graphics/Minimap.cs
index f81c6860b4..3db0bd3117 100644
--- a/OpenRA.Game/Graphics/Minimap.cs
+++ b/OpenRA.Game/Graphics/Minimap.cs
@@ -36,17 +36,16 @@ namespace OpenRA.Graphics
 
 			unsafe
 			{
-				var c = (int*)bitmapData.Scan0;
-
-				for (var x = 0; x < b.Width; x++)
+				var colors = (int*)bitmapData.Scan0;
+				var stride = bitmapData.Stride / 4;
+				for (var y = 0; y < b.Height; y++)
 				{
-					for (var y = 0; y < b.Height; y++)
+					for (var x = 0; x < b.Width; x++)
 					{
 						var mapX = x + b.Left;
 						var mapY = y + b.Top;
 						var type = tileset.GetTerrainInfo(mapTiles[mapX, mapY]);
-
-						*(c + (y * bitmapData.Stride >> 2) + x) = type.Color.ToArgb();
+						colors[y * stride + x] = type.Color.ToArgb();
 					}
 				}
 			}
@@ -67,11 +66,11 @@ namespace OpenRA.Graphics
 
 			unsafe
 			{
-				var c = (int*)bitmapData.Scan0;
-
-				for (var x = 0; x < b.Width; x++)
+				var colors = (int*)bitmapData.Scan0;
+				var stride = bitmapData.Stride / 4;
+				for (var y = 0; y < b.Height; y++)
 				{
-					for (var y = 0; y < b.Height; y++)
+					for (var x = 0; x < b.Width; x++)
 					{
 						var mapX = x + b.Left;
 						var mapY = y + b.Top;
@@ -85,7 +84,7 @@ namespace OpenRA.Graphics
 						if (res == null)
 							continue;
 
-						*(c + (y * bitmapData.Stride >> 2) + x) = tileset[tileset.GetTerrainIndex(res)].Color.ToArgb();
+						colors[y * stride + x] = tileset[tileset.GetTerrainIndex(res)].Color.ToArgb();
 					}
 				}
 			}
@@ -107,19 +106,18 @@ namespace OpenRA.Graphics
 
 			unsafe
 			{
-				var c = (int*)bitmapData.Scan0;
-
-				for (var x = 0; x < b.Width; x++)
+				var colors = (int*)bitmapData.Scan0;
+				var stride = bitmapData.Stride / 4;
+				for (var y = 0; y < b.Height; y++)
 				{
-					for (var y = 0; y < b.Height; y++)
+					for (var x = 0; x < b.Width; x++)
 					{
 						var mapX = x + b.Left;
 						var mapY = y + b.Top;
 						var custom = map.CustomTerrain[mapX, mapY];
 						if (custom == -1)
 							continue;
-
-						*(c + (y * bitmapData.Stride >> 2) + x) = world.TileSet[custom].Color.ToArgb();
+						colors[y * stride + x] = world.TileSet[custom].Color.ToArgb();
 					}
 				}
 			}
@@ -140,8 +138,8 @@ namespace OpenRA.Graphics
 
 			unsafe
 			{
-				var c = (int*)bitmapData.Scan0;
-
+				var colors = (int*)bitmapData.Scan0;
+				var stride = bitmapData.Stride / 4;
 				foreach (var t in world.ActorsWithTrait<IRadarSignature>())
 				{
 					if (world.FogObscures(t.Actor))
@@ -152,7 +150,7 @@ namespace OpenRA.Graphics
 					{
 						var uv = Map.CellToMap(map.TileShape, cell);
 						if (b.Contains(uv.X, uv.Y))
-							*(c + ((uv.Y - b.Top) * bitmapData.Stride >> 2) + uv.X - b.Left) = color.ToArgb();
+							colors[(uv.Y - b.Top) * stride + uv.X - b.Left] = color.ToArgb();
 					}
 				}
 			}
@@ -180,15 +178,17 @@ namespace OpenRA.Graphics
 
 			unsafe
 			{
-				var c = (int*)bitmapData.Scan0;
-
+				var colors = (int*)bitmapData.Scan0;
+				var stride = bitmapData.Stride / 4;
+				var shroudObscured = world.ShroudObscuresTest(map.Cells);
+				var fogObscured = world.FogObscuresTest(map.Cells);
 				foreach (var cell in map.Cells)
 				{
 					var uv = Map.CellToMap(map.TileShape, cell) - offset;
-					if (world.ShroudObscures(cell))
-						*(c + (uv.Y * bitmapData.Stride >> 2) + uv.X) = shroud;
-					else if (world.FogObscures(cell))
-						*(c + (uv.Y * bitmapData.Stride >> 2) + uv.X) = fog;
+					if (shroudObscured(cell))
+						colors[uv.Y * stride + uv.X] = shroud;
+					else if (fogObscured(cell))
+						colors[uv.Y * stride + uv.X] = fog;
 				}
 			}
 
diff --git a/OpenRA.Game/Map/CellLayer.cs b/OpenRA.Game/Map/CellLayer.cs
index 3c0f444840..9b18333442 100644
--- a/OpenRA.Game/Map/CellLayer.cs
+++ b/OpenRA.Game/Map/CellLayer.cs
@@ -12,7 +12,6 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Drawing;
-using OpenRA.Graphics;
 
 namespace OpenRA
 {
@@ -21,7 +20,7 @@ namespace OpenRA
 	{
 		public readonly Size Size;
 		public readonly TileShape Shape;
-		T[] entries;
+		readonly T[] entries;
 
 		public CellLayer(Map map)
 			: this(map.TileShape, new Size(map.MapSize.X, map.MapSize.Y)) { }
@@ -37,35 +36,27 @@ namespace OpenRA
 		int Index(CPos cell)
 		{
 			var uv = Map.CellToMap(Shape, cell);
-			return uv.Y * Size.Width + uv.X;
+			return Index(uv.X, uv.Y);
+		}
+
+		// Resolve an array index from map coordinates
+		int Index(int u, int v)
+		{
+			return v * Size.Width + u;
 		}
 
 		/// <summary>Gets or sets the <see cref="OpenRA.CellLayer"/> using cell coordinates</summary>
 		public T this[CPos cell]
 		{
-			get
-			{
-				return entries[Index(cell)];
-			}
-
-			set
-			{
-				entries[Index(cell)] = value;
-			}
+			get { return entries[Index(cell)]; }
+			set { entries[Index(cell)] = value; }
 		}
 
 		/// <summary>Gets or sets the layer contents using raw map coordinates (not CPos!)</summary>
 		public T this[int u, int v]
 		{
-			get
-			{
-				return entries[v * Size.Width + u];
-			}
-
-			set
-			{
-				entries[v * Size.Width + u] = value;
-			}
+			get { return entries[Index(u, v)]; }
+			set { entries[Index(u, v)] = value; }
 		}
 
 		/// <summary>Clears the layer contents with a known value</summary>
diff --git a/OpenRA.Game/Map/CellRegion.cs b/OpenRA.Game/Map/CellRegion.cs
index ce7316ecd1..ef7fc6debd 100644
--- a/OpenRA.Game/Map/CellRegion.cs
+++ b/OpenRA.Game/Map/CellRegion.cs
@@ -11,8 +11,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
-using System.Drawing;
-using OpenRA.Graphics;
+using System.Linq;
 
 namespace OpenRA
 {
@@ -50,29 +49,69 @@ namespace OpenRA
 			return new CellRegion(region.shape, tl, br);
 		}
 
+		/// <summary>Returns the minimal region that covers at least the specified cells.</summary>
+		public static CellRegion BoundingRegion(TileShape shape, IEnumerable<CPos> cells)
+		{
+			if (cells == null || !cells.Any())
+				throw new ArgumentException("cells must not be null or empty.", "cells");
+
+			var minX = int.MaxValue;
+			var minY = int.MaxValue;
+			var maxX = int.MinValue;
+			var maxY = int.MinValue;
+			foreach (var cell in cells)
+			{
+				if (minX > cell.X)
+					minX = cell.X;
+				if (maxX < cell.X)
+					maxX = cell.X;
+				if (minY > cell.Y)
+					minY = cell.Y;
+				if (maxY < cell.Y)
+					maxY = cell.Y;
+			}
+
+			return new CellRegion(shape, new CPos(minX, minY), new CPos(maxX, maxY));
+		}
+
+		public bool Contains(CellRegion region)
+		{
+			return
+				TopLeft.X <= region.TopLeft.X && TopLeft.Y <= region.TopLeft.Y &&
+				BottomRight.X >= region.BottomRight.X && BottomRight.Y >= region.BottomRight.Y;
+		}
+
 		public bool Contains(CPos cell)
 		{
 			var uv = Map.CellToMap(shape, cell);
 			return uv.X >= mapTopLeft.X && uv.X <= mapBottomRight.X && uv.Y >= mapTopLeft.Y && uv.Y <= mapBottomRight.Y;
 		}
 
-		public IEnumerator<CPos> GetEnumerator()
+		public CellRegionEnumerator GetEnumerator()
 		{
 			return new CellRegionEnumerator(this);
 		}
 
+		IEnumerator<CPos> IEnumerable<CPos>.GetEnumerator()
+		{
+			return GetEnumerator();
+		}
+
 		IEnumerator IEnumerable.GetEnumerator()
 		{
 			return GetEnumerator();
 		}
 
-		class CellRegionEnumerator : IEnumerator<CPos>
+		public class CellRegionEnumerator : IEnumerator<CPos>
 		{
 			readonly CellRegion r;
 
 			// Current position, in map coordinates
 			int u, v;
 
+			// Current position, in cell coordinates
+			CPos current;
+
 			public CellRegionEnumerator(CellRegion region)
 			{
 				r = region;
@@ -94,6 +133,7 @@ namespace OpenRA
 						return false;
 				}
 
+				current = Map.MapToCell(r.shape, new CPos(u, v));
 				return true;
 			}
 
@@ -104,7 +144,7 @@ namespace OpenRA
 				v = r.mapTopLeft.Y;
 			}
 
-			public CPos Current { get { return Map.MapToCell(r.shape, new CPos(u, v)); } }
+			public CPos Current { get { return current; } }
 			object IEnumerator.Current { get { return Current; } }
 			public void Dispose() { }
 		}
diff --git a/OpenRA.Game/Traits/Player/FrozenActorLayer.cs b/OpenRA.Game/Traits/Player/FrozenActorLayer.cs
index dea1db7658..544bc3ea43 100755
--- a/OpenRA.Game/Traits/Player/FrozenActorLayer.cs
+++ b/OpenRA.Game/Traits/Player/FrozenActorLayer.cs
@@ -24,11 +24,12 @@ namespace OpenRA.Traits
 	public class FrozenActor
 	{
 		public readonly CPos[] Footprint;
+		public readonly CellRegion FootprintRegion;
 		public readonly WPos CenterPosition;
 		public readonly Rectangle Bounds;
 		readonly Actor actor;
 
-		public IRenderable[] Renderables { set; private get; }
+		public IRenderable[] Renderables { private get; set; }
 		public Player Owner;
 
 		public string TooltipName;
@@ -39,10 +40,12 @@ namespace OpenRA.Traits
 
 		public bool Visible;
 
-		public FrozenActor(Actor self, IEnumerable<CPos> footprint)
+		public FrozenActor(Actor self, CPos[] footprint, CellRegion footprintRegion)
 		{
 			actor = self;
-			Footprint = footprint.ToArray();
+			Footprint = footprint;
+			FootprintRegion = footprintRegion;
+
 			CenterPosition = self.CenterPosition;
 			Bounds = self.Bounds.Value;
 		}
@@ -55,16 +58,7 @@ namespace OpenRA.Traits
 		int flashTicks;
 		public void Tick(World world, Shroud shroud)
 		{
-			Visible = true;
-			foreach (var pos in Footprint)
-			{
-				if (shroud.IsVisible(pos))
-				{
-					Visible = false;
-					break;
-				}
-			}
-
+			Visible = !Footprint.Any(shroud.IsVisibleTest(FootprintRegion));
 			if (flashTicks > 0)
 				flashTicks--;
 		}
@@ -85,6 +79,7 @@ namespace OpenRA.Traits
 				return Renderables.Concat(Renderables.Where(r => !r.IsDecoration)
 					.Select(r => r.WithPalette(highlight)));
 			}
+
 			return Renderables;
 		}
 
diff --git a/OpenRA.Game/Traits/World/Shroud.cs b/OpenRA.Game/Traits/World/Shroud.cs
index 0928f0e159..6e51f639f4 100644
--- a/OpenRA.Game/Traits/World/Shroud.cs
+++ b/OpenRA.Game/Traits/World/Shroud.cs
@@ -10,7 +10,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Drawing;
 using System.Linq;
 
 namespace OpenRA.Traits
@@ -23,31 +22,37 @@ namespace OpenRA.Traits
 
 	public class Shroud
 	{
-		[Sync] public bool Disabled = false;
+		[Sync] public bool Disabled;
 
-		Actor self;
-		Map map;
+		readonly Actor self;
+		readonly Map map;
 
-		CellLayer<int> visibleCount;
-		CellLayer<int> generatedShroudCount;
-		CellLayer<bool> explored;
+		readonly CellLayer<short> visibleCount;
+		readonly CellLayer<short> generatedShroudCount;
+		readonly CellLayer<bool> explored;
 
 		readonly Lazy<IFogVisibilityModifier[]> fogVisibilities;
 
 		// Cache of visibility that was added, so no matter what crazy trait code does, it
 		// can't make us invalid.
-		Dictionary<Actor, CPos[]> visibility = new Dictionary<Actor, CPos[]>();
-		Dictionary<Actor, CPos[]> generation = new Dictionary<Actor, CPos[]>();
+		readonly Dictionary<Actor, CPos[]> visibility = new Dictionary<Actor, CPos[]>();
+		readonly Dictionary<Actor, CPos[]> generation = new Dictionary<Actor, CPos[]>();
 
 		public int Hash { get; private set; }
 
+		static readonly Func<CPos, bool> TruthPredicate = cell => true;
+		readonly Func<CPos, bool> fastExploredTest;
+		readonly Func<CPos, bool> slowExploredTest;
+		readonly Func<CPos, bool> fastVisibleTest;
+		readonly Func<CPos, bool> slowVisibleTest;
+
 		public Shroud(Actor self)
 		{
 			this.self = self;
 			map = self.World.Map;
 
-			visibleCount = new CellLayer<int>(map);
-			generatedShroudCount = new CellLayer<int>(map);
+			visibleCount = new CellLayer<short>(map);
+			generatedShroudCount = new CellLayer<short>(map);
 			explored = new CellLayer<bool>(map);
 
 			self.World.ActorAdded += AddVisibility;
@@ -57,6 +62,11 @@ namespace OpenRA.Traits
 			self.World.ActorRemoved += RemoveShroudGeneration;
 
 			fogVisibilities = Exts.Lazy(() => self.TraitsImplementing<IFogVisibilityModifier>().ToArray());
+
+			fastExploredTest = IsExploredCore;
+			slowExploredTest = IsExplored;
+			fastVisibleTest = IsVisibleCore;
+			slowVisibleTest = IsVisible;
 		}
 
 		void Invalidate()
@@ -187,14 +197,17 @@ namespace OpenRA.Traits
 
 		public void Explore(World world, CPos center, WRange range)
 		{
-			foreach (var q in FindVisibleTiles(world, center, range))
-				explored[q] = true;
+			foreach (var c in FindVisibleTiles(world, center, range))
+				explored[c] = true;
 
 			Invalidate();
 		}
 
 		public void Explore(Shroud s)
 		{
+			if (map.Bounds != s.map.Bounds)
+				throw new ArgumentException("The map bounds of these shrouds do not match.", "s");
+
 			foreach (var cell in map.Cells)
 				if (s.explored[cell])
 					explored[cell] = true;
@@ -222,10 +235,32 @@ namespace OpenRA.Traits
 			if (!map.Contains(cell))
 				return false;
 
-			if (Disabled || !self.World.LobbyInfo.GlobalSettings.Shroud)
+			if (!ShroudEnabled)
 				return true;
 
-			return explored[cell] && (generatedShroudCount[cell] == 0 || visibleCount[cell] > 0);
+			return IsExploredCore(cell);
+		}
+
+		bool ShroudEnabled { get { return !Disabled && self.World.LobbyInfo.GlobalSettings.Shroud; } }
+
+		bool IsExploredCore(CPos cell)
+		{
+			var uv = Map.CellToMap(map.TileShape, cell);
+			return explored[uv.X, uv.Y] && (generatedShroudCount[uv.X, uv.Y] == 0 || visibleCount[uv.X, uv.Y] > 0);
+		}
+
+		public Func<CPos, bool> IsExploredTest(CellRegion region)
+		{
+			// If the region to test extends outside the map we must use the slow test that checks the map boundary every time.
+			if (!map.Cells.Contains(region))
+				return slowExploredTest;
+
+			// If shroud isn't enabled, then we can see everything.
+			if (!ShroudEnabled)
+				return TruthPredicate;
+
+			// If shroud is enabled, we can use the fast test that just does the core check.
+			return fastExploredTest;
 		}
 
 		public bool IsExplored(Actor a)
@@ -238,12 +273,33 @@ namespace OpenRA.Traits
 			if (!map.Contains(cell))
 				return false;
 
-			if (Disabled || !self.World.LobbyInfo.GlobalSettings.Fog)
+			if (!FogEnabled)
 				return true;
 
+			return IsVisibleCore(cell);
+		}
+
+		bool FogEnabled { get { return !Disabled && self.World.LobbyInfo.GlobalSettings.Fog; } }
+
+		bool IsVisibleCore(CPos cell)
+		{
 			return visibleCount[cell] > 0;
 		}
 
+		public Func<CPos, bool> IsVisibleTest(CellRegion region)
+		{
+			// If the region to test extends outside the map we must use the slow test that checks the map boundary every time.
+			if (!map.Cells.Contains(region))
+				return slowVisibleTest;
+
+			// If fog isn't enabled, then we can see everything.
+			if (!FogEnabled)
+				return TruthPredicate;
+
+			// If fog is enabled, we can use the fast test that just does the core check.
+			return fastVisibleTest;
+		}
+
 		// Actors are hidden under shroud, but not under fog by default
 		public bool IsVisible(Actor a)
 		{
diff --git a/OpenRA.Game/World.cs b/OpenRA.Game/World.cs
index 6d1aa45475..50b04194e5 100644
--- a/OpenRA.Game/World.cs
+++ b/OpenRA.Game/World.cs
@@ -10,7 +10,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Drawing;
 using System.Linq;
 using OpenRA.Effects;
 using OpenRA.FileFormats;
@@ -25,6 +24,7 @@ namespace OpenRA
 {
 	public class World
 	{
+		static readonly Func<CPos, bool> FalsePredicate = cell => false;
 		internal readonly TraitDictionary traitDict = new TraitDictionary();
 		readonly HashSet<Actor> actors = new HashSet<Actor>();
 		readonly List<IEffect> effects = new List<IEffect>();
@@ -54,6 +54,24 @@ namespace OpenRA
 		public bool FogObscures(CPos p) { return RenderPlayer != null && !RenderPlayer.Shroud.IsVisible(p); }
 		public bool ShroudObscures(Actor a) { return RenderPlayer != null && !RenderPlayer.Shroud.IsExplored(a); }
 		public bool ShroudObscures(CPos p) { return RenderPlayer != null && !RenderPlayer.Shroud.IsExplored(p); }
+		
+		public Func<CPos, bool> FogObscuresTest(CellRegion region)
+		{
+			var rp = RenderPlayer;
+			if (rp == null)
+				return FalsePredicate;
+			var predicate = rp.Shroud.IsVisibleTest(region);
+			return cell => !predicate(cell);
+		}
+
+		public Func<CPos, bool> ShroudObscuresTest(CellRegion region)
+		{
+			var rp = RenderPlayer;
+			if (rp == null)
+				return FalsePredicate;
+			var predicate = rp.Shroud.IsExploredTest(region);
+			return cell => !predicate(cell);
+		}
 
 		public bool IsReplay
 		{
diff --git a/OpenRA.Mods.RA/Modifiers/FrozenUnderFog.cs b/OpenRA.Mods.RA/Modifiers/FrozenUnderFog.cs
index 5ee0ab5384..2baaf8ae8e 100644
--- a/OpenRA.Mods.RA/Modifiers/FrozenUnderFog.cs
+++ b/OpenRA.Mods.RA/Modifiers/FrozenUnderFog.cs
@@ -29,19 +29,24 @@ namespace OpenRA.Mods.RA
 	{
 		[Sync] public int VisibilityHash;
 
-		bool initialized, startsRevealed;
+		readonly bool startsRevealed;
 		readonly CPos[] footprint;
-		Lazy<IToolTip> tooltip;
-		Lazy<Health> health;
+		readonly CellRegion footprintRegion;
 
-		Dictionary<Player, bool> visible;
-		Dictionary<Player, FrozenActor> frozen;
+		readonly Lazy<IToolTip> tooltip;
+		readonly Lazy<Health> health;
+
+		readonly Dictionary<Player, bool> visible;
+		readonly Dictionary<Player, FrozenActor> frozen;
+
+		bool initialized;
 
 		public FrozenUnderFog(ActorInitializer init, FrozenUnderFogInfo info)
 		{
 			// Spawned actors (e.g. building husks) shouldn't be revealed
 			startsRevealed = info.StartsRevealed && !init.Contains<ParentActorInit>();
 			footprint = FootprintUtils.Tiles(init.self).ToArray();
+			footprintRegion = CellRegion.BoundingRegion(init.world.Map.TileShape, footprint);
 			tooltip = Exts.Lazy(() => init.self.TraitsImplementing<IToolTip>().FirstOrDefault());
 			tooltip = Exts.Lazy(() => init.self.TraitsImplementing<IToolTip>().FirstOrDefault());
 			health = Exts.Lazy(() => init.self.TraitOrDefault<Health>());
@@ -63,15 +68,7 @@ namespace OpenRA.Mods.RA
 			VisibilityHash = 0;
 			foreach (var p in self.World.Players)
 			{
-				var isVisible = false;
-				foreach (var pos in footprint)
-				{
-					if (p.Shroud.IsVisible(pos))
-					{
-						isVisible = true;
-						break;
-					}
-				}
+				var isVisible = footprint.Any(p.Shroud.IsVisibleTest(footprintRegion));
 				visible[p] = isVisible;
 				if (isVisible)
 					VisibilityHash += p.ClientIndex;
@@ -82,7 +79,7 @@ namespace OpenRA.Mods.RA
 				foreach (var p in self.World.Players)
 				{
 					visible[p] |= startsRevealed;
-					p.PlayerActor.Trait<FrozenActorLayer>().Add(frozen[p] = new FrozenActor(self, footprint));
+					p.PlayerActor.Trait<FrozenActorLayer>().Add(frozen[p] = new FrozenActor(self, footprint, footprintRegion));
 				}
 
 				initialized = true;
diff --git a/OpenRA.Mods.RA/ShroudRenderer.cs b/OpenRA.Mods.RA/ShroudRenderer.cs
index 10a44b1fc5..3a726b1c1b 100644
--- a/OpenRA.Mods.RA/ShroudRenderer.cs
+++ b/OpenRA.Mods.RA/ShroudRenderer.cs
@@ -9,9 +9,6 @@
 #endregion
 
 using System;
-using System.Drawing;
-using System.Linq;
-using OpenRA;
 using OpenRA.Graphics;
 using OpenRA.Traits;
 
@@ -47,52 +44,81 @@ namespace OpenRA.Mods.RA
 
 	public class ShroudRenderer : IRenderShroud, IWorldLoaded
 	{
-		class ShroudTile
+		[Flags]
+		enum Edges : byte
+		{
+			None = 0,
+			TopLeft = 0x01,
+			TopRight = 0x02,
+			BottomRight = 0x04,
+			BottomLeft = 0x08,
+			AllCorners = TopLeft | TopRight | BottomRight | BottomLeft,
+			TopSide = 0x10,
+			RightSide = 0x20,
+			BottomSide = 0x40,
+			LeftSide = 0x80,
+			AllSides = TopSide | RightSide | BottomSide | LeftSide,
+			Top = TopSide | TopLeft | TopRight,
+			Right = RightSide | TopRight | BottomRight,
+			Bottom = BottomSide | BottomRight | BottomLeft,
+			Left = LeftSide | TopLeft | BottomLeft,
+			All = Top | Right | Bottom | Left
+		}
+
+		struct ShroudTile
 		{
-			public readonly CPos Position;
 			public readonly float2 ScreenPosition;
-			public readonly int Variant;
+			public readonly byte Variant;
 
 			public Sprite Fog;
 			public Sprite Shroud;
 
-			public ShroudTile(CPos position, float2 screenPosition, int variant)
+			public ShroudTile(float2 screenPosition, byte variant)
 			{
-				Position = position;
 				ScreenPosition = screenPosition;
 				Variant = variant;
+
+				Fog = null;
+				Shroud = null;
 			}
 		}
 
 		readonly ShroudRendererInfo info;
 		readonly Sprite[] shroudSprites, fogSprites;
-		readonly int[] spriteMap;
+		readonly byte[] spriteMap;
 		readonly CellLayer<ShroudTile> tiles;
-		readonly int variantStride;
+		readonly byte variantStride;
 		readonly Map map;
+		readonly Edges notVisibleEdges;
+
+		bool clearedForNullShroud;
+		int lastShroudHash;
+		CellRegion updatedRegion;
 
 		PaletteReference fogPalette, shroudPalette;
-		int shroudHash;
 
 		public ShroudRenderer(World world, ShroudRendererInfo info)
 		{
+			if (info.ShroudVariants.Length != info.FogVariants.Length)
+				throw new ArgumentException("ShroudRenderer must define the same number of shroud and fog variants!", "info");
+
+			if ((info.OverrideFullFog == null) ^ (info.OverrideFullShroud == null))
+				throw new ArgumentException("ShroudRenderer cannot define overrides for only one of shroud or fog!", "info");
+
+			if (info.ShroudVariants.Length > byte.MaxValue)
+				throw new ArgumentException("ShroudRenderer cannot define this many shroud and fog variants.", "info");
+
+			if (info.Index.Length >= byte.MaxValue)
+				throw new ArgumentException("ShroudRenderer cannot define this many indexes for shroud directions.", "info");
+
 			this.info = info;
 			map = world.Map;
 
 			tiles = new CellLayer<ShroudTile>(map);
 
-			// Force update on first render
-			shroudHash = -1;
-
 			// Load sprite variants
-			if (info.ShroudVariants.Length != info.FogVariants.Length)
-				throw new InvalidOperationException("ShroudRenderer must define the same number of shroud and fog variants!");
-
-			if ((info.OverrideFullFog == null) ^ (info.OverrideFullShroud == null))
-				throw new InvalidOperationException("ShroudRenderer cannot define overrides for only one of shroud or fog!");
-
 			var variantCount = info.ShroudVariants.Length;
-			variantStride = info.Index.Length + (info.OverrideFullShroud != null ? 1 : 0);
+			variantStride = (byte)(info.Index.Length + (info.OverrideFullShroud != null ? 1 : 0));
 			shroudSprites = new Sprite[variantCount * variantStride];
 			fogSprites = new Sprite[variantCount * variantStride];
 
@@ -115,31 +141,33 @@ namespace OpenRA.Mods.RA
 			}
 
 			// Mapping of shrouded directions -> sprite index
-			spriteMap = new int[info.UseExtendedIndex ? 256 : 16];
+			spriteMap = new byte[(byte)(info.UseExtendedIndex ? Edges.All : Edges.AllCorners) + 1];
 			for (var i = 0; i < info.Index.Length; i++)
-				spriteMap[info.Index[i]] = i;
+				spriteMap[info.Index[i]] = (byte)i;
 
 			if (info.OverrideFullShroud != null)
-				spriteMap[info.OverrideShroudIndex] = variantStride - 1;
+				spriteMap[info.OverrideShroudIndex] = (byte)(variantStride - 1);
+
+			notVisibleEdges = info.UseExtendedIndex ? Edges.AllSides : Edges.AllCorners;
 		}
 
-		static int FoggedEdges(Shroud s, CPos p, bool useExtendedIndex)
+		Edges GetEdges(CPos p, Func<CPos, bool> isVisible)
 		{
-			if (!s.IsVisible(p))
-				return useExtendedIndex ? 240 : 15;
+			if (!isVisible(p))
+				return notVisibleEdges;
 
 			// If a side is shrouded then we also count the corners
-			var u = 0;
-			if (!s.IsVisible(p + new CVec(0, -1))) u |= 0x13;
-			if (!s.IsVisible(p + new CVec(1, 0))) u |= 0x26;
-			if (!s.IsVisible(p + new CVec(0, 1))) u |= 0x4C;
-			if (!s.IsVisible(p + new CVec(-1, 0))) u |= 0x89;
+			var u = Edges.None;
+			if (!isVisible(p + new CVec(0, -1))) u |= Edges.Top;
+			if (!isVisible(p + new CVec(1, 0))) u |= Edges.Right;
+			if (!isVisible(p + new CVec(0, 1))) u |= Edges.Bottom;
+			if (!isVisible(p + new CVec(-1, 0))) u |= Edges.Left;
 
-			var uside = u & 0x0F;
-			if (!s.IsVisible(p + new CVec(-1, -1))) u |= 0x01;
-			if (!s.IsVisible(p + new CVec(1, -1))) u |= 0x02;
-			if (!s.IsVisible(p + new CVec(1, 1))) u |= 0x04;
-			if (!s.IsVisible(p + new CVec(-1, 1))) u |= 0x08;
+			var ucorner = u & Edges.AllCorners;
+			if (!isVisible(p + new CVec(-1, -1))) u |= Edges.TopLeft;
+			if (!isVisible(p + new CVec(1, -1))) u |= Edges.TopRight;
+			if (!isVisible(p + new CVec(1, 1))) u |= Edges.BottomRight;
+			if (!isVisible(p + new CVec(-1, 1))) u |= Edges.BottomLeft;
 
 			// RA provides a set of frames for tiles with shrouded
 			// corners but unshrouded edges. We want to detect this
@@ -147,51 +175,24 @@ namespace OpenRA.Mods.RA
 			// in other combinations. The XOR turns off the corner
 			// bits that are enabled twice, which gives the behavior
 			// we want here.
-			return useExtendedIndex ? u ^ uside : u & 0x0F;
+			return info.UseExtendedIndex ? u ^ ucorner : u & Edges.AllCorners;
 		}
 
-		static int ShroudedEdges(Shroud s, CPos p, bool useExtendedIndex)
+		Edges GetObserverEdges(CPos p)
 		{
-			if (!s.IsExplored(p))
-				return useExtendedIndex ? 240 : 15;
+			var u = Edges.None;
+			if (!map.Contains(p + new CVec(0, -1))) u |= Edges.Top;
+			if (!map.Contains(p + new CVec(1, 0))) u |= Edges.Right;
+			if (!map.Contains(p + new CVec(0, 1))) u |= Edges.Bottom;
+			if (!map.Contains(p + new CVec(-1, 0))) u |= Edges.Left;
 
-			// If a side is shrouded then we also count the corners
-			var u = 0;
-			if (!s.IsExplored(p + new CVec(0, -1))) u |= 0x13;
-			if (!s.IsExplored(p + new CVec(1, 0))) u |= 0x26;
-			if (!s.IsExplored(p + new CVec(0, 1))) u |= 0x4C;
-			if (!s.IsExplored(p + new CVec(-1, 0))) u |= 0x89;
+			var ucorner = u & Edges.AllCorners;
+			if (!map.Contains(p + new CVec(-1, -1))) u |= Edges.TopLeft;
+			if (!map.Contains(p + new CVec(1, -1))) u |= Edges.TopRight;
+			if (!map.Contains(p + new CVec(1, 1))) u |= Edges.BottomRight;
+			if (!map.Contains(p + new CVec(-1, 1))) u |= Edges.BottomLeft;
 
-			var uside = u & 0x0F;
-			if (!s.IsExplored(p + new CVec(-1, -1))) u |= 0x01;
-			if (!s.IsExplored(p + new CVec(1, -1))) u |= 0x02;
-			if (!s.IsExplored(p + new CVec(1, 1))) u |= 0x04;
-			if (!s.IsExplored(p + new CVec(-1, 1))) u |= 0x08;
-
-			// RA provides a set of frames for tiles with shrouded
-			// corners but unshrouded edges. We want to detect this
-			// situation without breaking the edge -> corner enabling
-			// in other combinations. The XOR turns off the corner
-			// bits that are enabled twice, which gives the behavior
-			// we want here.
-			return useExtendedIndex ? u ^ uside : u & 0x0F;
-		}
-
-		static int ObserverShroudedEdges(Map map, CPos p, bool useExtendedIndex)
-		{
-			var u = 0;
-			if (!map.Contains(p + new CVec(0, -1))) u |= 0x13;
-			if (!map.Contains(p + new CVec(1, 0))) u |= 0x26;
-			if (!map.Contains(p + new CVec(0, 1))) u |= 0x4C;
-			if (!map.Contains(p + new CVec(-1, 0))) u |= 0x89;
-
-			var uside = u & 0x0F;
-			if (!map.Contains(p + new CVec(-1, -1))) u |= 0x01;
-			if (!map.Contains(p + new CVec(1, -1))) u |= 0x02;
-			if (!map.Contains(p + new CVec(1, 1))) u |= 0x04;
-			if (!map.Contains(p + new CVec(-1, 1))) u |= 0x08;
-
-			return useExtendedIndex ? u ^ uside : u & 0x0F;
+			return info.UseExtendedIndex ? u ^ ucorner : u & Edges.AllCorners;
 		}
 
 		public void WorldLoaded(World w, WorldRenderer wr)
@@ -201,14 +202,15 @@ namespace OpenRA.Mods.RA
 			foreach (var cell in CellRegion.Expand(w.Map.Cells, 1))
 			{
 				var screen = wr.ScreenPosition(w.Map.CenterOfCell(cell));
-				var variant = Game.CosmeticRandom.Next(info.ShroudVariants.Length);
-				tiles[cell] = new ShroudTile(cell, screen, variant);
+				var variant = (byte)Game.CosmeticRandom.Next(info.ShroudVariants.Length);
+				tiles[cell] = new ShroudTile(screen, variant);
 
 				// Set the cells outside the border so they don't need to be touched again
 				if (!map.Contains(cell))
 				{
-					var index = info.UseExtendedIndex ? 240 : 15;
-					tiles[cell].Shroud = shroudSprites[variant * variantStride + spriteMap[index]];
+					var shroudTile = tiles[cell];
+					shroudTile.Shroud = GetTile(shroudSprites, notVisibleEdges, variant);
+					tiles[cell] = shroudTile;
 				}
 			}
 
@@ -216,50 +218,68 @@ namespace OpenRA.Mods.RA
 			shroudPalette = wr.Palette(info.ShroudPalette);
 		}
 
-		Sprite GetTile(int flags, int variant)
+		Sprite GetTile(Sprite[] sprites, Edges edges, int variant)
 		{
-			if (flags == 0)
+			if (edges == Edges.None)
 				return null;
 
-			return shroudSprites[variant * variantStride + spriteMap[flags]];
+			return sprites[variant * variantStride + spriteMap[(byte)edges]];
 		}
 
-		void Update(Shroud shroud)
+		void Update(Shroud shroud, CellRegion region)
 		{
-			var hash = shroud != null ? shroud.Hash : 0;
-			if (shroudHash == hash)
-				return;
-
-			shroudHash = hash;
-			if (shroud == null)
+			if (shroud != null)
 			{
-				// Players with no shroud see the whole map so we only need to set the edges
-				foreach (var cell in map.Cells)
-				{
-					var t = tiles[cell];
-					var shrouded = ObserverShroudedEdges(map, t.Position, info.UseExtendedIndex);
+				// If the current shroud hasn't changed and we have already updated the specified area, we don't need to do anything.
+				if (lastShroudHash == shroud.Hash && !clearedForNullShroud && updatedRegion.Contains(region))
+					return;
 
-					t.Shroud = shrouded != 0 ? shroudSprites[t.Variant * variantStride + spriteMap[shrouded]] : null;
-					t.Fog = shrouded != 0 ? fogSprites[t.Variant * variantStride + spriteMap[shrouded]] : null;
-				}
+				lastShroudHash = shroud.Hash;
+				clearedForNullShroud = false;
+				updatedRegion = region;
+				UpdateShroud(shroud);
 			}
-			else
+			else if (!clearedForNullShroud)
 			{
-				foreach (var cell in map.Cells)
-				{
-					var t = tiles[cell];
-					var shrouded = ShroudedEdges(shroud, t.Position, info.UseExtendedIndex);
-					var fogged = FoggedEdges(shroud, t.Position, info.UseExtendedIndex);
+				// We need to clear any applied shroud.
+				clearedForNullShroud = true;
+				updatedRegion = new CellRegion(map.TileShape, new CPos(0, 0), new CPos(-1, -1));
+				UpdateNullShroud();
+			}
+		}
 
-					t.Shroud = shrouded != 0 ? shroudSprites[t.Variant * variantStride + spriteMap[shrouded]] : null;
-					t.Fog = fogged != 0 ? fogSprites[t.Variant * variantStride + spriteMap[fogged]] : null;
-				}
+		void UpdateShroud(Shroud shroud)
+		{
+			var visibleUnderShroud = shroud.IsExploredTest(updatedRegion);
+			var visibleUnderFog = shroud.IsVisibleTest(updatedRegion);
+			foreach (var cell in updatedRegion)
+			{
+				var shrouded = GetEdges(cell, visibleUnderShroud);
+				var fogged = GetEdges(cell, visibleUnderFog);
+				var shroudTile = tiles[cell];
+				var variant = shroudTile.Variant;
+				shroudTile.Shroud = GetTile(shroudSprites, shrouded, variant);
+				shroudTile.Fog = GetTile(fogSprites, fogged, variant);
+				tiles[cell] = shroudTile;
+			}
+		}
+
+		void UpdateNullShroud()
+		{
+			foreach (var cell in map.Cells)
+			{
+				var edges = GetObserverEdges(cell);
+				var shroudTile = tiles[cell];
+				var variant = shroudTile.Variant;
+				shroudTile.Shroud = GetTile(shroudSprites, edges, variant);
+				shroudTile.Fog = GetTile(fogSprites, edges, variant);
+				tiles[cell] = shroudTile;
 			}
 		}
 
 		public void RenderShroud(WorldRenderer wr, Shroud shroud)
 		{
-			Update(shroud);
+			Update(shroud, wr.Viewport.VisibleCells);
 
 			foreach (var cell in CellRegion.Expand(wr.Viewport.VisibleCells, 1))
 			{
diff --git a/OpenRA.Mods.RA/World/ResourceLayer.cs b/OpenRA.Mods.RA/World/ResourceLayer.cs
index 02eac34b10..b7bf86fbe5 100644
--- a/OpenRA.Mods.RA/World/ResourceLayer.cs
+++ b/OpenRA.Mods.RA/World/ResourceLayer.cs
@@ -34,9 +34,10 @@ namespace OpenRA.Mods.RA
 
 		public void Render(WorldRenderer wr)
 		{
+			var shroudObscured = world.ShroudObscuresTest(wr.Viewport.VisibleCells);
 			foreach (var cell in wr.Viewport.VisibleCells)
 			{
-				if (world.ShroudObscures(cell))
+				if (shroudObscured(cell))
 					continue;
 
 				var c = render[cell];