Allow sequences to define depth offsets via sprites.

OpenRA.Game/Graphics/SpriteRenderable.cs

@@ -52,7 +52,9 @@ namespace OpenRA.Graphics
 		float3 ScreenPosition(WorldRenderer wr)
 		{
 			var xy = wr.ScreenPxPosition(pos) + wr.ScreenPxOffset(offset) - (0.5f * scale * sprite.Size.XY).ToInt2();
-			return new float3(xy, wr.ScreenZPosition(pos, 0) - 0.5f * scale * sprite.Size.Z);
+
+			// HACK: The z offset needs to be applied somewhere, but this probably is the wrong place.
+			return new float3(xy, sprite.Offset.Z + wr.ScreenZPosition(pos, 0) - 0.5f * scale * sprite.Size.Z);
 		}
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }

OpenRA.Mods.Common/Graphics/DefaultSpriteSequence.cs

@@ -161,7 +161,7 @@ namespace OpenRA.Mods.Common.Graphics
 
 						// Allow per-sprite offset, flipping, start, and length
 						var subStart = LoadField(sd, "Start", 0);
-						var subOffset = LoadField(sd, "Offset", float2.Zero);
+						var subOffset = LoadField(sd, "Offset", float3.Zero);
 						var subFlipX = LoadField(sd, "FlipX", false);
 						var subFlipY = LoadField(sd, "FlipY", false);
 
@@ -169,7 +169,7 @@ namespace OpenRA.Mods.Common.Graphics
 						var subSprites = cache[subSrc].Select(
 							s => new Sprite(s.Sheet,
 								FlipRectangle(s.Bounds, subFlipX, subFlipY), ZRamp,
-								new float2(subFlipX ? -s.Offset.X : s.Offset.X, subFlipY ? -s.Offset.Y : s.Offset.Y) + subOffset + offset,
+								new float3(subFlipX ? -s.Offset.X : s.Offset.X, subFlipY ? -s.Offset.Y : s.Offset.Y, s.Offset.Z) + subOffset + offset,
 								s.Channel, blendMode));
 
 						var subLength = 0;
@@ -192,10 +192,47 @@ namespace OpenRA.Mods.Common.Graphics
 					sprites = cache[src].Select(
 						s => new Sprite(s.Sheet,
 							FlipRectangle(s.Bounds, flipX, flipY), ZRamp,
-							new float2(flipX ? -s.Offset.X : s.Offset.X, flipY ? -s.Offset.Y : s.Offset.Y) + offset,
+							new float3(flipX ? -s.Offset.X : s.Offset.X, flipY ? -s.Offset.Y : s.Offset.Y, s.Offset.Z) + offset,
 							s.Channel, blendMode)).ToArray();
 				}
 
+				var depthSprite = LoadField<string>(d, "DepthSprite", null);
+				if (!string.IsNullOrEmpty(depthSprite))
+				{
+					var depthSpriteFrame = LoadField(d, "DepthSpriteFrame", 0);
+					var depthOffset = LoadField(d, "DepthSpriteOffset", float2.Zero);
+					var depthSprites = cache.AllCached(depthSprite)
+						.Select(s => s[depthSpriteFrame]);
+
+					sprites = sprites.Select(s =>
+					{
+						// The depth sprite must live on the same sheet as the main sprite
+						var ds = depthSprites.FirstOrDefault(dss => dss.Sheet == s.Sheet);
+						if (ds == null)
+						{
+							// The sequence has probably overflowed onto a new sheet.
+							// Allocating a new depth sprite on this sheet will almost certainly work
+							ds = cache.Reload(depthSprite)[depthSpriteFrame];
+							depthSprites = cache.AllCached(depthSprite)
+								.Select(ss => ss[depthSpriteFrame]);
+
+							// If that doesn't work then we may be referencing a cached sprite from an earlier sheet
+							// TODO: We could try and reallocate the main sprite, but that requires more complicated code and a perf hit
+							// We'll only cross that bridge if this becomes a problem in reality
+							if (ds.Sheet != s.Sheet)
+								throw new SheetOverflowException("Cross-sheet depth sprite reference: {0}.{1}: {2}");
+						}
+
+						var cw = (ds.Bounds.Left + ds.Bounds.Right) / 2 + (int)(s.Offset.X + depthOffset.X);
+						var ch = (ds.Bounds.Top + ds.Bounds.Bottom) / 2 + (int)(s.Offset.Y + depthOffset.Y);
+						var w = s.Bounds.Width / 2;
+						var h = s.Bounds.Height / 2;
+
+						var r = Rectangle.FromLTRB(cw - w, ch - h, cw + w, ch + h);
+						return new SpriteWithSecondaryData(s, r, ds.Channel);
+					}).ToArray();
+				}
+
 				MiniYaml length;
 				if (d.TryGetValue("Length", out length) && length.Value == "*")
 					Length = sprites.Length - Start;