Find "ideal way" to split tile into triangles

terrain/Terrain.cs

@@ -143,17 +143,33 @@ public partial class Terrain
 			AddPoint(v3, uv3);
 		}
 
-		for (var x = 0; x < Size.X; x++) {
+		for (var x = 0; x < Size.X; x++)
 			for (var z = 0; z < Size.Y; z++) {
 				var corners = GetCornersPosition(new(x, z));
-				AddTriangle(corners.TopLeft    , new(0.0f, 0.0f),
-				            corners.TopRight   , new(1.0f, 0.0f),
-				            corners.BottomLeft , new(0.0f, 1.0f));
-				AddTriangle(corners.TopRight   , new(1.0f, 0.0f),
-				            corners.BottomRight, new(1.0f, 1.0f),
-				            corners.BottomLeft , new(0.0f, 1.0f));
+
+				// Find the "ideal way" to split the quad for the tile into two triangles.
+				// This is done by finding the corner with the least variance between its neighboring corners.
+				var sorted  = corners.ToSorted((a, b) => a.Y.CompareTo(b.Y));
+				var minDiff = Abs(sorted[0].Value.Y - sorted[2].Value.Y); // Difference between lowest and 3rd lowest point.
+				var maxDiff = Abs(sorted[3].Value.Y - sorted[1].Value.Y); // Difference between highest and 3rd highest point.
+
+				var first = sorted[(minDiff > maxDiff) ? 0 : 3].Corner;
+				if (first is Corner.TopLeft or Corner.BottomRight) {
+					AddTriangle(corners.TopLeft    , new(0.0f, 0.0f),
+								corners.TopRight   , new(1.0f, 0.0f),
+								corners.BottomLeft , new(0.0f, 1.0f));
+					AddTriangle(corners.TopRight   , new(1.0f, 0.0f),
+								corners.BottomRight, new(1.0f, 1.0f),
+								corners.BottomLeft , new(0.0f, 1.0f));
+				} else {
+					AddTriangle(corners.TopRight   , new(1.0f, 0.0f),
+								corners.BottomRight, new(1.0f, 1.0f),
+								corners.TopLeft    , new(0.0f, 0.0f));
+					AddTriangle(corners.BottomRight, new(1.0f, 1.0f),
+								corners.BottomLeft , new(0.0f, 1.0f),
+								corners.TopLeft    , new(0.0f, 0.0f));
+				}
 			}
-		}
 
 		mesh.SurfaceEnd();
 		mesh.SurfaceSetMaterial(0, _terrainMaterial);
@@ -213,7 +229,7 @@ public partial class Terrain
 		(T TopLeft, T TopRight, T BottomRight, T BottomLeft)
 	{
 		public T this[Corner corner] {
-			readonly get => corner switch {
+			get => corner switch {
 				Corner.TopLeft     => TopLeft,
 				Corner.TopRight    => TopRight,
 				Corner.BottomRight => BottomRight,
@@ -240,8 +256,20 @@ public partial class Terrain
 			       (corner == Corner.BottomRight) ? value : BottomRight,
 			       (corner == Corner.BottomLeft)  ? value : BottomLeft);
 
-		public readonly T[] ToArray()
+		public T[] ToArray()
 			=> [ TopLeft, TopRight, BottomRight, BottomLeft ];
+
+		public (Corner Corner, T Value)[] ToSorted(Comparison<T> comparison)
+		{
+			var result = new (Corner Corner, T Value)[] {
+				(Corner.TopLeft     , TopLeft    ),
+				(Corner.TopRight    , TopRight   ),
+				(Corner.BottomRight , BottomRight),
+				(Corner.BottomLeft  , BottomLeft ),
+			};
+			Array.Sort(result, (a, b) => comparison(a.Value, b.Value));
+			return result;
+		}
 	}
 }