[unity] Fixed SpineVisualElement not rendering back-faces by providing a Flip Back Faces property (UI Elements). Closes #3016.

Author: HaraldCsaszar

spine-unity/Assets/Spine/Runtime/spine-unity/Mesh Generation/MeshGenerator.cs

@@ -1230,6 +1230,33 @@ void ResizeOptionalUVBuffer (ref ExposedList<Vector2> uvBuffer, int vertexCount)
 				}
 			}
 		}
+
+		public void FlipBackfaceWindingOrder () {
+			int submeshCount = submeshes.Count;
+			ExposedList<int>[] submeshesItems = submeshes.Items;
+			Vector3[] vertexPositions = vertexBuffer.Items;
+
+			for (int s = 0; s < submeshCount; ++s) {
+				ExposedList<int> submeshIndices = submeshesItems[s];
+				int indexCount = submeshIndices.Count;
+				int[] indices = submeshIndices.Items;
+				for (int i = 0; i < indexCount; i += 3) {
+					int iA = indices[i];
+					int iB = indices[i + 1];
+					int iC = indices[i + 2];
+					Vector3 a = vertexPositions[iA];
+					Vector3 b = vertexPositions[iB];
+					Vector3 c = vertexPositions[iC];
+					Vector2 d1 = new Vector2(b.x - a.x, b.y - a.y);
+					Vector2 d2 = new Vector2(c.x - b.x, c.y - b.y);
+					float z = d1.x * d2.y - d1.y * d2.x;
+					if (z < 0.0f) {
+						indices[i + 1] = iC;
+						indices[i + 2] = iB;
+					}
+				}
+			}
+		}
 		#endregion
 
 		#region Step 3 : Transfer vertex and triangle data to UnityEngine.Mesh

spine-unity/Assets/Spine/package.json

@@ -2,7 +2,7 @@
 	"name": "com.esotericsoftware.spine.spine-unity",
 	"displayName": "spine-unity Runtime",
 	"description": "This plugin provides the spine-unity runtime core and examples. Spine Examples can be installed via the Samples tab.",
-	"version": "4.3.40",
+	"version": "4.3.41",
 	"unity": "2018.3",
 	"author": {
 		"name": "Esoteric Software",

spine-unity/Modules/com.esotericsoftware.spine.ui-toolkit/Runtime/SpineVisualElement.cs

@@ -95,6 +95,9 @@ public string InitialSkinName {
 		}
 		public string initialSkinName;
 
+		/// <summary>Flip indices of back-faces to correct winding order during mesh generation.
+		/// UI Elements otherwise does not draw back-faces.</summary>
+		[UxmlAttribute] public bool flipBackFaces { get; set; } = true;
 		[UxmlAttribute] public bool startingLoop { get; set; } = true;
 		[UxmlAttribute] public float timeScale { get; set; } = 1.0f;
 
@@ -335,6 +338,8 @@ protected void GenerateVisualContents (MeshGenerationContext context) {
 				// clipping is done, vertex counts are final.
 
 				PrepareUISubmesh(uiSubmesh, meshGenerator.VertexCount, meshGenerator.SubmeshIndexCount(0));
+				if (flipBackFaces)
+					meshGenerator.FlipBackfaceWindingOrder();
 				meshGenerator.FillVertexData(ref uiSubmesh.verticesSlice);
 				meshGenerator.FillTrianglesSingleSubmesh(ref uiSubmesh.indicesSlice);
 

spine-unity/Modules/com.esotericsoftware.spine.ui-toolkit/package.json

@@ -2,7 +2,7 @@
   "name": "com.esotericsoftware.spine.ui-toolkit",
   "displayName": "Spine UI Toolkit [Experimental]",
   "description": "This plugin provides UI Toolkit integration for the spine-unity runtime.\n\nPrerequisites:\nIt requires a working installation of the spine-unity runtime, version 4.3.0 or newer and Unity 6000.0.16 or newer (requires [this bugfix](https://issuetracker.unity3d.com/issues/some-default-uxmlconverters-are-dependent-on-the-current-culture)).\n(See http://esotericsoftware.com/git/spine-runtimes/spine-unity)",
-  "version": "4.3.0-preview.3",
+  "version": "4.3.0-preview.4",
   "unity": "6000.0",
   "author": {
     "name": "Esoteric Software",