From 4a6d94f0fb817810d14a9920130db7bada5ff47c Mon Sep 17 00:00:00 2001
From: ackwell <saxon@ackwell.com.au>
Date: Sat, 20 Apr 2024 20:02:43 +1000
Subject: [PATCH] Avoid inclusion of zero-weighted bones in name mapping

---
 Penumbra/Import/Models/Import/MeshImporter.cs | 18 ++++++++++++++----
 .../Import/Models/Import/VertexAttribute.cs   | 19 +++++++++++--------
 2 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/Penumbra/Import/Models/Import/MeshImporter.cs b/Penumbra/Import/Models/Import/MeshImporter.cs
index 1d4b223d..5d5df948 100644
--- a/Penumbra/Import/Models/Import/MeshImporter.cs
+++ b/Penumbra/Import/Models/Import/MeshImporter.cs
@@ -189,15 +189,25 @@ public class MeshImporter(IEnumerable<Node> nodes, IoNotifier notifier)
         foreach (var (primitive, primitiveIndex) in node.Mesh.Primitives.WithIndex())
         {
             // Per glTF specification, an asset with a skin MUST contain skinning attributes on its meshes.
-            var jointsAccessor = primitive.GetVertexAccessor("JOINTS_0")
-             ?? throw notifier.Exception($"Primitive {primitiveIndex} is skinned but does not contain skinning vertex attributes.");
+            var jointsAccessor = primitive.GetVertexAccessor("JOINTS_0")?.AsVector4Array();
+            var weightsAccessor = primitive.GetVertexAccessor("WEIGHTS_0")?.AsVector4Array();
+
+            if (jointsAccessor == null || weightsAccessor == null)
+                throw notifier.Exception($"Primitive {primitiveIndex} is skinned but does not contain skinning vertex attributes.");
 
             // Build a set of joints that are referenced by this mesh.
-            // TODO: Would be neat to omit 0-weighted joints here, but doing so will require some further work on bone mapping behavior to ensure the unweighted joints can still be resolved to valid bone indices during vertex data construction.
-            foreach (var joints in jointsAccessor.AsVector4Array())
+            for (var i = 0; i < jointsAccessor.Count; i++) 
             {
+                var joints = jointsAccessor[i];
+                var weights = weightsAccessor[i];
                 for (var index = 0; index < 4; index++)
+                {
+                    // If a joint has absolutely no weight, we omit the bone entirely.
+                    if (weights[index] == 0)
+                        continue;
+
                     usedJoints.Add((ushort)joints[index]);
+                }
             }
         }
 
diff --git a/Penumbra/Import/Models/Import/VertexAttribute.cs b/Penumbra/Import/Models/Import/VertexAttribute.cs
index 3cfedd6f..b7f5dcf1 100644
--- a/Penumbra/Import/Models/Import/VertexAttribute.cs
+++ b/Penumbra/Import/Models/Import/VertexAttribute.cs
@@ -144,10 +144,10 @@ public class VertexAttribute
 
     public static VertexAttribute? BlendIndex(Accessors accessors, IDictionary<ushort, ushort>? boneMap, IoNotifier notifier)
     {
-        if (!accessors.TryGetValue("JOINTS_0", out var accessor))
+        if (!accessors.TryGetValue("JOINTS_0", out var jointsAccessor))
             return null;
 
-        if (!accessors.ContainsKey("WEIGHTS_0"))
+        if (!accessors.TryGetValue("WEIGHTS_0", out var weightsAccessor))
             throw notifier.Exception("Mesh contained JOINTS_0 attribute but no corresponding WEIGHTS_0 attribute.");
 
         if (boneMap == null)
@@ -160,18 +160,21 @@ public class VertexAttribute
             Usage  = (byte)MdlFile.VertexUsage.BlendIndices,
         };
 
-        var values = accessor.AsVector4Array();
+        var joints = jointsAccessor.AsVector4Array();
+        var weights = weightsAccessor.AsVector4Array();
 
         return new VertexAttribute(
             element,
             index =>
             {
-                var gltfIndices = values[index];
+                var gltfIndices = joints[index];
+                var gltfWeights = weights[index]; 
+
                 return BuildUByte4(new Vector4(
-                    boneMap[(ushort)gltfIndices.X],
-                    boneMap[(ushort)gltfIndices.Y],
-                    boneMap[(ushort)gltfIndices.Z],
-                    boneMap[(ushort)gltfIndices.W]
+                    gltfWeights.X == 0 ? 0 : boneMap[(ushort)gltfIndices.X],
+                    gltfWeights.Y == 0 ? 0 : boneMap[(ushort)gltfIndices.Y],
+                    gltfWeights.Z == 0 ? 0 : boneMap[(ushort)gltfIndices.Z],
+                    gltfWeights.W == 0 ? 0 : boneMap[(ushort)gltfIndices.W]
                 ));
             }
         );