export bones in original position instead of bindpose

When exporting bones of skinned meshes, their transform values are first set to the bindpose, so that the skin and bindpose are exported properly.

However, afterwards we need to set the bones back to their original transform values so that if the character is not in the bindpose on export, the same pose is maintained in the FBX.

Author: vkovec

com.unity.formats.fbx/Editor/FbxExporter.cs

@@ -938,6 +938,29 @@ SkinnedMeshRenderer unitySkin
 
                 // add bind pose
                 ExportBindPose (unitySkin, fbxNode, fbxScene, skinnedMeshToBonesMap);
+
+                // now that the skin and bindpose are set, make sure that each of the bones
+                // is set to its original position
+                var bones = unitySkin.bones;
+                foreach (var bone in bones)
+                {
+                    var fbxBone = MapUnityObjectToFbxNode[bone.gameObject];
+                    ExportTransform(bone, fbxBone, newCenter: Vector3.zero, TransformExportType.Local);
+
+                    // cancel out the pre-rotation from the exported rotation
+                    var fbxPreRotationEuler = fbxBone.GetPreRotation(FbxNode.EPivotSet.eSourcePivot);
+                    // Get the inverse of the prerotation
+                    var fbxPreRotationInverse = ModelExporter.EulerToQuaternion(fbxPreRotationEuler);
+                    fbxPreRotationInverse.Inverse();
+
+                    var fbxFinalQuat = fbxPreRotationInverse * EulerToQuaternion(new FbxVector4(fbxBone.LclRotation.Get()));
+
+                    var finalUnityQuat = new FbxQuaternion((float)fbxFinalQuat.X, (float)fbxFinalQuat.Y, (float)fbxFinalQuat.Z, (float)fbxFinalQuat.W);
+
+                    var quatToEulerMatrix = new FbxAMatrix();
+                    quatToEulerMatrix.SetQ(finalUnityQuat);
+                    fbxBone.LclRotation.Set(ToFbxDouble3(quatToEulerMatrix.GetR()));
+                }
             }
 
             return true;