Implemented a Mirror Transforms utility menu.

Assets/Editor.meta

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Assets/Editor/HelperMenu.cs

+using System;
+using System.Linq;
+using System.Collections.Generic;
+using UnityEditor;
+using UnityEngine;
+
+public class MirrorTransformWindow : EditorWindow
+{
+    private bool m_FlipX = true;
+    private bool m_FlipY = false;
+    private bool m_FlipZ = false;
+
+    private float m_Distance = 0f;
+
+    private Vector4 m_Plane = new Vector4(1f, 0f, 0f, 0f);
+
+    [MenuItem("Animation Rigging/Mirror Transforms", false, 0)]
+    static void ShowWindow()
+    {
+        MirrorTransformWindow window = EditorWindow.GetWindow<MirrorTransformWindow>();
+        window.position = new Rect(Screen.width / 2, Screen.height / 2, 250, 150);
+        window.ShowUtility();
+    }
+
+    public void OnGUI()
+    {
+        EditorGUIUtility.labelWidth = 75;
+
+        EditorGUILayout.LabelField("Plane");
+        if (EditorGUILayout.Toggle("X", m_FlipX, EditorStyles.radioButton))
+        {
+            m_FlipX = true;
+            m_FlipY = m_FlipZ = false;
+
+        }
+        if (EditorGUILayout.Toggle("Y", m_FlipY, EditorStyles.radioButton))
+        {
+            m_FlipY = true;
+            m_FlipX = m_FlipZ = false;
+        }
+        if (EditorGUILayout.Toggle("Z", m_FlipZ, EditorStyles.radioButton))
+        {
+            m_FlipZ = true;
+            m_FlipX = m_FlipY = false;
+        }
+
+        m_Distance = EditorGUILayout.FloatField("Distance", m_Distance);
+
+        GUILayout.Space(30);
+
+        GUILayout.BeginHorizontal();
+        if (GUILayout.Button("Apply"))
+            Apply();
+        if (GUILayout.Button("Close"))
+            Close();
+        GUILayout.EndHorizontal();
+    }
+
+    public void Apply()
+    {
+        Transform[] transforms = Selection.transforms;
+
+        List<Transform> allTransforms = new List<Transform>(transforms.Length);
+        foreach(var transform in transforms)
+        {
+            allTransforms.Add(transform);
+            allTransforms.AddRange(transform.GetComponentsInChildren<Transform>(true));
+        }
+
+        Vector4 plane = new Vector4(m_FlipX ? 1f : 0f, m_FlipY ? 1f : 0f, m_FlipZ ? 1f : 0f, -m_Distance);
+        float xMult = m_FlipX ? 1f : -1f;
+        float yMult = m_FlipY ? 1f : -1f;
+        float zMult = m_FlipZ ? 1f : -1f;
+
+        Matrix4x4 matrix = CalculateReflectionMatrix(plane);
+
+        List<Vector3> positions = new List<Vector3>(allTransforms.Select((transform) => transform.position));
+        List<Vector3> eulerAngles = new List<Vector3>(allTransforms.Select((transform) => transform.eulerAngles));
+
+        for (int i = 0; i < allTransforms.Count; ++i)
+        {
+            Transform transform = allTransforms[i];
+            Vector3 position = positions[i];
+            Vector3 angles = eulerAngles[i];
+
+            Undo.RecordObject(transform, "Mirror transforms");
+            transform.position = (matrix.MultiplyPoint(positions[i]));
+            transform.eulerAngles = new Vector3(xMult * angles.x, yMult * angles.y, zMult * angles.z);
+        }
+    }
+
+    // Calculates reflection matrix around the given plane
+	private static Matrix4x4 CalculateReflectionMatrix (Vector4 plane)
+	{
+        var reflectionMat = Matrix4x4.zero;
+
+		reflectionMat.m00 = (1F - 2F*plane[0]*plane[0]);
+		reflectionMat.m01 = (   - 2F*plane[0]*plane[1]);
+		reflectionMat.m02 = (   - 2F*plane[0]*plane[2]);
+		reflectionMat.m03 = (   - 2F*plane[3]*plane[0]);
+
+		reflectionMat.m10 = (   - 2F*plane[1]*plane[0]);
+		reflectionMat.m11 = (1F - 2F*plane[1]*plane[1]);
+		reflectionMat.m12 = (   - 2F*plane[1]*plane[2]);
+		reflectionMat.m13 = (   - 2F*plane[3]*plane[1]);
+
+		reflectionMat.m20 = (   - 2F*plane[2]*plane[0]);
+		reflectionMat.m21 = (   - 2F*plane[2]*plane[1]);
+		reflectionMat.m22 = (1F - 2F*plane[2]*plane[2]);
+		reflectionMat.m23 = (   - 2F*plane[3]*plane[2]);
+
+		reflectionMat.m30 = 0F;
+		reflectionMat.m31 = 0F;
+		reflectionMat.m32 = 0F;
+		reflectionMat.m33 = 1F;
+
+        return reflectionMat;
+	}
+}

Assets/Editor/HelperMenu.cs.meta

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