/*
 * Version: 1.0
 * Author:  Yilmaz Kiymaz (@VoxelBoy)
 * Purpose: To be able to change the pivot of Game Objects
 *          without needing to use a separate 3D application.
 * License: Free to use and distribute, in both free and commercial projects.
 *          Do not try to sell as your own work. Simply put, play nice
 * Contact: VoxelBoy on Unity Forums
 */

/*
 * TODO:
 * - Doesn't work properly with rotated objects.
 * - Can't compensate for the positioning of Mesh Colliders.
 * - Need to figure out if the "Instantiating mesh" error in Editor is a big issue, if not, how to supress it.
 * - Allowing the pivot to move outside the bounds of the mesh, 
 * - ideally using the movement gizmo but only affecting the pivot.
 */

using UnityEngine;
using UnityEditor;

public class SetPivot : EditorWindow {

    Vector3 p; //Pivot value -1..1, calculated from Mesh bounds
    Vector3 last_p; //Last used pivot

    GameObject obj; //Selected object in the Hierarchy
    MeshFilter meshFilter; //Mesh Filter of the selected object
    Mesh mesh; //Mesh of the selected object
    Collider col; //Collider of the selected object

    bool pivotUnchanged; //Flag to decide when to instantiate a copy of the mesh

    [MenuItem ("GameObject/Set Pivot")] //Place the Set Pivot menu item in the GameObject menu
    static void Init () {
        SetPivot window = (SetPivot)EditorWindow.GetWindow (typeof (SetPivot));
        // Initialize the variables by calling RecognizeSelectedObject on the class instance
        window.RecognizeSelectedObject(); 
        window.Show ();
    }

    void OnGUI() {
        if(obj) {
            if(mesh) {
                p.x = EditorGUILayout.Slider("X", p.x, -1.0f, 1.0f);
                p.y = EditorGUILayout.Slider("Y", p.y, -1.0f, 1.0f);
                p.z = EditorGUILayout.Slider("Z", p.z, -1.0f, 1.0f);
                if(p != last_p) { //Detects user input on any of the three sliders
                    //Only create instance of mesh when user changes pivot
                    if(pivotUnchanged) mesh = meshFilter.sharedMesh;
                    pivotUnchanged = false;
                    UpdatePivot();
                    last_p = p;
                }
                if(GUILayout.Button("Center")) { //Set pivot to the center of the mesh bounds
                    //Only create instance of mesh when user changes pivot
                    if(pivotUnchanged) mesh = meshFilter.sharedMesh;
                    pivotUnchanged = false;
                    p = Vector3.zero;
                    UpdatePivot();
                    last_p = p;
                }
                GUILayout.Label("Bounds " + mesh.bounds.ToString());
            } else {
                GUILayout.Label("Selected object does not have a Mesh specified.");
            }
        } else {
            GUILayout.Label("No object selected in Hierarchy.");
        }
    }

    //Achieve the movement of the pivot by moving the transform position in the specified direction
    //and then moving all vertices of the mesh in the opposite direction back to where they were in world-space
    void UpdatePivot() {
        Vector3 diff = Vector3.Scale(mesh.bounds.extents, last_p - p); //Calculate difference in 3d position
        obj.transform.position -= Vector3.Scale(diff, obj.transform.localScale); //Move object position
        //Iterate over all vertices and move them in the opposite direction of the object position movement
        Vector3[] verts = mesh.vertices;
        for(int i = 0; i < verts.Length; i++) {
            verts[i] += diff;
        }
        mesh.vertices = verts; //Assign the vertex array back to the mesh
        mesh.RecalculateBounds(); //Recalculate bounds of the mesh, for the renderer's sake
        //The 'center' parameter of certain colliders needs to be adjusted
        //when the transform position is modified
        if(col) {
            if(col is BoxCollider) {
                ((BoxCollider) col).center += diff;
            } else if(col is CapsuleCollider) {
                ((CapsuleCollider) col).center += diff;
            } else if(col is SphereCollider) {
                ((SphereCollider) col).center += diff;
            }
        }
    }

    //Look at the object's transform position in comparison to the center of its mesh bounds
    //and calculate the pivot values for xyz
    void UpdatePivotVector() {
        Bounds b = mesh.bounds;
        Vector3 offset = -1 * b.center;
        p = last_p = new Vector3(offset.x / b.extents.x, offset.y / b.extents.y, offset.z / b.extents.z);
    }

    //When a selection change notification is received
    //recalculate the variables and references for the new object
    void OnSelectionChange() {
        RecognizeSelectedObject();
    }

    //Gather references for the selected object and its components
    //and update the pivot vector if the object has a Mesh specified
    void RecognizeSelectedObject() {
        Transform t = Selection.activeTransform;
        obj = t ? t.gameObject : null;
        if(obj) {
            meshFilter = obj.GetComponent(typeof(MeshFilter)) as MeshFilter;
            mesh = meshFilter ? meshFilter.sharedMesh : null;
            if(mesh)
                UpdatePivotVector();
            col = obj.GetComponent(typeof(Collider)) as Collider;
            pivotUnchanged = true;
        } else {
            mesh = null;
        }
    }
}

效果如图：

本文参考：http://wiki.unity3d.com/index.php?title=SetPivot