在Canvas不同的渲染模式（RenderMode）下實現UI跟隨3D物體

當Canvas.RenderMode為Screen Space-Overlay時

利用WorldToScreenPoint(worldPos)將物體的世界座標轉換成螢幕座標,實時更新UI的座標：

using UnityEngine;
using System.Collections;

public class FollowWorldObj : MonoBehaviour {
    [SerializeField]
    GameObject worldPos;//3D物體（人物）
    [SerializeField]
    RectTransform rectTrans;//UI元素（如：血條等）
 

    public Vector2 offset;//偏移量

    // Update is called once per frame
    void Update () {
        Vector2 screenPos=Camera.main.WorldToScreenPoint(worldPos.transform.position);
        rectTrans.position = screenPos + offset;
    }
}

當Canvas.RenderMode為Screen Space-Camera時

利用RectTransformUtility.ScreenPointToLocalPointInRectangle換算出UI元素在Canvas的2D座標:

using UnityEngine;
using System.Collections;
using UnityEngine.EventSystems;

public class UI_FollowObj : MonoBehaviour {
    [SerializeField]
    Camera UI_Camera;//UI相機
    [SerializeField]
    RectTransform image;//UI元素
    [SerializeField]
    GameObject obj;//3D物體
    [SerializeField]
    Canvas ui_Canvas;
    // Update is called once per frame
 

    void Update () {
        UpdateNamePosition();
    }
    /// <summary>
    /// 更新image位置
    /// </summary>
    void UpdateNamePosition()
    {
        Vector2 mouseDown = Camera.main.WorldToScreenPoint(obj.transform.position);
        Vector2 mouseUGUIPos = new Vector2();
        bool isRect = RectTransformUtility.ScreenPointToLocalPointInRectangle(ui_Canvas.transform as RectTransform, mouseDown, UI_Camera, out mouseUGUIPos);
        if (isRect)
        {
            image.anchoredPosition = mouseUGUIPos;
        }
    }
}

效果如下：