Hololens开发笔记之Gaze凝视射线

凝视是HoloLens首要输入方式，形式功能类似于桌面系统的光标，用于选择操作全息对象。然而在Unity中并没有明确的Gaze API或者组件。

概念上来说，Gaze是通过用户头部两眼之间发出一条向前方的射线来实现的，射线可以识别它所碰撞的物体。在Unity中，使用Main Camera来表示用户头部的位置和朝向。准确的说，是指UnityEngine.Camera.main.transform.forward 和 UnityEngine.Camera.main.transform.position.调用Physics.RayCast 发出射线后可以得到RaycastHit结果，该结果包含了碰撞点的3D位置参数和碰撞对象。

实现Gaze的例子

---

void Update()
{
       RaycastHit hitInfo;
       if (Physics.Raycast(
               Camera.main.transform.position,
               Camera.main.transform.forward,
               out hitInfo,
               20.0f,
               Physics.DefaultRaycastLayers))
       {
           // 如果射线成功击中物体
           // hitInfo.point代表了射线碰撞的位置
           // hitInfo.collider.gameObject代表了射线注视的全息对象
       }
}

在使用Gaze的时候，尽量避免每个物体都发出凝视射线，而是使用单例对象来管理凝视射线和其结果。

可视化凝视的具体例子

---

可以直接使用HoloToolkit中的GazeManager.cs脚本来实现凝视射线。

可以参考或直接使用HoloToolkit-Unity项目中的GazeManager.cs和预制的各种指针资源，包括Cursor.prefab 和 CursorWithFeedback.prefab 等。

1、添加GazeManager.cs

点击“ Create Empty” 创建一个空游戏对象，并将其命名为 Manager，为 Manager对象添加核心脚本组件GazeManager.cs

// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License. See LICENSE in the project root for license information.  

using UnityEngine;
using UnityEngine.VR.WSA;  

namespace HoloToolkit.Unity
{
    /// <summary>
    /// GazeManager determines the location of the user's gaze, hit position and normals.
    /// </summary>
    public partial class GazeManager : Singleton<GazeManager>
    {
        [Tooltip("Maximum gaze distance, in meters, for calculating a hit.")]
        public float MaxGazeDistance = 15.0f;  

        [Tooltip("Select the layers raycast should target.")]
        public LayerMask RaycastLayerMask = Physics.DefaultRaycastLayers;  

        /// <summary>
        /// Physics.Raycast result is true if it hits a hologram.
        /// </summary>
        public bool Hit { get; private set; }  

        /// <summary>
        /// HitInfo property gives access
        /// to RaycastHit public members.
        /// </summary>
        public RaycastHit HitInfo { get; private set; }  

        /// <summary>
        /// Position of the intersection of the user's gaze and the holograms in the scene.
        /// </summary>
        public Vector3 Position { get; private set; }  

        /// <summary>
        /// RaycastHit Normal direction.
        /// </summary>
        public Vector3 Normal { get; private set; }  

        /// <summary>
        /// Object currently being focused on.
        /// </summary>
        public GameObject FocusedObject { get; private set; }  

        [Tooltip("Checking enables SetFocusPointForFrame to set the stabilization plane.")]
        public bool SetStabilizationPlane = true;
        [Tooltip("Lerp speed when moving focus point closer.")]
        public float LerpStabilizationPlanePowerCloser = 4.0f;
        [Tooltip("Lerp speed when moving focus point farther away.")]
        public float LerpStabilizationPlanePowerFarther = 7.0f;  

        private Vector3 gazeOrigin;
        private Vector3 gazeDirection;
        private float lastHitDistance = 15.0f;  

        private void Update()
        {
            gazeOrigin = Camera.main.transform.position;
            gazeDirection = Camera.main.transform.forward;  

            UpdateRaycast();
            UpdateStabilizationPlane();
        }  

        /// <summary>
        /// Calculates the Raycast hit position and normal.
        /// </summary>
        private void UpdateRaycast()
        {
            // Get the raycast hit information from Unity's physics system.
            RaycastHit hitInfo;
            Hit = Physics.Raycast(gazeOrigin,
                           gazeDirection,
                           out hitInfo,
                           MaxGazeDistance,
                           RaycastLayerMask);  

            GameObject oldFocusedObject = FocusedObject;
            // Update the HitInfo property so other classes can use this hit information.
            HitInfo = hitInfo;  

            if (Hit)
            {
                // If the raycast hits a hologram, set the position and normal to match the intersection point.
                Position = hitInfo.point;
                Normal = hitInfo.normal;
                lastHitDistance = hitInfo.distance;
                FocusedObject = hitInfo.collider.gameObject;
            }
            else
            {
                // If the raycast does not hit a hologram, default the position to last hit distance in front of the user,
                // and the normal to face the user.
                Position = gazeOrigin + (gazeDirection * lastHitDistance);
                Normal = -gazeDirection;
                FocusedObject = null;
            }  

            // Check if the currently hit object has changed
            if (oldFocusedObject != FocusedObject)
            {
                if (oldFocusedObject != null)
                {
                    oldFocusedObject.SendMessage("OnGazeLeave", SendMessageOptions.DontRequireReceiver);
                }
                if (FocusedObject != null)
                {
                    FocusedObject.SendMessage("OnGazeEnter", SendMessageOptions.DontRequireReceiver);
                }
            }
        }  

        /// <summary>
        /// Adds the stabilization plane modifier if it's enabled and if it doesn't exist yet.
        /// </summary>
        private void UpdateStabilizationPlane()
        {
            // We want to use the stabilization logic.
            if (SetStabilizationPlane)
            {
                // Check if it exists in the scene.
                if (StabilizationPlaneModifier.Instance == null)
                {
                    // If not, add it to us.
                    gameObject.AddComponent<StabilizationPlaneModifier>();
                }
            }  

            if (StabilizationPlaneModifier.Instance)
            {
                StabilizationPlaneModifier.Instance.SetStabilizationPlane = SetStabilizationPlane;
            }
        }
    }
}  

2、创建一个新的游戏对象Cube,用来测试凝视效果

3、添加Cursor

就像PC使用鼠标来选中和交互图标一样，你可以为凝视也实现一个指针来更好的代表用户的凝视。

从 HoloToolkit/Input/Prefabs/ 目录下拖拽 Cursor Prefab 组件到场景中。这样当凝视在全息对象时，其表面会出现一个蓝色的光圈，表示当前凝视该对象，当射线离开该游戏对象时，Cursor变成一个点光源，以此来区分是否凝视游戏对象。

可以查看到Cursor中存在两个光标对象，分别是凝视在对象上及离开光息对象时分别显示的光标

CursorManager.cs

// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License. See LICENSE in the project root for license information.  

using HoloToolkit.Unity;
using UnityEngine;  

/// <summary>
/// CursorManager class takes Cursor GameObjects.
/// One that is on Holograms and another off Holograms.
/// 1. Shows the appropriate Cursor when a Hologram is hit.
/// 2. Places the appropriate Cursor at the hit position.
/// 3. Matches the Cursor normal to the hit surface.
/// </summary>
public partial class CursorManager : Singleton<CursorManager>
{
    //凝视射线在全息对象上时显示的光标对象
    [Tooltip("Drag the Cursor object to show when it hits a hologram.")]
    public GameObject CursorOnHolograms;
    //凝视射线离开全息对象时显示的光标对象
    [Tooltip("Drag the Cursor object to show when it does not hit a hologram.")]
    public GameObject CursorOffHolograms;  

    [Tooltip("Distance, in meters, to offset the cursor from the collision point.")]
    public float DistanceFromCollision = 0.01f;  

    void Awake()
    {
        //当未设定光标对象时直接返回
        if (CursorOnHolograms == null || CursorOffHolograms == null)
        {
            return;
        }  

        // Hide the Cursors to begin with.
        CursorOnHolograms.SetActive(false);
        CursorOffHolograms.SetActive(false);
    }  

    void LateUpdate()
    {
        if (GazeManager.Instance == null || CursorOnHolograms == null || CursorOffHolograms == null)
        {
            return;
        }
        //当凝视射线在全息对象上及离开全息对象时，分别显示不同的光标对象，以此来进行区分
        if (GazeManager.Instance.Hit)
        {
            CursorOnHolograms.SetActive(true);
            CursorOffHolograms.SetActive(false);
        }
        else
        {
            CursorOffHolograms.SetActive(true);
            CursorOnHolograms.SetActive(false);
        }  

        //计算并安置光标
        // Place the cursor at the calculated position.
        this.gameObject.transform.position = GazeManager.Instance.Position + GazeManager.Instance.Normal * DistanceFromCollision;  

        // Orient the cursor to match the surface being gazed at.
        gameObject.transform.up = GazeManager.Instance.Normal;
    }
}  

4、运行测试

当凝视射线在Cube上时，出现蓝色的光圈，表示当前凝视的点在该位置

当凝视射线离开Cube时，光标显示为一个点光源