实现一个简单的Unity3D三皮卡—

实现一个简单的Unity3D三皮卡——3D Picking (1)

3D Picking 其原理是从摄像机位置到空间发射的射线。基于光线碰到物体回暖。

这里我们使用了触摸屏拿起触摸，鼠标选择相同的原理，仅仅是可选API不同。

从unity3D官网Manual里找到下面Input内容：

http://docs.unity3d.com/Documentation/Manual/Input.html

当中有段样例程序：

Following is an example script which will shoot a ray whenever the user taps on the screen:

var particle : GameObject;
function Update () {
    for (var touch : Touch in Input.touches) {
        if (touch.phase == TouchPhase.Began) {
            // Construct a ray from the current touch coordinates
            var ray = Camera.main.ScreenPointToRay (touch.position);
            if (Physics.Raycast (ray)) {
                // Create a particle if hit
                Instantiate (particle, transform.position, transform.rotation);
            }
        }
　　}
}

var ray = Camera.main.ScreenPointToRay (touch.position);
            if (Physics.Raycast (ray))

这两句代码是关键代码，我们从这里入手。

查找ScreenPointToRay文档：

http://docs.unity3d.com/Documentation/ScriptReference/Camera.ScreenPointToRay.html

节选文档中主要描写叙述和一段样例

Ray ScreenPointToRay(Vector3 position);

Returns a ray going from camera through a screen point.

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        Ray ray = camera.ScreenPointToRay(new Vector3(200, 200, 0));
        Debug.DrawRay(ray.origin, ray.direction * 10, Color.yellow);
    }
}

上面的Debug.DrawRay留着后面调试时使用，能够看到发射的射线。

接下来Camera.main.ScreenPointToRay中的Camera.main是什么意思呢，查找Camera.main

static Camera main;

The first enabled camera tagged "MainCamera" (Read Only).

即unity新建一个project后，默认的一个main Camera

查找Physics.Raycast文档。

http://docs.unity3d.com/Documentation/ScriptReference/Physics.Raycast.html

文档内容非常多。当中有一个

static bool Raycast(Vector3origin, Vector3direction, RaycastHithitInfo, float distance = Mathf.Infinity, int layerMask = DefaultRaycastLayers);

红色的两个參数我们后面将会用到，在查看RaycastHithitInfo，

http://docs.unity3d.com/Documentation/ScriptReference/RaycastHit.html

至此。我们能够实现一个简单的Picking了，创建sphere, plane, Direction Light,和一个Empty GameObject取名GameController。并新建一个脚本与其绑定。

打开GameController.cs。输入下面代码：

using UnityEngine;
using System.Collections;

public class GameController : MonoBehaviour
{
	void Update () 
	{
		if(Input.touchCount == 1)
		{
			Ray ray = Camera.main.ScreenPointToRay (Input.GetTouch(0).position);

			Debug.DrawRay (ray.origin, ray.direction * 10, Color.yellow);  // ray needs a origin, and a dir

			if(Physics.Raycast(ray, 10))
				Debug.Log("Hit something");
		}
	}
}

在移动设备上使用Unity remote，用手触摸屏幕，将会看到一道黄线，若触摸到小球Sphere或地面Plane后。可看到在Console中有Hit something信息，即射线击中了物体。

以下我们想分别选中物体。即仅仅picking小球而忽略地面plane。查看Layer文档：

http://docs.unity3d.com/Documentation/Components/Layers.html

当中讲的非常仔细。关键就是要新建Tags和设定layerMask

打开Edit，选择Project Settings->Tags and Layers

在Layer 8输入Player。Layer 9输入Background

将Sphere 和 Plane的Layer 设为对应的层

改动GameController.cs，

using UnityEngine;
using System.Collections;

public class GameController : MonoBehaviour
{
	void Update () 
	{
		if(Input.touchCount == 1)
		{

			Ray ray = Camera.main.ScreenPointToRay (Input.GetTouch(0).position);
			
			int playersLayerMask = 1 << 8;
			
			RaycastHit hit;
			if(Physics.Raycast(ray, out hit, Mathf.Infinity, playersLayerMask))
			{
				Debug.DrawLine(ray.origin, hit.point, Color.yellow);			// line needs two points
			}
		}
	}
}

此时，仅仅有当我们触摸到Sphere时才绘制黄色射线提示，因为设置了LayerMask射线将忽略Plane。

以下我们尝试当射线击中Spheres时绘制黄色提示线，先击中Sphere后击中Plane时绘制红色提示线。

改动GameController.cs，

using UnityEngine;
using System.Collections;

public class GameController : MonoBehaviour
{
	void Update () 
	{
		if(Input.touchCount == 1)
		{
			Ray ray = Camera.main.ScreenPointToRay (Input.GetTouch(0).position);
			
			int playersLayerMask = 1 << 8;
			int backgroundLayerMask = 1 << 9;
			
			RaycastHit hit;
			if(Physics.Raycast(ray, out hit, Mathf.Infinity, playersLayerMask))
			{
				Debug.DrawLine(ray.origin, hit.point, Color.yellow);			// line needs two points
				
				RaycastHit groundHit;
				if(Physics.Raycast(ray, out groundHit, Mathf.Infinity, backgroundLayerMask))
				{
					Debug.DrawLine(ray.origin, groundHit.point, Color.red);
					
				}

			}
		}
	}
}

点击Play，效果例如以下，