Unity论坛问答-如何让Camera.ScreenToWroldPoint在XZ平面上移动

原贴地址: How to make "Camera.ScreenToWroldPoint" work on XZ plane?

题主luwenwan问道:

public class DragOnPlanXY : MonoBehaviour
{
    void Update()
    {
        var screenPoint = Camera.main.WorldToScreenPoint(gameObject.transform.position);
        transform.position = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
    }
}

这个代码可以让物件随着鼠标在XZ平面上移动, 但是怎么能让物件在XY平面上移动呢?

neginfinity提供了一个使用Vector3.ProjectOnPlane的方法

using UnityEngine;

public class DragByProjectOnPlane : MonoBehaviour
{
    float timer = 0;

    void Update()
    {
        if (timer > 0)
        {
            timer--;
            return;
        }

        timer = 120;

        var screenPoint = Camera.main.WorldToScreenPoint(gameObject.transform.position);
        var targetTransform = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));

        Vector3 planePoint = Vector3.zero;//point on the plane
        Vector3 planeNormal = Vector3.up;//normal vector of the plane
        transform.position = Vector3.ProjectOnPlane(targetTransform - planePoint, planeNormal) + planePoint;
    }
}

当然他只提供了最后3行代码, 这个方法问题就是与镜头的深度无法立即获得, 所以能明显感到物件有一个延时.
这里加了一个计数器能更清楚地看到这个问题.

exiguous提供了一个使用Plane.RayCast的方法

public static class ExtensionMethods_Camera
{
    private static Plane xzPlane = new Plane(Vector3.up, Vector3.zero);

    public static Vector3 MouseOnPlane(this Camera camera)
    {
        // calculates the intersection of a ray through the mouse pointer with a static x/z plane for example for movement etc, 
        // source: http://unifycommunity.com/wiki/index.php?title=Click_To_Move
        Ray mouseray = camera.ScreenPointToRay(Input.mousePosition);
        float hitdist = 0.0f;
        if (xzPlane.Raycast(mouseray, out hitdist))
        {
            // check for the intersection point between ray and plane
            return mouseray.GetPoint(hitdist);
        }
        if (hitdist < -1.0f)
        {
            // when point is "behind" plane (hitdist != zero, fe for far away orthographic camera) simply switch sign 
            // https://docs.unity3d.com/ScriptReference/Plane.Raycast.html
            return mouseray.GetPoint(-hitdist);
        }
        // both are parallel or plane is behind camera so write a log and return zero vector
        Debug.Log("ExtensionMethods_Camera.MouseOnPlane: plane is behind camera or ray is parallel to plane! " + hitdist);       
        
        return Vector3.zero;
    }
}