起因

之前偶然项目中看到过 API Vector3.ProjectOnPlane API 的使用

然在这篇文章也看到：Unity3d那些你不常用但确有其用的方法—Vector3.ProjectOnPlane，该作者使用的是将 UI 的面向和相机的 forward 正向的反向，有点 BB（Billboard）的功能想过

关于 BB，如果你感兴趣，可以参考我之前写的两篇：

Unity Shader - Billboard 广告板/广告牌 - BB树,BB投影
Unity Shader - Billboard火堆热扭曲

好奇

然后我就好奇，这个函数的作用是啥，返回的 Vector3 是什么位置

所以我就使用 Gizmos 来绘制查看

绘制 Vector3.ProjectOnPlane 的参数，与返回结果

// jave.lin 2021/11/13
// 绘制 project on plane 的 gizmosusing UnityEngine;public class DrawProjectOnPlaneGizmos : MonoBehaviour
{public Transform transForward;public Transform targetPos;private void OnDrawGizmos(){var forwardStart = transForward.position;var forwardEnd = forwardStart + transForward.forward;var srcCol = Gizmos.color;// forwardGizmos.color = Color.blue;Gizmos.DrawLine(forwardStart, forwardEnd);// point to target posvar p2pStart = forwardStart;var p2pEnd = targetPos.position;Gizmos.color = Color.yellow;Gizmos.DrawLine(p2pStart, p2pEnd);Vector3 projPos = Vector3.ProjectOnPlane(p2pEnd - p2pStart, transForward.forward);Gizmos.color = Color.green;Gizmos.DrawLine(forwardEnd, projPos);// jave.lin : 从 gizmos 视图上看：p2pEnd->projPos 平行与 transform.forwardGizmos.DrawLine(p2pEnd, projPos);Gizmos.color = srcCol;}
}

绘制结果

在这里插入图片描述

定位作用、优化

从上面的效果图可以看出来，Vector3.ProjectOnPlane(Vector3 vector, Vector3 planeNormal) 的作用就是返回 vector 向量相对 planeNormal 上的反向投影

先来看看该 API Unity C# 的源码：

        //// 摘要://     Projects a vector onto a plane defined by a normal orthogonal to the plane.//// 参数://   planeNormal://     The direction from the vector towards the plane.////   vector://     The location of the vector above the plane.//// 返回结果://     The location of the vector on the plane.public static Vector3 ProjectOnPlane(Vector3 vector, Vector3 planeNormal){float num = Dot(planeNormal, planeNormal);if (num < Mathf.Epsilon){return vector;}float num2 = Dot(vector, planeNormal);return new Vector3(vector.x - planeNormal.x * num2 / num, vector.y - planeNormal.y * num2 / num, vector.z - planeNormal.z * num2 / num);}

在这里插入图片描述

我能吐槽的是，这运算量，和这可读性，真的不敢恭维~

那么，其实可以优化（减少一个除法）：

    private static Vector3 MyProjectOnPlane(Vector3 vector, Vector3 planeNormal){var dot = Vector3.Dot(vector, planeNormal);return vector + -planeNormal * dot;}

完整脚本

// jave.lin 2021/11/13
// 绘制 project on plane 的 gizmos
// 顺便优化using UnityEngine;public class DrawProjectOnPlaneGizmos : MonoBehaviour
{public Transform transForward;public Transform targetPos;private float forwardAddLength = 10.0f;public bool usingMyFunc = false;private void OnDrawGizmos(){var forwardStart = transForward.position;var forwardEnd = forwardStart + transForward.forward;var srcCol = Gizmos.color;// forwardGizmos.color = Color.blue;Gizmos.DrawLine(forwardStart, forwardEnd);Gizmos.DrawLine(forwardEnd, forwardEnd + transForward.forward * forwardAddLength);// point to target posvar p2pStart = forwardStart;var p2pEnd = targetPos.position;Gizmos.color = Color.yellow;Gizmos.DrawLine(p2pStart, p2pEnd);Vector3 projPos;if (usingMyFunc){projPos = MyProjectOnPlane(p2pEnd - p2pStart, transForward.forward);}else{projPos = Vector3.ProjectOnPlane(p2pEnd - p2pStart, transForward.forward);}Gizmos.color = Color.green;Gizmos.DrawLine(forwardEnd, projPos);// jave.lin : 从 gizmos 视图上看：p2pEnd->projPos 平行与 transform.forwardGizmos.color = Color.red;Gizmos.DrawLine(p2pEnd, projPos);Gizmos.color = srcCol;}private static Vector3 MyProjectOnPlane(Vector3 vector, Vector3 planeNormal){var dot = Vector3.Dot(vector, planeNormal); // 得到投影长度return vector + -planeNormal * dot; // planeNormal 的反向 * 投影长度}
}