Unity - 优化 Vector3.ProjectOnPlane

• 起因
• 好奇
• 绘制 Vector3.ProjectOnPlane 的参数，与返回结果
• 绘制结果
• 定位作用、优化
• 完整脚本
• Project
• References

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

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

// jave.lin 2021/11/13
// 绘制 project on plane 的 gizmos

using 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;

        // forward
        Gizmos.color = Color.blue;
        Gizmos.DrawLine(forwardStart, forwardEnd);

        // point to target pos
        var 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.forward
        Gizmos.DrawLine(p2pEnd, projPos);

        Gizmos.color = srcCol;
    }
}

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);
        }

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;
    }

// 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;

        // forward
        Gizmos.color = Color.blue;
        Gizmos.DrawLine(forwardStart, forwardEnd);
        Gizmos.DrawLine(forwardEnd, forwardEnd + transForward.forward * forwardAddLength);

        // point to target pos
        var 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.forward
        Gizmos.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
    }
}

• TestingProjectOnPlane_AndOptimzie 提取码: mnxd

• Unity shader (hlsl) equivalent of Vector3.ProjectOnPlane - 写完 blog 后，发现 stackoverflow 上也有问，在 hlsl shader 上如何实现类似此函数的功能，也有人回答了