基于UnityShader的面模型射线检测与表面曲线测量

实现流程：

首先需要准备一个shader来做屏幕特效，shader中输出屏幕上面模型的深度信息，用来计算模型像素的坐标。

Shader "Custom/Depth" {
	SubShader{
	Tags{ "RenderType" = "Opaque" }

	Pass{

	ZTest Always Cull Off ZWrite Off

	CGPROGRAM

		// Use shader model 3.0 target, to get nicer looking lighting
		#pragma glsl
		#pragma fragmentoption ARB_precision_hint_fastest
		#pragma target 3.0
		#pragma vertex vert
		#pragma fragment frag
		#include "unityCG.cginc"
		sampler2D _CameraDepthTexture;

	struct v2f {
		float4 pos : SV_POSITION;
		float4 scrPos:TEXCOORD0;
		float2 uv : TEXCOORD1;
	};
	//Vertex Shader
	v2f vert(appdata_base v) {
		v2f o;
		o.pos = UnityObjectToClipPos(v.vertex);
		o.scrPos = ComputeScreenPos(o.pos);
		o.uv = v.texcoord;
		return o;
	}

	//Fragment Shader
	float4 frag(v2f i) :COLOR{
		float depthValue = Linear01Depth(tex2Dproj(_CameraDepthTexture, UNITY_PROJ_COORD(i.scrPos)).r);
		return float4(depthValue, depthValue, depthValue, 1.0f);
	}
		ENDCG
	}
	}
		FallBack "Diffuse"
}

我们生成一个射线检测摄像机，只渲染需要参与射线检测的模型，摄像机指向鼠标的位置。在OnRenderImage()中使用上面的shader获取一个像素的深度输出rgb值，然后以此计算当前像素的坐标，即得到鼠标在模型上的位置。下面包含了模型上取两点测量它们在面上的最短距离。

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class RaycastTest : MonoBehaviour
{
    private float degree = 0;
    private Shader depthShader;
    private RenderTexture hitTex;
    private int hitWidth = 1;
    private int hitHeight = 1;
    private LayerMask meshLayerMask;
    private Texture2D tex;
    private Color c;
    public  Transform dot;
    private bool OnMeasure;
    private bool isEnd;
    private Vector3 startPos;
    private Vector3 endPos;
    private Vector3 upVector;
    private Vector3 forwardVector;
    private List pointsPos = new List();
    private List minPointsPos = new List();
    private const int pointNum = 40;
    public  LineRenderer lineRender;
    private List hitResult = new List();
    private List hitCam = new List();
    private int _hitCamNum = 1;
    public  Transform cameraContent;
    public int hitCamNum
    {
        set
        {
            _hitCamNum = value;
            for (int i = 0; i < hitCam.Count; i++)
            {
                Destroy(hitCam[i].gameObject);
            }
            hitCam.Clear();
            hitResult.Clear();
            for (int i = 0; i < value; ++i)
            {
                GameObject hitCamObj = new GameObject("HitCam" + i.ToString());
                hitCamObj.transform.SetParent(cameraContent);
                Camera tempcam = hitCamObj.AddComponent();
                tempcam.stereoTargetEye = StereoTargetEyeMask.None;
                tempcam.enabled = false;
                hitCam.Add(tempcam);
                hitResult.Add(Vector3.zero);
            }
            for (int j = 0; j < hitCamNum; ++j)
            {
                hitCam[j].clearFlags = CameraClearFlags.SolidColor;
                hitCam[j].backgroundColor = Color.black;
            }
        }
        get
        {
            return _hitCamNum;
        }
    }
    void Start()
    {
        hitCamNum = 1;
        for (int j = 0; j < hitCamNum; ++j)
        {
            hitCam[j].clearFlags = CameraClearFlags.SolidColor;
            hitCam[j].backgroundColor = Color.black;
        }
        meshLayerMask = LayerMask.GetMask("objMesh");
        tex = new Texture2D(hitWidth, hitHeight, TextureFormat.RGBAFloat, false);
        hitTex = RenderTexture.GetTemporary(hitWidth, hitHeight, 24, RenderTextureFormat.ARGBFloat);
        depthShader = Shader.Find("Custom/Depth");
    }
    private void Update()
    {
        if (OnMeasure)
        {
            GetAllPoints();
        }
        else
        {
            dot.transform.position = RayCastBody();
            if (Input.GetMouseButtonDown(0) && RayCastBody().magnitude > 0.0001f)
            {
                if (isEnd)
                {
                    endPos = RayCastBody();
                    OnMeasure = true;
                    //得到垂直于起点和终点向量的平面上互相垂直的两个向量
                    upVector = Vector3.Cross(Vector3.up, startPos - endPos).normalized;
                    forwardVector = Vector3.Cross(upVector, startPos - endPos).normalized;
                    hitCamNum = pointNum;
                    isEnd = false;
                }
                else
                {
                    startPos = RayCastBody();
                    isEnd = true;
                }
            }
        }
    }
    private Vector3 RayCastBody()
    {
        Ray ray = new Ray();
        ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        hitCam[0].transform.position = ray.origin - ray.direction * 0.01f;
        hitCam[0].transform.forward = ray.direction;
        return hitResult[0];
    }
    private void OnRenderImage(RenderTexture source, RenderTexture destination)
    {
        for (int j = 0; j < hitCamNum; j++)
        {
            hitCam[j].targetTexture = hitTex;
            hitCam[j].clearFlags = CameraClearFlags.SolidColor;
            hitCam[j].backgroundColor = Color.black;
            hitCam[j].cullingMask = meshLayerMask;//只渲染objMesh层的物体
            hitCam[j].RenderWithShader(depthShader, "RenderType");//用depthShader渲染模型像素深度图
            RenderTexture.active = hitTex;
            tex.ReadPixels(new Rect(0, 0, hitWidth, hitHeight), 0, 0);
            c = tex.GetPixels()[0];//读取该像素深度图r=g=b=该摄像机到该像素点的距离/摄像机的maxdistance;
            //如果检测到了像素，则计算该像素的坐标
            if (c.r > 0)
            {
                Vector3 temp = hitCam[j].transform.position + hitCam[j].transform.forward * c.r * 1000;
                hitResult[j] = temp;
            }
            else
            {
                hitResult[j] = Vector3.zero;
            }
        }
        Graphics.Blit(source, destination);
    }
    private void GetAllPoints()
    {
        if (degree > 360)
        {
            degree = 0;
            currentLength = Mathf.Infinity;
            OnMeasure = false;
            return;
        }
        //以起点和终点的线段为基准，计算该线段在曲面上的投影，旋转一圈得最小值即为最短距离
        Vector3 v = upVector * Mathf.Cos(Mathf.Deg2Rad * degree) + forwardVector * Mathf.Sin(Mathf.Deg2Rad * degree);
        v.Scale(Vector3.one * 10);
        degree += 5;
        pointsPos.Clear();
        //防止生成的曲线和原模型表面重合，向外拉一点点，保证曲线附着在模型外表面上
        pointsPos.Add(startPos + v.normalized * 0.0005f);
        for (int i = 1; i < pointNum - 1; i++)
        {
            Vector3 temp = startPos + (endPos - startPos) * i / pointNum;
            hitCam[i - 1].transform.position = temp + v.normalized * 1.5f;
            hitCam[i - 1].transform.forward = -v.normalized;
            pointsPos.Add(hitResult[i - 1] + v.normalized * 0.0005f);
        }
        pointsPos.Add(endPos + v.normalized * 0.0005f);
        GetMinLine();

    }
    float currentLength = Mathf.Infinity;
    /// 

    /// 获得曲面上两点之间的曲线最短长度
    /// 
    private void GetMinLine()
    {
        float sum = 0;
        for (int i = 0; i < pointNum - 1; i++)
        {
            sum += Vector3.Distance(pointsPos[i], pointsPos[i + 1]);
        }
        if (sum < currentLength)
        {
            currentLength = sum;
            minPointsPos = pointsPos;
            Drawline(minPointsPos);
        }
    }
    void Drawline(List pois)
    {
        for (int i = 0; i < pois.Count; i++)
        {
            lineRender.SetPosition(i, pois[i]);
        }
    }
}