OpenGL 鼠标拾取模型

1.简介

在我们的场景中，使用鼠标光标点击或“挑选”一个3d对象是很有用的。一种方法是从鼠标投射3d光线，通过相机，进入场景，然后检查光线是否与任何物体相交。这通常被称为光线投射。

我们不是从局部空间中的网格开始，而是从视口空间中的2d鼠标光标位置开始。我们用逆矩阵来逆向进行变换，得到世界空间中的一条射线。

• 首先在局部空间中有一个局部坐标(0.5,1,0)
• 然后乘以model矩阵变换到世界空间坐标(10.5,1,-20,1)，这里增加了齐次坐标w=1
• 然后世界空间坐标乘以view矩阵转换到我们摄像机看到的坐标(5,-1,-9.5,1)
• 然后观察坐标乘以projection矩阵转换到裁剪空间(9.05,-2.41,7.67,9.5)，这里齐次坐标变为-z
• 然后标准化设备坐标到(-1,1)的空间中，使用x/w，y/w，z/w得到(0.95,-0.25,0.81)
• 最后转换到像素坐标，转换到(1024,768)这个坐标下，得到(998,288)，以下为公式。

 

但是实际上，我们要点击到模型，首先是从视口空间坐标开始，也就是从(998,288)开始，进行反推。

2.计算世界空间坐标

float _near=0.1f,_far=100.0f;
QVector4D AXBOpemglWidget::worldPosFromViewPort(int posX, int posY)
{
    float winZ;
    glReadPixels(
                posX,
                this->height()-posY
                ,1,1
                ,GL_DEPTH_COMPONENT,GL_FLOAT
                ,&winZ);
    qDebug()<<"z = "<width()-1.0f; //转化为标准设备坐标(-1,1)
    float y=1.0f-(2.0f*posY)/this->height();//转化为标准设备坐标(-1,1)
    float z=winZ*2.0-1.0f;//转化为标准设备坐标(-1,1)

    float w = (2.0 * _near * _far) / (_far + _near - z * (_far - _near));//计算齐次坐标
    //float w= _near*_far/(_near*winZ-_far*winZ+_far);
    QVector4D wolrdPostion(x,y,z,1);
    wolrdPostion=w*wolrdPostion;//裁剪空间的坐标
    return view.inverted()*projection.inverted()*wolrdPostion; //获得世界空间的坐标
}

• 首先传入像素坐标x，y
•  glReadPixels获取到深度值
• 然后标准化坐标(x,y,z)
• 计算齐次坐标
• 使用w*x，w*y，w*z得到裁剪空间坐标
• 最后乘以projection逆矩阵 view的逆矩阵得到世界空间坐标

3.如何判断点中了模型

这里给出了一个简单的算法，把模型比作一个球体，计算点击位置P点到模型中心O点的距离，如果小于半径r（使用世界坐标来计算距离），则选中物体。当然这个算法并不完美。

 

        for(QMap::iterator iter=m_Models.begin();iter!=m_Models.end();iter++){
            ModelInfo *modelInfo=&iter.value();
            float r=(modelInfo->model->m_maxY-modelInfo->model->m_minY)/2;
            if(modelInfo->worldPos.distanceToPoint(QVector3D(wolrdPostion))isSelected=true;
                hasSelected=true;
            }
        }

4.源码

#include "axbopemglwidget.h"
#include "vertices.h"
const unsigned int timeOutmSec=50;

QVector3D viewInitPos(0.0f,5.0f,20.0f);
float _near=0.1f,_far=100.0f;
QMatrix4x4 model;
QMatrix4x4 view;
QMatrix4x4 projection;
AXBOpemglWidget::AXBOpemglWidget(QWidget *parent) : QOpenGLWidget(parent)
{
    connect(&m_timer,SIGNAL(timeout()),this,SLOT(on_timeout()));
    m_timer.start(timeOutmSec);
    m_time.start();
    m_camera.Position=viewInitPos;
    setFocusPolicy(Qt::StrongFocus);
    //setMouseTracking(true);
}

AXBOpemglWidget::~AXBOpemglWidget()
{
    for(auto iter=m_Models.begin();iter!=m_Models.end();iter++){
        ModelInfo *modelInfo=&iter.value();
        delete modelInfo->model;
    }
}

void AXBOpemglWidget::loadModel(string path)
{
    static int i=0;
    makeCurrent();
    Model * _model=new Model(QOpenGLContext::currentContext()->versionFunctions()
                             ,path.c_str());
    qDebug()<<"miny = "<<_model->m_minY;
    //m_camera.Position=cameraPosInit(_model->m_maxY,_model->m_minY);
    m_Models["张三"+QString::number(i++)]=
            ModelInfo{_model,QVector3D(0,0-_model->m_minY,0),0.0,0.0,0.0,false,"张三"};
    doneCurrent();
}

void AXBOpemglWidget::initializeGL()
{
    initializeOpenGLFunctions();
    //创建VBO和VAO对象，并赋予ID
    bool success;
    m_ShaderProgram.addShaderFromSourceFile(QOpenGLShader::Vertex,":/shaders/shaders/shapes.vert");
    m_ShaderProgram.addShaderFromSourceFile(QOpenGLShader::Fragment,":/shaders/shaders/shapes.frag");
    success=m_ShaderProgram.link();
    if(!success) qDebug()<<"ERR:"<textureId());
}

void AXBOpemglWidget::resizeGL(int w, int h)
{
    Q_UNUSED(w);
    Q_UNUSED(h);
}

void AXBOpemglWidget::paintGL()
{
    model.setToIdentity();
    view.setToIdentity();
    projection.setToIdentity();
    // float time=m_time.elapsed()/50.0;
    projection.perspective(m_camera.Zoom,(float)width()/height(),_near,_far);
    view=m_camera.GetViewMatrix();
    glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LESS);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    m_ShaderProgram.bind();
    m_ShaderProgram.setUniformValue("projection", projection);
    m_ShaderProgram.setUniformValue("view", view);
    //model.rotate(time, 1.0f, 1.0f, 0.5f);

    m_ShaderProgram.setUniformValue("viewPos",m_camera.Position);

    // light properties, note that all light colors are set at full intensity
    m_ShaderProgram.setUniformValue("light.ambient", 0.7f, 0.7f, 0.7f);
    m_ShaderProgram.setUniformValue("light.diffuse", 0.9f, 0.9f, 0.9f);
    m_ShaderProgram.setUniformValue("light.specular", 1.0f, 1.0f, 1.0f);
    // material properties
    m_ShaderProgram.setUniformValue("material.shininess", 32.0f);
    m_ShaderProgram.setUniformValue("light.direction", -0.2f, -1.0f, -0.3f);
    m_ShaderProgram.setUniformValue("model", model);

    m_PlaneMesh->Draw(m_ShaderProgram);

    foreach(auto modelInfo,m_Models){
        model.setToIdentity();
        model.translate(modelInfo.worldPos);

        model.rotate(modelInfo.pitch,QVector3D(1.0,0.0,0.0));
        model.rotate(modelInfo.yaw,QVector3D(0.0,1.0,0.0));
        model.rotate(modelInfo.roll,QVector3D(0.0,0.0,1.0));
        m_ShaderProgram.setUniformValue("model", model);
        modelInfo.model->Draw(m_ShaderProgram);
    }

}

void AXBOpemglWidget::wheelEvent(QWheelEvent *event)
{
    m_camera.ProcessMouseScroll(event->angleDelta().y()/120);
}

void AXBOpemglWidget::keyPressEvent(QKeyEvent *event)
{
    float deltaTime=timeOutmSec/1000.0f;

    switch (event->key()) {
    case Qt::Key_W: m_camera.ProcessKeyboard(FORWARD,deltaTime);break;
    case Qt::Key_S: m_camera.ProcessKeyboard(BACKWARD,deltaTime);break;
    case Qt::Key_D: m_camera.ProcessKeyboard(RIGHT,deltaTime);break;
    case Qt::Key_A: m_camera.ProcessKeyboard(LEFT,deltaTime);break;
    case Qt::Key_Q: m_camera.ProcessKeyboard(DOWN,deltaTime);break;
    case Qt::Key_E: m_camera.ProcessKeyboard(UP,deltaTime);break;
    case Qt::Key_Space: m_camera.Position=viewInitPos;break;

    default:break;
    }
}

void AXBOpemglWidget::mouseMoveEvent(QMouseEvent *event)
{
    if(event->buttons() & Qt::RightButton){
        static QPoint lastPos(width()/2,height()/2);
        auto currentPos=event->pos();
        QPoint deltaPos=currentPos-lastPos;
        lastPos=currentPos;

        m_camera.ProcessMouseMovement(deltaPos.x(),-deltaPos.y());
    }
}

void AXBOpemglWidget::mousePressEvent(QMouseEvent *event)
{
    makeCurrent();
    if(event->buttons()&Qt::LeftButton){

        QVector4D wolrdPostion=worldPosFromViewPort(event->pos().x(),
                                                    event->pos().y());

        mousePickingPos(QVector3D(wolrdPostion));

        for(QMap::iterator iter=m_Models.begin();iter!=m_Models.end();iter++){
            ModelInfo *modelInfo=&iter.value();
            float r=(modelInfo->model->m_maxY-modelInfo->model->m_minY)/2;
            if(modelInfo->worldPos.distanceToPoint(QVector3D(wolrdPostion))=0)
        temp.setY(height/2.0);
    viewInitPos=temp;
    return temp;
}

Mesh* AXBOpemglWidget::processMesh(float *vertices, int size, unsigned int textureId)
{
    vector _vertices;
    vector _indices;
    vector _textures;
    //memcpy(&_vertices[0],vertices,8*size*sizeof(float));
    for(int i=0;iversionFunctions()
                ,_vertices,_indices,_textures);
}

QVector4D AXBOpemglWidget::worldPosFromViewPort(int posX, int posY)
{
    float winZ;
    glReadPixels(
                posX,
                this->height()-posY
                ,1,1
                ,GL_DEPTH_COMPONENT,GL_FLOAT
                ,&winZ);
    float x=(2.0f*posX)/this->width()-1.0f;
    float y=1.0f-(2.0f*posY)/this->height();
    float z=winZ*2.0-1.0f;

    float w = (2.0 * _near * _far) / (_far + _near - z * (_far - _near));
    //float w= _near*_far/(_near*winZ-_far*winZ+_far);
    QVector4D wolrdPostion(x,y,z,1);
    wolrdPostion=w*wolrdPostion;
    return view.inverted()*projection.inverted()*wolrdPostion;
}

5.完整工程
https://download.csdn.net/download/wzz953200463/87941643https://download.csdn.net/download/wzz953200463/87941643

6.相关参考 

OpenGL 齐次坐标_Mr.codeee的博客-CSDN博客

OpenGL 坐标系统_Mr.codeee的博客-CSDN博客

OpenGL模型加载_Mr.codeee的博客-CSDN博客