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OpenGL picking shows nothing after the pick

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So I have 2 cubes in a display list and I want one of them be pickable so I can maybe change his color or something like that.

When I click on a cube then the screen turns black and nothing happens, the console gives me output of the closest hit but the screen turns black and doesn't show anything.

Here is my cpp file:

#include "glwidget.h"
#include <QDomDocument>
#include <QDebug>
#include <QFile>
#include <math.h>
#include <QString>
#include <stdlib.h>
GLWidget::GLWidget(QWidget *parent):QGLWidget(parent)
{

    camPosx = 0.0,  camPosy = 0.0,    camPosz = 1.0;
    camViewx = 0.0, camViewy = 0.0, camViewz = 0.0;
    camUpx = 0.0,   camUpy = 1.0,   camUpz = 0.0;
    camAngle = 0.0;
    camViewz = -cos(camAngle);
    camViewx = sin(camAngle);
    mode = 1;
    timer = new QTimer();
    connect( timer, SIGNAL(timeout()), this, SLOT(updateGL()) );
}

void GLWidget::initializeGL() {

    loadGLTextures();
    glEnable(GL_TEXTURE_2D);                            // Enable Texture Mapping
    glShadeModel(GL_SMOOTH);                            // Enable Smooth Shading
    glClearColor(0.0f, 0.0f, 0.0f, 0.5f);               // Black Background
    glClearDepth(1.0f);                                 // Depth Buffer Setup
    glEnable(GL_DEPTH_TEST);                            // Enables Depth Testing
    glDepthFunc(GL_LEQUAL);                             // The Type Of Depth Testing To Do
    glEnable(GL_LIGHT0);                                // Quick And Dirty Lighting (Assumes Light0 Is Set Up)
    glEnable(GL_LIGHTING);                              // Enable Lighting
    glEnable(GL_COLOR_MATERIAL);                        // Enable Material Coloring
    glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);  // Perspective Calculations
    buildLists(2);                                      // Creating displaylist #
    glLoadIdentity();

    timer->start(50);
}

void GLWidget::resizeGL(int width, int height) {

    //set viewport
    glViewport(0,0,width,height);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    //set persepective
    //change the next line order to have a different perspective
    aspect_ratio=(GLdouble)width/(GLdouble)height;
    gluPerspective(45.0f, aspect_ratio, 0.1 , 100.0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}

void GLWidget::paintGL() {
    glEnable(GL_DEPTH_TEST);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glLoadIdentity();

    // store current matrix
    glMatrixMode( GL_MODELVIEW );
    glPushMatrix( );

    gluLookAt(camPosx ,camPosy ,camPosz,
              camPosx + camViewx,camViewy,camPosz + camViewz,
              camUpx, camUpy, camUpz );

    if (mode == 2) {
        startPicking();
    }

    glColor3f(1.0f,0.0f,0.0f);
    glCallList(displayList[0]);
    glTranslatef(5.0,0.0,0.0);
    glColor3f(0.0f,1.0f,1.0f);
    glCallList(displayList[0]);

    if (mode == 2)
        stopPicking();
    //    glEnable( GL_LIGHTING );
    //    glEnable( GL_LIGHT0 );
    //    glScalef(10.0,10.0,10.0);
    //    glBindTexture(GL_TEXTURE_2D, texture[0]);
    //    glBegin(GL_QUADS);
    //    // Front Face
    //    glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f,  1.0f);  // Bottom Left Of The Texture and Quad
    //    glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f,  1.0f);  // Bottom Right Of The Texture and Quad
    //    glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f,  1.0f,  1.0f);  // Top Right Of The Texture and Quad
    //    glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f,  1.0f,  1.0f);  // Top Left Of The Texture and Quad
    //    // Back Face
    //    glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f, -1.0f);  // Bottom Right Of The Texture and Quad
    //    glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f,  1.0f, -1.0f);  // Top Right Of The Texture and Quad
    //    glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f,  1.0f, -1.0f);  // Top Left Of The Texture and Quad
    //    glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f, -1.0f);  // Bottom Left Of The Texture and Quad
    //    // Top Face
    //    glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f,  1.0f, -1.0f);  // Top Left Of The Texture and Quad
    //    glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f,  1.0f,  1.0f);  // Bottom Left Of The Texture and Quad
    //    glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f,  1.0f,  1.0f);  // Bottom Right Of The Texture and Quad
    //    glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f,  1.0f, -1.0f);  // Top Right Of The Texture and Quad
    //    // Bottom Face
    //    glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f, -1.0f, -1.0f);  // Top Right Of The Texture and Quad
    //    glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f, -1.0f, -1.0f);  // Top Left Of The Texture and Quad
    //    glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f,  1.0f);  // Bottom Left Of The Texture and Quad
    //    glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f,  1.0f);  // Bottom Right Of The Texture and Quad
    //    // Right face
    //    glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f, -1.0f);  // Bottom Right Of The Texture and Quad
    //    glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f,  1.0f, -1.0f);  // Top Right Of The Texture and Quad
    //    glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f,  1.0f,  1.0f);  // Top Left Of The Texture and Quad
    //    glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f,  1.0f);  // Bottom Left Of The Texture and Quad
    //    // Left Face
    //    glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f, -1.0f);  // Bottom Left Of The Texture and Quad
    //    glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f,  1.0f);  // Bottom Right Of The Texture and Quad
    //    glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f,  1.0f,  1.0f);  // Top Right Of The Texture and Quad
    //    glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f,  1.0f, -1.0f);  // Top Left Of The Texture and Quad
    //    glEnd();


//    // XML
//    QDomDocument doc( "AdBookML" );
//    QDomNode n;
//    QDomElement e;
//    QFile file( "test2.xml" );
//    QString s;
//    QStringList sl;


////    if( !file.open(QIODevice::ReadOnly))
////        qDebug("probleem bij het openen");

//    if( !doc.setContent( &file ) )
//    {
//        file.close();
//    }
//    file.close();

//    QDomElement root = doc.documentElement();
//    if( root.tagName() != "playlist" )
//        // qDebug("root is different");

//        //qDebug( root.tagName() );

//        // doorheen u boom lopen
//        n = root;
//    float f1, f2, f3;

//    glDisable( GL_LIGHTING );
//    glBegin(GL_TRIANGLES);
//    for(int i = 0; i< n.childNodes().length(); i++) // voor alle triangles
//    {
//        if(n.childNodes().at(i).toElement().tagName() == "triangle")
//        {
//            for(int j =0; j < 4; j++) // voor alle punten
//            {
//                e = n.childNodes().at(i).childNodes().at(j).toElement(); // e is een punt
//                //qDebug(e.tagName());
//                s = e.text();
//                sl = s.split(" ");          // opsplitsen naar het x, y , z coordinaat;
//                f1 = sl.at(0).toFloat();
//                f2 = sl.at(1).toFloat();
//                f3 = sl.at(2).toFloat();
//                if( j > 0)
//                    glVertex3f(f1, f2, f3);     // de vertex tekenen
//                if(j == 0)
//                    glColor3f(f1,f2,f3);
//            }
//        }


//    }
//    glEnd();
//    glEnable(GL_LIGHTING);


    // restore current matrix
    glMatrixMode( GL_MODELVIEW );
    glPopMatrix( );

}


void GLWidget::loadGLTextures()
{
    QImage t;
    QImage b;

    if ( !b.load( "images/redbrick.png" ) )
    {
        qDebug("Didn't found the image.");
        b = QImage( 16, 16, QImage::Format_RGB32 );
        b.fill( 1 );
    }

    t = QGLWidget::convertToGLFormat( b );
    glGenTextures( 1, &texture[0] );
    glBindTexture( GL_TEXTURE_2D, texture[0] );
    glTexImage2D( GL_TEXTURE_2D, 0, 3, t.width(), t.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, t.bits() );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
}

//Functie die display lists kan aanmaken, het aantal ( is het aantal displaylists )
GLvoid GLWidget::buildLists(int aantal)
{
    displayList = new GLuint[aantal];
    for(int i = 0; i < aantal; i++)
    {
        displayList[i]=glGenLists(aantal);                                  // Maak x Aantal displaylists
        glNewList(displayList[i],GL_COMPILE);                               //start met de eerste display list te compile
        //Hieronder moet er xml worden ingeladen
        glBegin(GL_QUADS);
        // Bottom Face
        glNormal3f( 0.0f,-1.0f, 0.0f);
        glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f, -1.0f, -1.0f);
        glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f, -1.0f, -1.0f);
        glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f,  1.0f);
        glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f,  1.0f);
        // Front Face
        glNormal3f( 0.0f, 0.0f, 1.0f);
        glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f,  1.0f);
        glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f,  1.0f);
        glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f,  1.0f,  1.0f);
        glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f,  1.0f,  1.0f);
        // Back Face
        glNormal3f( 0.0f, 0.0f,-1.0f);
        glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f, -1.0f);
        glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f,  1.0f, -1.0f);
        glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f,  1.0f, -1.0f);
        glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f, -1.0f);
        // Right face
        glNormal3f( 1.0f, 0.0f, 0.0f);
        glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f, -1.0f);
        glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f,  1.0f, -1.0f);
        glTexCoord2f(0.0f, 1.0f); glVertex3f( 1.0f,  1.0f,  1.0f);
        glTexCoord2f(0.0f, 0.0f); glVertex3f( 1.0f, -1.0f,  1.0f);
        // Left Face
        glNormal3f(-1.0f, 0.0f, 0.0f);
        glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f, -1.0f);
        glTexCoord2f(1.0f, 0.0f); glVertex3f(-1.0f, -1.0f,  1.0f);
        glTexCoord2f(1.0f, 1.0f); glVertex3f(-1.0f,  1.0f,  1.0f);
        glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f,  1.0f, -1.0f);
        glEnd();
        glEndList();
    }
}

//functie die zorgt dat we renderen in selectiemode
void GLWidget::renderInSelectionMode() {
    glInitNames(); //Creates empty stack

    glPushName(1); //Push a name on the stack
    //draw something
    glColor3f(1.0f,0.0f,0.0f);
    glCallList(displayList[0]);
    glPopName(); //pop a name from the stack

    glPushName(2); //Push a name on the stack
    //draw something
    glTranslatef(5.0,0.0,0.0);
    glColor3f(0.0f,1.0f,1.0f);
    glCallList(displayList[0]);
    glPopName(); //Pops a name from the stack
}


void GLWidget::startPicking() {

    GLint viewport[4];

    glSelectBuffer(BUFSIZE,selectBuf);
    glRenderMode(GL_SELECT);

    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();

    glGetIntegerv(GL_VIEWPORT,viewport);
    gluPickMatrix(cursorX,viewport[3]-cursorY,5,5,viewport);
    gluPerspective(45,aspect_ratio,0.1,1000);
    glMatrixMode(GL_MODELVIEW);
    glInitNames();
}

void GLWidget::stopPicking() {

    int hits;

    // restoring the original projection matrix
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
    glMatrixMode(GL_MODELVIEW);
    glFlush();

    // returning to normal rendering mode
    hits = glRenderMode(GL_RENDER);

    // if there are hits process them
    if (hits != 0)
        processHits(hits,selectBuf);
}

void GLWidget::processHits (GLint hits, GLuint buffer[])
{
    unsigned int i, j;
    GLuint names, *ptr, minZ,*ptrNames, numberOfNames;

    printf ("hits = %d\n", hits);
    ptr = (GLuint *) buffer;
    minZ = 0xffffffff;
    for (i = 0; i < hits; i++) {
        names = *ptr;
        ptr++;
        if (*ptr < minZ) {
            numberOfNames = names;
            minZ = *ptr;
            ptrNames = ptr+2;
        }

        ptr += names+2;
    }
    printf ("The closest hit names are ");
    ptr = ptrNames;
    for (j = 0; j < numberOfNames; j++,ptr++) {
        printf ("%d ", *ptr);
    }
    printf ("\n");

}

void GLWidget::mousePressEvent(QMouseEvent * e)
{
    if(e->button() == Qt::LeftButton)
    {
        qDebug("mouse");
        qDebug("%d",QCursor::pos().x());
        this->cursorX = QCursor::pos().x();
        this->cursorY = QCursor::pos().y();
        this->mode = 2;
    }
}

void GLWidget::keyPressEvent( QKeyEvent * e ) {

    double fraction = 0.1f;
    if(e->key() == Qt::Key_Up)
    {
        camPosz += camViewz * fraction;
        camPosx += camViewx * fraction ;
    }
    if(e->key() == Qt::Key_Down)
    {
        camPosz -= camViewz * fraction;
        camPosx -= camViewx * fraction ;
    }
    if(e->key() == Qt::Key_Left)
    {
        camAngle -= 0.05f;
        camViewz = -cos(camAngle);
        camViewx = sin(camAngle);
    }


    if(e->key() == Qt::Key_Right)
    {
        qDebug("cam angle is %f", camAngle);
        camAngle +=0.05f;
        camViewz = -cos(camAngle);
        camViewx = sin(camAngle);
    }
}

#ifndef GLWIDGET_H
#define GLWIDGET_H

#include <QtOpenGL/QGLWidget>
#include <gl/GLU.h>
#include <QImage>
#include <QKeyEvent>
#include <QMouseEvent>
#include <QTimer>
#define BUFSIZE 512

class GLWidget: public QGLWidget
{
    Q_OBJECT

public:
    GLWidget(QWidget *parent = NULL);

private:
    double camPosx,camPosy,camPosz;
    double camUpx,camUpy,camUpz;
    double camViewx,camViewy,camViewz;
    double camAngle;


protected:
    void initializeGL();
    void resizeGL(int width, int height);
    void paintGL();
    void keyPressEvent(QKeyEvent * e);
    void mousePressEvent(QMouseEvent * e);
    QTimer* timer;
    void loadGLTextures();
    GLuint  texture[1];
    GLuint  * displayList;
    void renderInSelectionMode();
    GLvoid buildLists(int aantal);
    void startPicking();
    void stopPicking();
    void processHits (GLint hits, GLuint buffer[]);
    GLuint selectBuf[BUFSIZE];
    GLdouble aspect_ratio;
    int cursorY;
    int cursorX;
    int mode;
};

#endif // GLWIDGET_H

this is the normal view without clicking once http://imageshack.us/photo/my-images/233/31536776.png/

then when clicked it keeps getting in startpicking and stop picking and gives a black screen http://imageshack.us/photo/my-images/13/14180995.png/

and then after closing the window so it stops running it gives me this output http://imageshack.us/photo/my-images/861/13486229.png/

c++ qt opengl picking
Original Q&A
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Could be wrong, but it looks like you aren't resetting your mode-variable back to 1 after the first mouseclick anywhere, so the start- & stopPicking-methods are called on each frame from the first click onwards. If that's not the case, then you probably have some problem with gl-states/matrices not all being reset correctly after the picking.

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