color picking opengl with QT framework

I’ve been reading tutorials about picking and that color picking is far the most popular and simplest form.

Tried some tutorials with snowmans as example but it doesn’t work for me. When I run the program , it gives me just a black sceen without anything draw on it. When I click a couple of times nothing happens except when I close the window then it sais “you haven’t clicked on a snowman”.

Don’t know what is wrong with it can someone help me?

  void GLWidget::paintGL() {
        glEnable(GL_DEPTH_TEST);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    
        glLoadIdentity();
    
        gluLookAt(camPosx ,camPosy ,camPosz,
                  camPosx + camViewx,camViewy,camPosz + camViewz,
                  camUpx, camUpy, camUpz );
    
        draw(); //draw the normal scene
     
        if (mode == SELECT)
            drawPickingMode();
        else
            drawPickingMode();
    
        if (mode == SELECT) {
            processPick();
            mode = RENDER;
        }
        else
            QGLWidget::swapBuffers();
    
        // restore current matrix
        glMatrixMode( GL_MODELVIEW );
        glPopMatrix( );
    
    }
    
    void GLWidget::mousePressEvent(QMouseEvent * e)
    {
        if(e->button() == Qt::LeftButton)
        {
            qDebug("mouse");
            qDebug("%d %d",QCursor::pos().x(),QCursor::pos().y());
            this->cursorX = QCursor::pos().x(); // set x and y cord from mouse
            this->cursorY = QCursor::pos().y();
            mode = SELECT; // set the mode to select
        }
    }
    
    void GLWidget::draw() {
    
    // Draw ground
        glColor3f(0.9f, 0.9f, 0.9f);
        glBegin(GL_QUADS);
            glVertex3f(-100.0f, 0.0f, -100.0f);
            glVertex3f(-100.0f, 0.0f,  100.0f);
            glVertex3f( 100.0f, 0.0f,  100.0f);
            glVertex3f( 100.0f, 0.0f, -100.0f);
        glEnd();
    
    // Draw 4 Snowmen
    
        glColor3f(1.0f, 1.0f, 1.0f);
    
        for(int i = 0; i < 2; i++)
            for(int j = 0; j < 2; j++) {
                glPushMatrix();
                glTranslatef(i*3.0,0,-j * 3.0);
                glColor3f(1.0f, 1.0f, 1.0f);
                glCallList(snowman_display_list);
                glPopMatrix();
            }
    
    }
    
    void GLWidget::processPick ()
    {
        GLint viewport[4];
        GLubyte pixel[3];
    
        glGetIntegerv(GL_VIEWPORT,viewport);
    
        glReadPixels(cursorX,viewport[3]-cursorY,1,1,
            GL_RGB,GL_UNSIGNED_BYTE,(void *)pixel);
    
        printf("%d %d %d
",pixel[0],pixel[1],pixel[2]);
        if (pixel[0] == 255)
          printf ("You picked the 1st snowman on the 1st row");
        else if (pixel[1] == 255)
          printf ("You picked the 1st snowman on the 2nd row");
        else if (pixel[2] == 255)
          printf ("You picked the 2nd snowman on the 1st row");
        else if (pixel[0] == 250)
          printf ("You picked the 2nd snowman on the 2nd row");
        else
           printf("You didn't click a snowman!");
      printf ("
");
    
    }
    
    void GLWidget::drawPickingMode() {
    
    // Draw 4 SnowMen
    
    
        glDisable(GL_DITHER);
        for(int i = 0; i < 2; i++)
               for(int j = 0; j < 2; j++) {
            glPushMatrix();
    
    // A different color for each snowman
    
            switch (i*2+j) {
                case 0: glColor3ub(255,0,0);break;
                case 1: glColor3ub(0,255,0);break;
                case 2: glColor3ub(0,0,255);break;
                case 3: glColor3ub(250,0,250);break;
            }
    
            glTranslatef(i*3.0,0,-j * 3.0);
            glCallList(snowman_display_list);
            glPopMatrix();
           }
        glEnable(GL_DITHER);
    }
    
    void GLWidget::initializeGL() {
    
        loadGLTextures();
        //LoadXml();
        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);
        qDebug("Init");
    }


Projection matrix is set as follows:

    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();
    }

Kind regards,

Tried some tutorials with snowmans as example but it doesn’t work for me. When I run the program , it gives me just a black sceen without anything draw on it. When I click a couple of times nothing happens except when I close the window then it sais “you haven’t clicked on a snowman”.

Don’t know what is wrong with it can someone help me?

Have you tried running the Santa claus program; you know the one where he drops all the presents?

Seriously? You’ve cut and pasted some random code from a web site (no refernece given) and say it doesn’t even run properly then expect us to tell you what’s wrong? Think about it.