Color picking with QT in openGL

[browser90]

Hi guys,

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\n",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 ("\n");

}

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");

}

@

Kind regards,

[minimoog77]

How is the projection matrix set?

[browser90]

Like this:
@
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();

}

thanks for the help.
@

[minimoog77]

I can't see what's wrong. Everything looks fine....

[browser90]

that's weird, because I always get a black screen?

[minimoog77]

If you get always black screen then color you 'pick' will be always black.

Try changing the camera position.