Cannot see object when camera is placed perfectly in front

Question

I am trying to run a simple example of OpenGL and I am facing the following issue. When I place the camera perfectly in front on my object, the object is not displayed at all. But when I move the camera (even by 0.00001) the object is displayed.

class GLWidget : public QGLWidget{
    void initializeGL(){

    glEnable(GL_DEPTH_TEST);

    update_timer_ = new QTimer(this);
    connect(update_timer_, SIGNAL(timeout()), this, SLOT(update()));
    update_timer_->start(0.017);
}

/// @note camera decides renderer size
void resizeGL(int width, int height){
    if (height==0) height=1;
    glViewport(0,0,width,height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,0.1f,100.0f);
 }

void paintGL(){
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity( );

    gluLookAt(0,0, 10.0,0,0,0,0,0,1);
    glBegin(GL_QUADS);

    glColor3ub(255,0,0);

    glVertex3d(1,1,1);

    glVertex3d(1,1,-1);
    glVertex3d(-1,1,-1);
    glVertex3d(-1,1,1);

    glColor3ub(0,255,0);
    glVertex3d(1,-1,1);
    glVertex3d(1,-1,-1);
    glVertex3d(1,1,-1);
    glVertex3d(1,1,1);

    glColor3ub(0,0,255);
    glVertex3d(-1,-1,1);
    glVertex3d(-1,-1,-1);
    glVertex3d(1,-1,-1);
    glVertex3d(1,-1,1);

    glColor3ub(255,255,0);
    glVertex3d(-1,1,1);
    glVertex3d(-1,1,-1);
    glVertex3d(-1,-1,-1);
    glVertex3d(-1,-1,1);

    glColor3ub(0,255,255);
    glVertex3d(1,1,-1);
    glVertex3d(1,-1,-1);
    glVertex3d(-1,-1,-1);
    glVertex3d(-1,1,-1);

    glColor3ub(255,0,255);
    glVertex3d(1,-1,1);
    glVertex3d(1,1,1);
    glVertex3d(-1,1,1);
    glVertex3d(-1,-1,1);

    glEnd();

    glFlush();
}
private:
    QTimer* update_timer_;
};

int main(int argc, char *argv[]){
    QApplication app(argc, argv);
    GLWidget widget;
    widget.resize(800,600);
    widget.show();
    return app.exec();
}

In this case I cannot see the object but if I use:

gluLookAt(0,0.00001, 10.0,0,0,0,0,0,1);

In this case, I can see the cube (And it looks perfectly in the middle of the screen).

Did I forgot to enable something in OpenGL or is there something wrong in the way I am using the gluLookAt function? Thanks in advance.

Answer 1

gluLookAt takes 3 parameters:

• position of camera
• point to look at
• upvector

The upvector must not be parallel to the direction in which you're looking (aka. 2nd - 1st argument) which is the case in your first call.

Your camera is at {0,0,10} and you look at {0,0,0} so you look in -z direction. Your upvector is {0,0,1} which is the same direction as you're facing (*-1).

Try gluLookAt(0.0, 0.0, 10.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);