Question

I’m using QGlWidget to draw a couple of points. The problems I’m having is that I seem to fail to set up the perspective properly and establish correct view on the points. I must be misunderstanding the coordinates somewhere or doing something else stupid, but after reading a bunch of guides and tutotrials I’m still stuck. The screen is black, no points. Here’s the code:

 void CGLWidget::initializeGL()
 {
    glEnable(GL_DEPTH_TEST);
    glDisable(GL_ALPHA_TEST);
    glDisable(GL_BLEND);
    glEnable(GL_POINT_SPRITE);
    glClearColor(0, 0, 0, 1);
    assert (glGetError() == GL_NO_ERROR);
 }

void CGLWidget::resizeGL(int w, int h)
 {
    glViewport(-w/2, -h/2, w/2, h <= 0 ? 1 : h/2);

    glMatrixMode(GL_PROJECTION); //Switch to setting the camera perspective
    //Set the camera perspective
    glLoadIdentity(); //Reset the camera
    gluPerspective(80.0,   //The camera FoV
    w/(double)h, //The width-to-height ratio
    1,                   //The near z clipping coordinate
    100);                //The far z clipping coordinate
 }

 void CGLWidget::paintGL()
 {
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(0, 0, 5, 0, 0, 0, 0, 1, 0);
    glLoadIdentity();

    glColor3i(255, 255, 255);
    glBegin(GL_POINTS);
        glVertex3d(0,0, -2);
        glVertex3d(0,0, -3);
        glVertex3d(0,0, +3);
        glVertex3d(0,0, 0);
        glVertex3f(-0.75f, -0.25f, -5.0f);
    glEnd();

    assert (glGetError() == GL_NO_ERROR);
 }

I’ve tried manipulating z coordinate of the “eye” in gluLookAt to no avail, so I must be getting something else wrong.

Answer 1

I think what you’ve got there looks mostly correct, though your glViewport parameters look wrong. glViewport is supposed to be (left, bottom, width, height), but you’re using something else.

Set glViewport to glViewport(0,0,width,height);.

For more explanation:

After transformation by modelViewProjection matrix (and perspective divide), all coordinates lie in what’s known as Normalized Device Coordinates (abbreviated as NDC). NDC ranges from -1 to 1 on each axis, which puts (0,0,0) right in the center of the viewing region.

When your point lies directly in front of the camera, this gets transformed to xy (0,0) in normalized device coordinates.

If you look at the formulas on glViewport, these map NDC to the actual pixels on the screen.

So if you supply (0,0,1024,768) as the parameters, it gets mapped as the following:

screen X = ( Xnd + 1 ) ( width / 2) + x;
screen Y = ( Ynd + 1 ) ( height / 2) + y;

Substituting our glViewport values:

screen X = ( 0 + 1 ) ( 1024 / 2) + 0;
screen Y = ( 0 + 1 ) ( 768  / 2) + 0;

screen X = ( 1 ) ( 1024 / 2) ;
screen Y = ( 1 ) ( 768  / 2) ;

screen X = ( 1 ) ( 512 ) ;
screen Y = ( 1 ) ( 384 ) ;

screen X =  512 ; //center of screen X
screen Y =  384 ; //center of screen Y