Basics eluding me

OpenGL OpenGL: Basic Coding
1

Most of my work with OpenGL has been using orthographic projection. Now I need perspective projection, and things rather mysteriously aren’t working, and I’m not sure why.

I have some dummy code set up just to test things out:


    glViewport(0,0,500,500);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45,1,0,10);
    //glOrtho(-1,1,-1,1,0,2);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(1,0,1,
              0,0,0,
              0,0,1);

    glClearColor(1,0,0,1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_DEPTH_TEST);

    glPointSize(5);
    glColor3d(0,1,0);
    glBegin(GL_POINTS);
    glVertex3d(0,0,0);
    glVertex3d(.1,0,0);
    glVertex3d(0,.1,0);
    glEnd();

When I use the glOrtho line, it works fine, and I get three green dots on a red background. But when I switch to the gluPerspective instead, the dots don’t show up?

Curiously, one of the three dots (the one in the center) does show up if I comment the gluLookAt(). I think this is because (0,0,0) is right at the root of the frustum and this is also the untransformed camera location.

Any suggestions?

2

Ah-ha—I knew that as soon as I posted I’d solve my own problem…

When you consider that gluPerspective() is probably just called glFrustum() internally, it becomes clear that the math won’t work unless the near clipping distance is > 0.

3

I do that a lot. Your mind works too fast, so you have to slow down and type it out and re-read it. Then you’re like… oya.