Drawing 2D Points on a 3D Context

Question

private void init_video () {
    uint32 video_flags = SurfaceFlag.SWSURFACE | SurfaceFlag.OPENGL;
    screen = Screen.set_video_mode (SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, video_flags);
    if (screen == null) stderr.printf ("Could not set video mode.\n");

    glClearColor (0.0f, 0.0f, 0.0f, 1.0f);
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    glFrustum (-2.0f, 2.0f, -2.0f, 2.0f, 1.0f, 300.0f);
    glMatrixMode (GL_MODELVIEW);
    glEnable (GL_DEPTH_TEST);
    glEnable (GL_TEXTURE_2D);

    SDL.WindowManager.set_caption ("Title", "Title");
}

That is the function where I set up the camera for a game I'm working on. I have this function to draw a cube:

public void draw () {
    glLoadIdentity ();
    glTranslatef (x, y, z);
    glScalef (size, size, size);
    glRotatef (x_angle, 1.0f, 0.0f, 0.0f);
    glRotatef (y_angle, 0.0f, 1.0f, 0.0f);
    glRotatef (z_angle, 0.0f, 0.0f, 1.0f);
    glBegin (GL_QUADS);

    /* Front face */
    glColor3f (1.0f, 0.0f, 0.0f);
    glVertex3f (0.5f, 0.5f, 0.5f);
    glVertex3f (-0.5f, 0.5f, 0.5f);
    glVertex3f (-0.5f, -0.5f, 0.5f);
    glVertex3f (0.5f, -0.5f, 0.5f);

    /* Left face */
    glColor3f (0.0f, 1.0f, 0.0f);
    glVertex3f (-0.5f, 0.5f, 0.5f);
    glVertex3f (-0.5f, -0.5f, 0.5f);
    glVertex3f (-0.5f, -0.5f, -0.5f);
    glVertex3f (-0.5f, 0.5f, -0.5f);

    /* Back face */
    glColor3f (0.0f, 0.0f, 1.0f);
    glVertex3f (0.5f, 0.5f, -0.5f);
    glVertex3f (-0.5f, 0.5f, -0.5f);
    glVertex3f (-0.5f, -0.5f, -0.5f);
    glVertex3f (0.5f, -0.5f, -0.5f);

    /* Right face */
    glColor3f (1.0f, 1.0f, 0.0f);
    glVertex3f (0.5f, 0.5f, 0.5f);
    glVertex3f (0.5f, -0.5f, 0.5f);
    glVertex3f (0.5f, -0.5f, -0.5f);
    glVertex3f (0.5f, 0.5f, -0.5f);

    /* Top face */
    glColor3f (0.0f, 1.0f, 1.0f);
    glVertex3f (0.5f, 0.5f, 0.5f);
    glVertex3f (-0.5f, 0.5f, 0.5f);
    glVertex3f (-0.5f, 0.5f, -0.5f);
    glVertex3f (0.5f, 0.5f, -0.5f);

    /* Bottom face */
    glColor3f (1.0f, 0.0f, 1.0f);
    glVertex3f (0.5f, -0.5f, 0.5f);
    glVertex3f (-0.5f, -0.5f, 0.5f);
    glVertex3f (-0.5f, -0.5f, -0.5f);
    glVertex3f (0.5f, -0.5f, -0.5f);
    glEnd ();
}

How can I draw points as if it was a 2D Context? Any ideas?

Answer 1

There is no thing as a 2D context in OpenGL. Everything is 3d. But you can emulate 2d using a parallel projection with glOrtho

// Setup Orthogonal projection to window coordiantes
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, WindowWidth-1, 0, WindowHeight-1, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

// draw quad in screen coodinates
glBegin(GL_QUADS);
    glColor3f(1.0f, 1.0f, 1.0f);
    glVertex2i(10, 10);
    glVertex2i(10, 80);
    glVertex2i(80, 80);
    glVertex2i(80, 10);
glEnd (); 

glVertex2 is like glVertex3 with the third coorinate set to 0.

With OpenGL 4 all the above commands are removed. But the glOrtho documentation shows you the matrix you need to create to set up rendering in windows coordinates.