Setting the coordinate system for drawing in OpenGL

Question

I just started reading initial chapters of Blue book and got to understand that the projection matrix can be used to modify the mapping of our desired coordinate system to real screen coordinates. It can be used to reset the coordinate system and change it from -1 to 1 on left, right, top and bottom by the following (as an example)

glMatrixMode(GL_PROJECTION);
glLoadIdentity(); //With 1's in the diagonal of the identity matrix, the coordinate system is rest from -1 to 1 (and the drawing should happen then inside those coordinates which be mapped later to the screen)

Another example: (Width: 1024, Height: 768, Aspect Ratio: 1.33) and to change the coordinate system, do:

glOrtho (-100.0 * aspectRatio, 100.0 * aspectRatio, -100.0, 100.0, 100.0, 1000.0);

I expected the coordinate system for OpenGL to change to -133 on left, 133 on right, -100 on bottom and 100 on top. Using these coordinates, I understand that the drawing will be done inside these coordinate and anything outside these coordinates will be clipped.

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
    glOrtho(-100 * aspectRatio, 100 * aspectRatio, -100, 100, 100, 1000);
glMatrixMode(GL_MODELVIEW);
    glRectf(-50.0, 50.0, 200, 100);

However, the above command doesn't give me any output on the screen. What am I missing here?

Answer 1

I see two problems here:

1. The rect should not by show at all, since glRectf() draws at depth z=0, but you set up your orthorgraphic projection to cover the z range [100,1000], so the object lies before the near plane and should be clipped away.
2. You do not specifiy waht MODELVIEW matrix you use. In the comments, you mention that the object does show up, but not in the place where you expect it. This also violates my first point, but could be explained if the ModelView matrix is not identity.

So I suggest to first use a different projection matrix with like glOrtho(..., -1.0f, 1.0f); so that z=0 is actually covered, and second insert a glLoadIdentity() call after the glMatrixMode(GL_MODELVIEW) in the above code.

Another approach would be to keep the glOrtho() as it is and to specify a translation matrix wich moves the rect somewhere between z=100 and z=1000.