something like this
Code:
void init(GLint w,GLint h){
glViewport (0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluOrtho2D (0.0, (GLdouble) w, 0.0, (GLdouble) h);
}
I don't really understand your custome coordinates question. I think you mean how do I know where the points are. Well, if you look at the GLortho2D function you will notice that the w axis goes from 0 to w(usually 640, 800, 1024), the same for h. Then you just pick points from there. Is that what you mean?