Here is a simple code:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glOrtho(0, 800, 0, 480, -1, 1);
glRasterPos(0,0,0);
glBegin(GL_LINES);
glVertex2i(10, 10);
glVertex2i(100, 10);
glEnd();
These coordinates should be equivalent to screen ones and the line should be a simple horizontal from x=10 to x=100 screen coord. Unfortunately from time to time the line is drawn to x=99 or x=101, depending on the y position (!!!). Any idea, please ??