void display() {
    glEnable(GL_DEPTH_TEST);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    //gluOrtho2D(0, 800, 0, 800);
    glLineWidth(10.0f);

    glBegin(GL_LINES);
    glColor3f(1.0, 0.0, 0.0);
    glVertex2f(-1.0f, -1.0f);
    glVertex2f(1.0f, 1.0f);

    glColor3f(0.0, 1.0, 0.0);
    glVertex2f(-1.0f, 1.0f);
    glVertex2f(1.0f, -1.0f);

 

    glEnd();

    gluOrtho2D(0, 800, 0, 800);
    glBegin(GL_LINES);
    glColor3f(1.0, 1.0, 0.0);
    glVertex2i(400, 800);
    glVertex2i(400, 0);
    glEnd();
    glFlush();
    glutSwapBuffers();

}

void main(int argc, char** argv) {
    glutInit(&argc, argv);
//    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
    glutInitWindowSize(800, 800);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("Points");
    //glutDisplayFunc(display);

    glutDisplayFunc(display);

    glutMainLoop();
}

#endif

//https://elixir.bootlin.com/linux/v5.6/source

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

After initialization, the entire coordinate plane will be initialized to [-1.0, 1.0] [-1.0, 1.0]

and

gluOrtho2D(0, 800, 0, 800)

Will initialize the range of the coordinate plane to [0, 800] [0, 800]

The results are shown in the figure.