#include <GL/glut.h>

float X = 0, Y = 25;

void draw_line(float xa, float ya, float xb, float yb)
{
    glPushAttrib(GL_ALL_ATTRIB_BITS);
    glColor3ub(128, 128, 128);
    glLineWidth(2);
    glEnable(GL_LINE_STIPPLE);
    glLineStipple(1, 0xFF00);
    glBegin(GL_LINES);
        glVertex2f(xa, ya);
        glVertex2f(xb, yb);
    glEnd();
    glPopAttrib();
}

void draw_square(float x, float y)
{
    glPushAttrib(GL_ALL_ATTRIB_BITS);
    glColor3ub(255, 0, 0);
    glLineWidth(1);
    glRectf(x - 25, y - 25, x + 25, y + 25);
    glPopAttrib();
}

void display()
{
    // clear screen
    glClearColor(1, 1, 1, 0);
    glClear(GL_COLOR_BUFFER_BIT);

    // draw grey line, 2px width, stippled
    draw_line(-240, 0, 240, 0);

    // draw red square
    draw_square(X, Y);

    glutSwapBuffers();
}

void normal_keys(unsigned char key, int x, int y)
{
    if(key == 'a') X -= 4;
    if(key == 'd') X += 4;
    glutPostRedisplay();
}

void timer(int)
{
    glutPostRedisplay();
    glutTimerFunc(1000.0 / 60.0, timer, 0);
}

int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitWindowPosition(200, 50);
   glutInitWindowSize(480, 360);
   glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
   glutCreateWindow("OpenGL");
   gluOrtho2D(-240, 240, -180, 180);
   glutDisplayFunc(display);
   glutKeyboardFunc(normal_keys);
   timer(0);
   glutMainLoop();
   return 0;
}