pendulum-3.c

#include <gtk/gtk.h>
#include <math.h>

#define WIDTH 320
#define HEIGHT 240
#define GAP (HEIGHT / 20)
#define r GAP
#define visual_length (HEIGHT - 2 * GAP - r)

#define phi0 (-0.5)
#define omega0 0.0
#define t0 0.0
#define g 9.81
#define l 1.0
#define dt 0.02

double t = t0;
double phi_with_sin = phi0;
double omega_with_sin= omega0;
double phi_without_sin = phi0;
double omega_without_sin= omega0;

void draw_pendulum (cairo_t *c, double phi, GdkRGBA *colour)
{
  int x = WIDTH / 2 + visual_length * sin (phi);
  int y = GAP + visual_length * cos (phi);

  gdk_cairo_set_source_rgba (c, colour);
  cairo_move_to (c, WIDTH / 2, 10);
  cairo_line_to (c, x, y);
  cairo_stroke (c);
  cairo_arc (c, x, y, r, 0, 2 * G_PI);
  cairo_fill (c);
}

gboolean draw (GtkWidget *widget, cairo_t *c, gpointer data)
{
  GdkRGBA blue   = { 0.0, 0.5, 1.0, 1.0 };
  GdkRGBA orange = { 1.0, 0.5, 0.0, 1.0 };
  GdkRGBA green  = { 0.0, 0.5, 0.0, 1.0 };

  double A = phi0;
  double B = 0.5 * M_PI;  /* 90° */
  double phi_analytic = A * sin (sqrt (g / l) * t + B);

  draw_pendulum (c, phi_with_sin, &blue);
  draw_pendulum (c, phi_without_sin, &orange);
  draw_pendulum (c, phi_analytic, &green);

  return FALSE;  /* TRUE to stop other handlers from being invoked for the event.
                    FALSE to propagate the event further. */
}

gboolean timer (GtkWidget *widget)
{
  t += dt;
  phi_with_sin += omega_with_sin * dt;
  omega_with_sin += - dt * g / l * sin (phi_with_sin);
  phi_without_sin += omega_without_sin * dt;
  omega_without_sin += - dt * g / l * phi_without_sin;

  gtk_widget_queue_draw_area (widget, 0, 0, WIDTH, HEIGHT);
  g_timeout_add (50, (GSourceFunc) timer, widget);
  return FALSE;
}

int main (int argc, char **argv)
{
  gtk_init (&argc, &argv);

  GtkWidget *window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_widget_show (window);
  gtk_window_set_title (GTK_WINDOW (window), "Hello");
  g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);

  GtkWidget *drawing_area = gtk_drawing_area_new ();
  gtk_widget_show (drawing_area);
  gtk_container_add (GTK_CONTAINER (window), drawing_area);
  gtk_widget_set_size_request (drawing_area, WIDTH, HEIGHT);
  g_signal_connect (drawing_area, "draw", G_CALLBACK (draw), NULL);

  g_timeout_add (50, (GSourceFunc) timer, drawing_area);

  gtk_main ();
  return 0;
}