cannot use clock() for realtime graphical simulations

[edward hage <edha xs4all nl>]

Hello,

I have a simulation running in a drawing_area, I start this simulation with 
gtk_timeout_add (50, Repaint, da).

gint Repaint (gpointer data)
{
  GtkWidget *da = (GtkWidget *) data;
  GdkRectangle update_rect;
  static double t = 0.0;

etc. etc. define graphics in pixmap and then repaint the window with the pixmap.

}


To get the timing I have defined a static double t = 0.0;
which I let count with t +=t + 0.050;
Then I can look up the correct value in a table for the given time.

This works okay, well the block move but not exactly at the right time. It does not work 
very accurate because I assume the function is called every 50 ms, but when I move my 
mouse around then that can not be quaranteed. It slows down the simulation.

My idea was to overcome this problem by calling clock() (using <time.h>) in Repaint. This 
does not work properly. The time seems to freeze, and only when I move my mouse around or 
drag the window the time is updated.

Why does clock() not work properly in this case and how can I overcome this problem ? Does 
anybody have experience with graphical simulations?

The source looks like this (inside Repaint)

static double t = 0;
static double tstart;

ClockResult = clock();
if (t==0)  tstart = (double) ClockResult / (double) CLOCKS_PER_SEC;
t = (double) ClockResult  / (double) CLOCKS_PER_SEC - tstart + 0.00001;


Thank you in advance for your help,
Edward