Verily does qtmux do this:


  /* bring frame numerator into a range that ensures both reasonable resolution
   * as well as a fair duration */
  rate = adjust_rate (framerate_num);

after having pulled framerate_num from caps. adjust_rate is this:

/* scale rate up or down by factor of 10 to fit into [1000,10000] interval */
static guint32
adjust_rate (guint64 rate)
{
  while (rate >= 10000)
    rate /= 10;

  while (rate < 1000)
    rate *= 10;

  return (guint32) rate;
}

Negative numerators are excluded by caps. Positive numerators work fine here. But numerators of 0, on the other hand, do not :)