

if (ctime >= start) {
  if (ctime >= mid) {
    if (ctime >= end) { /* after the end of interpolation (src F alone) */
    
    }
    else { /* interpolation of b*a to b */

    }
  }
  else { /* interpolation of a to a*b */

  }
}
else { /* before the beginning of interpolation (src f alone) */

}