/* Fix defaults for scale and offset: */
    provide_scale_offset_defaults(o->op, o->squared, o->verbose, &(o->scale), &(o->offset));
    
void provide_scale_offset_defaults
  ( image_window_op_t op, 
    bool_t squared, 
    bool_t verbose, 
    double *scaleP, 
    double *offsetP
  );
  /* If any of {*scaleP} and {*offsetP} is {NAN}, sets it to the default most appropriate
    for the given operator {op} and the given {squared} option. */

void provide_scale_offset_defaults
  ( image_window_op_t op, 
    bool_t squared, 
    bool_t verbose, 
    double *scaleP, 
    double *offsetP
  )
  { 
    /* Determine the natural range {[lo_hi]} of the operator: */
    double lo, hi;
    image_window_op_get_range(op, squared, &lo, &hi);
    if (verbose)
      { fprintf(stderr, "unscaled operator range = [ %24.16f  _ %24.16f]\n", lo, hi); } 
    assert(lo < hi);
    
    double scale = (*scaleP);
    double offset = (*offsetP);
    if (isnan(scale) && isnan(offset))
      { /* Map natural range to {[0_1]}: */
        scale = 1.0/(hi - lo);
        offset = 0.5 - scale*(hi + lo)/2;
      }
    else if (isnan(scale))
      { double rad = fmax(hi, -lo);
        scale = 0.5/rad;
      }
    else if (isnan(offset))
      { double ctr = scale*(hi + lo)/2;
        offset = 0.5 - ctr;
      }
    if (verbose)
      { fprintf(stderr, "using scale = %24.16f  offset = %24.16f\n", scale, offset); } 
    
    (*scaleP) = scale;
    (*offsetP) = offset;
  }