double yfrac = ypixfrac / im->ysize;
im->minval = pow(10, log10(im->minval) - yfrac * log10_range);
im->maxval = pow(10, log10(im->maxval) - yfrac * log10_range);
double yfrac = ypixfrac / im->ysize;
im->minval = pow(10, log10(im->minval) - yfrac * log10_range);
im->maxval = pow(10, log10(im->maxval) - yfrac * log10_range);