!------------------------------------------------------------------------ 
! LEVELS.GRAPHIC 
! Procedure to define contour levels for multichannel image 
! Variables defined in DEFINE.GRAPHIC 
! R.Lucas 10-may-1990 
!------------------------------------------------------------------------ 
on error return 
define real range expo mant 
define integer n1 n2 
! 
if spacing.lt.0 then 
  return 
endif 
if (g_extrema.eq.0) then        ! means no extrema section 
  header /extrema 
endif 
if (spacing.eq.0) then 
  let range = log10(g_max-g_min) 
  let expo = int(range+20.)-21 
  let mant = range-expo-1 
  if (mant.lt.0.25) then 
    let true_spacing 10**expo 
  else if (mant.lt.0.60) then 
    let true_spacing 2.*10**expo 
  else if (mant.lt.0.90) then 
    let true_spacing = 5.*10**expo 
  else 
    let true_spacing = 10**(expo+1) 
  endif 
else 
  let true_spacing = spacing 
endif 
let n1 max(-20,nint(g_min|true_spacing)) 
let n2 min(+20,nint(g_max|true_spacing)) 
if (n1.lt.0.and.n2.gt.0) then 
  levels true_spacing*n1 to -0.95*true_spacing by true_spacing true_spacing to true_spacing*(n2+0.05) by true_spacing 
else 
  let n1 max(n1,1) 
  levels true_spacing*n1 to true_spacing*(n2+0.05) by true_spacing 
endif