#!/usr/local/bin/wish
proc xydist {x1 y1 x2 y2} {
   return [expr {hypot(($x1 - $x2),($y1 - $y2))}]
}
proc line2spline {points} {
   lappend results [lindex $points 0] [lindex $points 1]
   for {set i 2} {$i < [expr [llength $points] - 2]} {incr i 2} {
      set h [expr $i - 1]
      set j [expr $i + 1]
      set x1 [lindex $points [expr $i - 2]]
      set y1 [lindex $points [expr $i - 1]]
      set x2 [lindex $points [expr $i + 0]]
      set y2 [lindex $points [expr $i + 1]]
      set x3 [lindex $points [expr $i + 2]]
      set y3 [lindex $points [expr $i + 3]]
      set xm1 [expr ($x1 + $x2)*0.5]
      set ym1 [expr ($y1 + $y2)*0.5]
      set xm2 [expr ($x3 + $x2)*0.5]
      set ym2 [expr ($y3 + $y2)*0.5]
      set dx [expr $x3 - $x1]
      set dy [expr $y3 - $y1]
      set dist1 [xydist $xm1 $ym1 $x2 $y2]
      set dist2 [xydist $xm2 $ym2 $x2 $y2]
      if {$dist1 < $dist2} {
         set dist [expr $dist1 * 0.5]
      } else {
         set dist [expr $dist2 * 0.5]
      }
      set angle atan2($dy,$dx)
      set deltax [expr $dist * cos($angle)]
      set deltay [expr $dist * sin($angle)]
      lappend results [expr $x2 - $deltax] [expr $y2 - $deltay] [expr $x2 + $deltax] [expr $y2 + $deltay]
   }
   lappend results [lindex $points end-1] [lindex $points end]
   return $results
}

proc poly2spoly points {
   lappend points [lindex $points 0] [lindex $points 1] [lindex $points 2] [lindex $points 3]
   for {set i 2} {$i < [expr [llength $points] - 2]} {incr i 2} {
      set h [expr $i - 1]
      set j [expr $i + 1]
      set x1 [lindex $points [expr $i - 2]]
      set y1 [lindex $points [expr $i - 1]]
      set x2 [lindex $points [expr $i + 0]]
      set y2 [lindex $points [expr $i + 1]]
      set x3 [lindex $points [expr $i + 2]]
      set y3 [lindex $points [expr $i + 3]]
      set xm1 [expr ($x1 + $x2)*0.5]
      set ym1 [expr ($y1 + $y2)*0.5]
      set xm2 [expr ($x3 + $x2)*0.5]
      set ym2 [expr ($y3 + $y2)*0.5]
      set dx [expr $x3 - $x1]
      set dy [expr $y3 - $y1]
      set dist1 [xydist $xm1 $ym1 $x2 $y2]
      set dist2 [xydist $xm2 $ym2 $x2 $y2]
      if {$dist1 < $dist2} {
         set dist [expr $dist1 * 0.5]
      } else {
         set dist [expr $dist2 * 0.5]
      }
      set angle atan2($dy,$dx)
      set deltax [expr $dist * cos($angle)]
      set deltay [expr $dist * sin($angle)]
      lappend results [expr $x2 - $deltax] [expr $y2 - $deltay] [expr $x2 + $deltax] [expr $y2 + $deltay]
   }
   return $results
}