This will become a "real" page - for now it's just a part of chat transcript (2003-07-09), so the info doesn't get lost... [RS]

 suchenwi	But one basic question: in barcodes like Code 128 (EAN?), each digit is a black and a white bar, the added thickness of which two is constant, right?
 kroc	Richard: each digit (or letter for some barcodes) is a set of bars and spaces...
 suchenwi	How many?
 kroc	it depends of barcode type.
 kroc	Six per char for Code 128 for a total of 11 units.
 kroc	 Here is a sample :
	B == bar and S == space
	digit represents width.
	char == BSBSBS
	a = 121124
	^ =  431111
	etc..
 kroc	 So, graphic representation will be:
	a = "X  X XX    "
	^ = "XXXX   X X "
 kroc	To build the barcode, you join all char bars and spaces. It also need start and end symbol, sometimes an ending control char, and must respect minimum width requirements to be read.
 suchenwi	Ah - and for a quick shot one could draw canvas lines for the bars...
 suchenwi	Check digit too, if I remember right.
 kroc	 It's more accurate to draw on a photo image like that :
	foreach {bar space} $code {
	        set larg [expr ($bar * $unit)+$debut]
	        img-$nbimg put #000000 -to $debut 0 $larg $fin
	        set debut [expr ($space*$unit) + $larg]
	}
 suchenwi	Do you have a link to specifications of the bar-space width patterns?
 kroc	mb: to answer your question, ean8 and Code 128 aren't the same thing at all.
    * kroc checking... 
 kroc	http://www.ean-int.org
 kroc	http://www.barcodeisland.com/symbolgy.phtml

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Here is a proc that returns Code 128 bars and spaces for a given string :

 proc String2C128 { signes } {
    set code ""
    foreach part [split $signes {}] {
        switch -- $part {
            \x20 {append code "212222" ; # 0 }
            \x21 {append code "222122"}
            \x22 {append code "222221"}
            \x23 {append code "121223"}
            \x24 {append code "121322"}
            \x25 {append code "131222"}
            \x26 {append code "122213"}
            \x27 {append code "122312"}
            \x28 {append code "132212"}
            \x29 {append code "221213"}
            \x2A {append code "221312" ; # 10 }
            \x2B {append code "231212"}
            \x2C {append code "112232"}
            \x2D {append code "122132"}
            \x2E {append code "122231"}
            \x2F {append code "113222"}
            \x30 {append code "123122"}
            \x31 {append code "123221"}
            \x32 {append code "223211"}
            \x33 {append code "221132"}
            \x34 {append code "221231" ; # 20 }
            \x35 {append code "213212"}
            \x36 {append code "223112"}
            \x37 {append code "312131"}
            \x38 {append code "311222"}
            \x39 {append code "321122"}
            \x3A {append code "321221"}
            \x3B {append code "312212"}
            \x3C {append code "322112"}
            \x3D {append code "322211"}
            \x3E {append code "212123" ; # 30 }
            \x3F {append code "212321"}
            \x40 {append code "232121"}
            \x41 {append code "111323"}
            \x42 {append code "131123"}
            \x43 {append code "131321"}
            \x44 {append code "112313"}
            \x45 {append code "132113"}
            \x46 {append code "132311"}
            \x47 {append code "211313"}
            \x48 {append code "231113" ; # 40 }
            \x49 {append code "231311"}
            \x4A {append code "112133"}
            \x4B {append code "112331"}
            \x4C {append code "132131"}
            \x4D {append code "113123"}
            \x4E {append code "113321"}
            \x4F {append code "133121"}
            \x50 {append code "313121"}
            \x51 {append code "211331"}
            \x52 {append code "231131" ; # 50 }
            \x53 {append code "213113"}
            \x54 {append code "213311"}
            \x55 {append code "213131"}
            \x56 {append code "311123"}
            \x57 {append code "311321"}
            \x58 {append code "331121"}
            \x59 {append code "312113"}
            \x5A {append code "312311"}
            \x5B {append code "332111"}
            \x5C {append code "314111" ; # 60 }
            \x5D {append code "221411"}
            \x5E {append code "431111"}
            \x5F {append code "111224"}
            \x60 {append code "111422"}
            \x61 {append code "121124"}
            \x62 {append code "121421"}
            \x63 {append code "141122"}
            \x64 {append code "141221"}
            \x65 {append code "112214"}
            \x66 {append code "112412" ; # 70 }
            \x67 {append code "122114"}
            \x68 {append code "122411"}
            \x69 {append code "142112"}
            \x6A {append code "142211"}
            \x6B {append code "241211"}
            \x6C {append code "221114"}
            \x6D {append code "413111"}
            \x6E {append code "241112"}
            \x6F {append code "134111"}
            \x70 {append code "111242" ; # 80 }
            \x71 {append code "121142"}
            \x72 {append code "121241"}
            \x73 {append code "114212"}
            \x74 {append code "124112"}
            \x75 {append code "124211"}
            \x76 {append code "411212"}
            \x77 {append code "421112"}
            \x78 {append code "421211"}
            \x79 {append code "212141"}
            \x7A {append code "214121" ; # 90 }
            \x7B {append code "412121"}
            \x7C {append code "111143"}
            \x7D {append code "111341"}
            \x7E {append code "131141"}
            \x7F {append code "114113"}
            \x80 {append code "114311"}
            \x81 {append code "411113"}
            \x82 {append code "411311"}
            \x83 {append code "113141"}
            \x84 {append code "114131" ; # 100 }
            \x85 {append code "311141"}
            \x86 {append code "411131" ; # FNC1 }
            \x87 {append code "211412" ; # Start A }
            \x88 {append code "211214" ; # Start B }
            \x89 {append code "211232" ; # Start C }
            \x8A {append code "2331112" ; # End }
        }
    }
    return [split $code {}]
 }
Spun off from here: [Code 39 generation]

Jan 19,2008 [SRIV] Heres the remainder of the code needed to generate Code128 B. 
#code 128
#One sees 6 parts in each symbol (3 bars, 3 gaps),  This part 
#converts an input string into a bar-gap sequence, with added start, 
#checksum and stop characters, followed by the termination bar

#This renders a bar-gap sequence, as from the above code, into a photo image

 proc c128img {c128} {
	set offset 20
	set scale 1
	set width [expr {round(([string length $c128] * $scale) + $offset)}]
	set height 60
	set im [image create photo -width $width -height $height]
	$im put white -to 0 0 $width $height
	set offset 20

	foreach {bar space} $c128 {
          set larg [expr ($bar * $scale)+$offset]
          $im put #000000 -to $offset 0 $larg $height
          set offset [expr ($space*$scale) + $larg]
	}
	set im
 }

 proc code128checksum {data} {
	set pos 1
	set sum 104
	foreach char [split $data ""] {
		scan $char %c decimal
		incr decimal -32
		set mult [expr $decimal * $pos]
		incr sum $mult
		#puts "$pos $mult $sum  $char $decimal"
		incr pos
	}
	
	set sum [expr $sum % 103]
	
	if {$sum < 95} {
		incr sum 32
	} else {
		incr sum 100
	}
	set sum [binary format c $sum]
	return "\x88${data}${sum}\x8a"
 }
	
	

 # Demo:
 set txt "ABCDE-12345"
 set bc [String2C128 [code128checksum $txt]]
 set im [c128img $bc]
 label .l1 -anchor c -text  $txt
 label .l2 -anchor c -image $im
 pack  .l1 .l2

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