Re: psychoceramics: barcode 666

[moroney @ world.std.com (Michael Moroney)]

In article <199603232126.NAA--@n--.netcom.com>, Anton Sherwood wrote:
> To help scanners detect and adjust for variation in speed,
> the Universal Products Code (UPC) block contains three "frame"
> patterns (at both ends and the middle) consisting of two narrow
> black bars separated by a narrow white bar.  It so happens that
> this is part of the pattern that represents `6'.

A little further explanation of such kookiness:
(it helps if you are looking at a UPC code while reading this)

The UPC on consumer products has a bar code such that each digit 0-9 is
represented by 4 bars, two light and two dark bars alternating.  Each digit is
7 times the width of the narrowest bar's width and each bar can be 1, 2, 3 or 4
times this width. Additionally there are two possible code for each digit, with
each code the "negative" of each other (light and dark reversed).  The patterns
that start with the light bar on the left side are on the left side group of 5
digits while patterns with the dark bar on the left side is on the right side
group.  The two types of patterns are so the scanning equipment can scan a code
backwards and know it has a valid backwards code rather than getting an error
and trying to scan the other way.

The two groups are separated by a sync code which helps the equipment know it
has a valid code.  This sync code is 5 narrow bars, light, dark, light, dark,
light.  There are also sync codes at each end, 3 bars, dark, light, dark.

Now it happens the pattern for the 6 is defined as 3 narrow bars and one extra
wide (4 widths) bar.  Like all other digits it must be 7 units wide.  The right
side 6 is therefore a thin dark, thin light, thin dark and wide light line.
To us humans it appears as two thin lines on a light background as this is the
type of printing we're used to reading.  But to the scanner the wide light bar
is just as important and must be present.  Now the sync patterns on each end
appear to people as 2 thin close dark lines on a light background.  Aha! A 6!
And three of them makes 666!  But no, each of these codes isn't the right width
to be a 6 (or any other digit)  The end sync patterns are 3 units wide and the
center 5 wide.  A valid digit *must* be 7 wide.  Also the number of transitions
(light to dark or dark to light) are wrong for the sync codes.  Valid digits
always have 4 transitions, the sync codes have 3 or 5.

I think I have a .GIF file of a valid UPC code for the code "66666 66666".
By looking at it it is obvious the pattern repeats only 5 times on each side.
If the sync codes were really 6s you'd see a pattern repeat a total of 13
times.

Now try explaining all this to one of those "Mark of the Beast" kooks.  You'll
fail.

-Mike