Xerox 4450 - Manual (Page 65)

TROUBLESHOOTING
Rounding variable data
Alignment problems Under certain circumstances, a form and variable data do not
align properly, even though it appears that both use the same
line spacing. This may be the result of the effects of rounding
on the line spacing.
As an example, construct a form with a grid unit of 13.6 cpi and
9 lpi. Horizontal lines are specified at 0, 4, 7, 10, 13, 16, 19, 22,
25, 28, 31, 34, 37, 40, 43, 46, 49, 52, 55, 58, 61, 64, and 67. The
data is printed with an override line spacing of “9.”
The natural assumption is that the form and the variable data line
up. However, when the variable data is merged with the form, it
tends to drift up the page. The reason for this is rounding.
Specifying 9 lines per inch for the variable data means that the
LPS is requested to place data at approximately 33.333333 dots.
The system rounds this number down to 33 dots per line.
Meanwhile, in FDL, rounding does not take place until it is time
to resolve specifications for the form to a dot address.
Note that variable data has a line spacing value that is computed
as dots per line, and rounding is done on that value. In FDL, the
rounding takes place only when it is needed to resolve to a dot
address and, therefore, might involve more than one line at a
time.
For example, the line drawn at 43 is assigned to scan line 1433
using the following formula:
(43 * 300) / 9 = 12900 / 9 = 1433.3333 or 1433
The equivalent line positioning for the variable data is computed
with the following method:
43 * (300 / 9) = 43 * 33 = 1419
This yields an error that increases as calculation proceeds down
the page.
Furthermore, in the example form, if we had drawn lines with a
REPEAT EVERY 3 LINES command, the uneven dot value
(33.333333 dots per line) would not have been rounded off
because rounding would not have been performed until it was
time to resolve the specification to a dot address. The dot
address for three lines is an even 100 dots. So, the lines for the
form would be drawn at exactly 100 dots, and the variable data,
on the other hand, would be using a 99-dot spacing.
Avoiding imperfect alignment The best way to ensure that a form and variable data share the
same coordinate system is to define both in terms of an integral
number of dots. It is also important to know when rounding will
affect the alignment of the variable data and the form. In the
example, the only way to make the form match the lines of data
is to set the line spacing for both at 33 dots.
If variable data does not line up properly with a form, and it
appears that both are using the same line spacing, find out what
the dot value is. It is most likely in such cases that the difficulty
is a variation in defining the coordinate system resulting from
rounding, and is not a software problem.
XEROX 4050/4090/4450/4650 LPS FORMS CREATION GUIDE 4-15