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 ª9.º
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
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 |
