PostScript points or pica points

You have probably seen this yourself. XPP pica points do not correspond to Adobe (or Acrobat) pica points.

Lets take a very simple XPP job with a page size of 51 picas x 66 picas.


But when you create a PDF and you ask for the document properties in Acrobat you get 50,888 x 65,855 pica and not 51 x 66 pica

And if you ask Acrobat what are the point sizes, you get the following:

• 10q becomes 9,98pt
• 12q becomes 11,97pt
• 24q becomes 23,95pt
• 48q becomes 47,9pt

Now this might seem like a rounding error on the side of Acrobat, but it is not. The points used in the Adobe PDF and PostScript world are bigger than the XPP pica points

What is a pica point?

The first official definition of the pica dates from the 19th century. In the year 1886 the US Type Founders Association defined the pica point to be exactly 0.013837 inch or 0.3514598 mm.

In the late seventies, early eighties, when Knuth was working on his TeX typesetting program, he redefined the pica point to be 1/72.27th of an (modern) inch, creating a slightly (very slighlty) bigger pica point as that value could be better expressed as a binary value:

Quoting from the TeXbook (chapter 10):

The units have been defined here so that precise conversion to sp is efficient on a wide variety of machines. In order to achieve this, TeX's “pt” has been made slightly larger than the official printer’s point, which was defined to equal exactly 0.013837 in by the American Typefounders Association in 1886 [cf. National Bureau of Standards Circular 570 (1956)]. In fact, one classical point is exactly 0.99999999 pt, so the “error” is essentially one part in 108. This is more than two orders of magnitude less than the amount by which the inch itself changed during 1959, when it shrank to 2.54 cm from its former value of (1/0.3937) cm; so there is no point in worrying about the difference. The new definition 72.27 pt = 1 in is not only better for calculation, it is also easier to remember.

The TeX pica point still computes to 0.351498mm.

Later in the eighties the Xyvision system came to live and defined the pica point to be an even bigger 352 micron. 

And then in the mid-1980s the desktop publishing revolution started with the Apple Macintosh, the LaserWriter and the PostScript language. Adobe simplified the definition of the PostScript pica point to be exactly 1/72th of an inch, not coincidentally matching the 72dpi resolution of the Mac screen. The PostScript point computes to 352,778 microns and is again bigger than a traditional pica point, the TeX pica point and even the XPP pica point.

Do we have to live with this?

I am afraid so.
Originally when I started writing this article I thought I would open up a new idea on the XPP Ideas page asking for a switch in the job ticket so you could ask for having 1q defined as the traditional XPP definition of a pica point or using 1q as the more modern PostScript point. Unfortunately the PostScript point does not compute to a round number of microns. So the best XPP could do would be setting 1 q = 353 microns. I created an XPP page using 353 as the base and got the following resulting PDF: page size now becomes 51,032 x 66,042 picas.
The point sizes became:

• 10q becomes 10,01pt
• 12q becomes 12,01pt
• 24q becomes 24,02pt
• 48q becomes 48,03pt

As you can see, closer but still not 100% correct.
It would be a good idea to have a switch that allows you to switch to PostScript units, but I am realist enough to understand that it will never happen as XPP is not designed to work with microns that are not whole numbers.

One question remains

One question remains though: why is a pica point defined as 352 microns and not 351 microns. After all 1 pica point computes to 351.498 microns and if you round that number you get 351 and not 352. But probably somebody rounded the 351.498 microns and came up with 351.5 microns as the answer. And if you round that you get 352 microns. Maybe somebody at engineering can still remember how we got stuck with 352.