[Xy-pic] xdvi not to blame in misalignment

[Alexander Perlis]

In an earlier message to this list, I included some sample code and a
discussion that led me to conclude that xdvi is (partially) to blame
for misalignment. But in preparing a report to the xdvi folks, I
realized I am probably mistaken. Here is some new sample code:

%&latex
\documentclass{article}
\usepackage[matrix,arrow,frame]{xy}
\def\exercise{\xymatrix{A\ar[d]\\C}}
\def\exer{\UseRules\exercise\NoRules\exercise
\hskip0.123456em\UseRules\exercise\NoRules\exercise
\hskip0.012345em\UseRules\exercise\NoRules\exercise}
\begin{document}
\[\exer\] \[\hskip1.23456em\exer\]
\[\hskip2.34567em\exer\] \[\hskip3.45678em\,\exer\]
\end{document}


There are lots of vertical arrows. The first vertical arrow in each
row uses \UseRules, and the arrows in each row alternate between
\UseRules and \NoRules. Subsequent rows are indented compared to
previous rows. That's it.

If you examine the output under full magnification in xdvi and
ghostview, you should see a lot of subtle misalignment in the arrows.
For me, all of the \NoRules arrows are messed up, and a few of the
\UseRules arrows are messed up. Significant is the fact that the same
arrow changes shape from one row to the next, indicating that: THE
ABSOLUTE POSITION ON THE PAGE MAKES A DIFFERENCE RE ALIGNMENT!

But if you compare the xdvi and ghostview output, you should see that
they are ESSENTIALLY THE SAME. Thus the blame cannot lie with xdvi,
and must lie either in the DVI file (perhaps the format cannot handle
the necessar precision, but I doubt it), or a calculation/precision
bug exists in TeX or in Xy-pic.

Perhaps everyone uses the PostScript driver, so no one cares. But
assuming the xy-fonts were put together for a reason, it might be
worthwhile to track down the source of the problem. I'm not certain
if I can be of further assistance (I don't know enough about Xy-pic
or TeX internals), but I'm open to suggestion.

Alex Perlis


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