Michael Barr mbarr at math.mcgill.ca
Thu Aug 17 01:56:42 CEST 2006

Please consider the following file:

\documentclass{article}
\usepackage[all]{xy}
\begin{document}

\let\*\otimes
$$\xy \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(50,0)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(40,30)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(30,40)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(0,50)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(-30,40)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(-40,30)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(-50,0)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(40,-30)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(30,-40)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(0,-50)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(-30,-40)*+!!<0ex,1ex>{A\* B}% \POS(0,0)*+!!<0ex,1ex>{A\* B}\ar@{->}(-40,-30)*+!!<0ex,1ex>{A\* B}% \endxy$$

\end{document}

Run it through tex and view it with the dvi viewer.  Perfect.  Now run
the dvi file through dvipdfm and look at in the Adobe viewer.  Suddenly
the A \otimes B in the middle is much darker than all the others.  If you
make all but one into \phantom's, this does not happen.  It is as though
the pdf file has 12 copies of A \otimes B in 12 slightly different
positions, kind of like Knuth's \pmb macro.  Does this arise from some
kind of rounding error in xy-pic?  Or is there another explanation?  In
any case, can it be cured?  My diagxy macros depend crucially on the
ability to put these things together without problems.

I understand from Markus Schmaus, who called this to my attention, that
the same problem arises with dvips (although I could not elicit it in
GS 8.3) and in pdftex (I didn't try it).  This suggests that the viewer
might be at fault, but who knows.

Michael