“fo”-ligature makes PDF code look strange in xetex

Question

I'm trying to extract table data from a PDF. To this end, I need the precise locations of the beginning and end of a piece of text, and that's where my question comes in. For testing, I generated some text using LaTeX. Consider the following minimal example:

\documentclass{article}
\begin{document}
foo
\end{document}

When compiled via latex -> dvips -> ps2pdf, I get (after decompression) the following PDF code, which looks fine:

10 0 0 10 0 0 cm          # concat to transformation matrix
BT                        # begin text mode
/R8 9.96264 Tf            # select font
1 0 0 1 148.68 657.24 Tm  # set text matrix
[(f)3.87556(o)-29.9875(o)-5.88993]TJ
# ^ print 'f', then go a little bit left, print 'o' (so f and o
# overlap a bit), go a little bit right, print 'o'. Then go right again,
# for whatever reason.

But when using xelatex, it looks like this:

 1 0 0 1 72 720 cm  # concat a DIFFERENT transformation matrix
 0 G 0 g            # set gray levels to 0
 BT                 # begin text mode
 /F1 9.963 Tf       # select font
 76.71 -62.76 Td    # move to text position (this was missing above)
 [(fo)-28(o)]TJ     # print 'fo', then go 28 right, then print 'o'

What confuses me is the last line in the second output: How come that 'f' and 'o' overlap? Where is that specified? I don't see any 'Tc' commands or the such. And then how come that the second 'o' doesn't overlap with the first one?

Thanks a lot!

Answer 1

Generating uncompressed files

Uncompressed files can be generated via TeX commands in case of pdfLaTeX or LuaLaTeX:

\ifx\pdfobjcompresslevel\undefined
\else
  \pdfobjcompresslevel=0
  \pdfcompresslevel=0
\fi
\documentclass{article}
\pagestyle{empty}
\begin{document} 
foo
\end{document}

With XeLaTeX uncompressed files can be generated via the command line:

xelatex --output-driver="xdvipdfmx -V4 -z0" test

Route latex/dvips/ps2pdf:

ps2pdf -dCompressPages=false test.ps

Page streams

pdfTeX:

BT
/F8 9.9626 Tf 148.712 707.125 Td [(fo)-28(o)]TJ
ET

LuaLaTeX:

BT
/F15 9.96264 Tf 1 0 0 1 148.712 707.125 Tm [(fo)-27(o)]TJ
ET

XeTeX:

 q 1 0 0 1 72 769.89 cm 0 G 0 g BT /F1 9.963 Tf 76.71 -62.76 Td[(fo)-28(o)]TJ ET
 Q

latex/dvips/ps2pdf:

q 0.1 0 0 0.1 0 0 cm
/R7 gs
0 g
q
10 0 0 10 0 0 cm BT
/R8 9.96264 Tf
1 0 0 1 148.68 707.14 Tm
[(f)3.87556(o)-29.9875(o)-5.88993]TJ
ET
Q
Q

Discussion

• In neither case I can't see any overlapping of the glyph shapes.

• There aren't any ligatures involved.

• Additonal space is inserted between the two os. This is called implicit kerning and the data are looked up by TeX in the TFM file (cmr10.tfm). A negative number (−28) means that space is inserted inbetween. A positive number means that the glyph bounding boxes overlap.

• Only in the case of latex/dvips/ps2pdf the f and the next o are moved a little bit closer by 0.00387556 (unit is per mill!), introduced by Ghostscript.

• Your case of XeLaTeX differs from the finding here: 0 G 0 g is usually added by package color several times. Perhaps you have used a different TeX file with a package for color and cleaned up the PDF outcome. Check, whether you are also using different fonts.