Vtyjkyuk

I was just messing around in LaTeX with the libertine and libertinus packages, and noticed that switching from LaTeX to XeLaTex to LuaLaTeX all produced PDFs that were mostly the same length, except spacing in paragraphs slightly changed.

What is causing this? Is there any “correct” output?

At first I thought it had to do with the fact that libertinus switches from Type 1 encoding with pdfLaTeX and OpenType with XeLaTeX/LuaLaTeX (not sure if libertine works in the same way), but even the outputs from XeLaTeX and LuaLaTeX are different.

documentclassturabian-researchpaper
usepackagebiblatex-chicago
usepackageturabian-formatting
usepackage[pass,letterpaper]geometry
usepackagelibertine % or libertinus %

begindocument

Afraid of intimacy, we use technology to remove ourselves from the real world because we
believe more will listen to us when we are online. The internet and our phones become an
addiction that is hard to break. Although our technological habits change our personal
relationships for the worst, we continue the cycle of needing to text or post to social
media to feel validated. We have been blinded by our own emotions into thinking we need to
be online so we don't have to self-reflect, even though the situation is the opposite. This
is extremely harmful to our social skills, and we form habits that, before technology, most
would have called awkward or antisocial. We sit with our friends without interacting. We go
to cafés alone to do nothing but text. We spew hateful comments online through the barrier
of anonymity. However, the habit of checking your phone, posting to social media, and
commenting on content is extremely hard to break and needs patience and the willingness to
take breaks often. Technology does not need to always be wasting resources in the back of
your mind.
footnoteBefore you start an unrelated argument, I'm not actually anti-technology like
this might suggest. This was an essay written to appease an English teacher.

enddocument

Sorry if this isn't minimal enough. I couldn't find a way reproduce the issue without these words and packages.

This GIF demonstrates the differences between pdfLaTeX, XeLaTeX, and LuaLaTeX.

(Added by David Carlisle) test file just using article class but the same width and font settings as in the original example, it shows the same three output variants for pdf/xe/lua latex as the original but has less dependencies so a bit easier to trace (perhaps)

documentclass[12pt]article


usepackagelibertine % or libertinus %

setlengthtextwidth469.755pt
setlengthparindent36.135pt
addtolengthoddsidemargin-1cm

begindocument

typeoutthefont
typeoutthefontdimen2font % space
typeoutthefontdimen3font % stretch
typeoutthefontdimen4font % shrink
typeoutthefontdimen7font % punct

Afraid of intimacy, we use technology to remove ourselves from the real world because we
believe more will listen to us when we are online. The internet and our phones become an
addiction that is hard to break. Although our technological habits change our personal
relationships for the worst, we continue the cycle of needing to text or post to social
media to feel validated. We have been blinded by our own emotions into thinking we need to
be online so we don't have to self-reflect, even though the situation is the opposite. This
is extremely harmful to our social skills, and we form habits that, before technology, most
would have called awkward or antisocial. We sit with our friends without interacting. We go
to cafés alone to do nothing but text. We spew hateful comments online through the barrier
of anonymity. However, the habit of checking your phone, posting to social media, and
commenting on content is extremely hard to break and needs patience and the willingness to
take breaks often. Technology does not need to always be wasting resources in the back of
your mind.


enddocument

If you add

typeoutthefont
typeoutthefontdimen2font %space
typeoutthefontdimen3font %stretch
typeoutthefontdimen4font % shrink
typeoutthefontdimen7font % punct

after begindocument then you will find

pdftex

OT1/LinuxLibertineT-TLF/m/n/12 
3.0pt
1.5pt
0.99959pt
0.5004pt

xetex

TU/LinLibertine(0)/m/n/12 
3.0pt
1.5pt
1.0pt
1.0pt

luatex

TU/LinLibertine(0)/m/n/12 
3.0pt
1.5pt
1.0pt
1.0pt

Note that fontdimen7 (extra space after punctuation) ends up twice as large in the OpenType font as in the tfm metrics used by pdftex. I note the libertinus font suport had an update yesterday in texlive this is with the latest version of that package (and a development build of luatex 1.09) running over texlive 2018.

So the big difference between pdftex and the others is not surprising, it's harder to see an obvious reason for a difference between luatex and xetex other than luatex explicitly documents that it uses a variant hyphenation algorithm that doesn't always choose the same points even using the same patterns.