Why does dvisvgm produce faulty svg code for some expressions involving fractions and certain fonts?

Question

I would like to ask for help with figuring out exactly what triggers a weird phenomenon concerning rendering of textstyle fractions in certain fonts with dvisvgm (version 2.6.3, distributed as part of the package texlive-bin 2019.51075-3 on Arch Linux).

(I stumbled across this when playing around with the animation library manim, which renders formulas by running them through LaTeX and then converting the dvi to svg using dvisvgm.)

Here's a very minimal example causing trouble, svgtest.tex:

\documentclass[preview]{standalone}
\usepackage{fourier}
\begin{document}
$y = \sin \frac{1}{x}$
\end{document}

Processing this with

latex svgtest.tex; dvisvgm svgtest.dvi -n -o svgtest.svg

produces the following file, svgtest.svg:

<?xml version='1.0' encoding='UTF-8'?>
<!-- This file was generated by dvisvgm 2.6.3 -->
<svg version='1.1' xmlns='http://www.w3.org/2000/svg' xmlns:xlink='http://www.w3.org/1999/xlink' width='33.871205pt' height='12.091864pt' viewBox='0.498134 -8.571499 33.871205 12.091864'>
<defs>
<path id='g3-49' d='M3.990416 0V-0.26481L3.22338 -0.32873C3.03162 -0.346993 2.922044 -0.420044 2.922044 -0.776168V-6.154555L2.876387 -6.209343L0.995321 -5.889745V-5.66146L1.84454 -5.561015C1.999774 -5.542752 2.063693 -5.469701 2.063693 -5.204891V-0.776168C2.063693 -0.602672 2.036299 -0.493095 1.981511 -0.429175C1.935854 -0.365255 1.862803 -0.337861 1.762358 -0.32873L0.995321 -0.26481V0H3.990416Z'/>
<path id='g1-198' d='M5.798431 -1.305788V-1.835409H0.776168V-1.305788H5.798431ZM5.798431 -3.314693V-3.844314H0.776168V-3.314693H5.798431Z'/>
<path id='g8-120' d='M3.518509 -0.957701C3.351952 -0.645407 3.164576 -0.277594 2.97026 -0.277594C2.838403 -0.277594 2.782884 -0.437211 2.630207 -1.054859L2.408131 -1.96398C2.657966 -2.408131 3.011899 -2.96332 3.213155 -2.96332C3.275614 -2.96332 3.331133 -2.94944 3.407471 -2.907801C3.47687 -2.880042 3.546268 -2.852282 3.629547 -2.852282C3.782224 -2.852282 3.927961 -2.998019 3.927961 -3.185396C3.927961 -3.400531 3.768344 -3.48381 3.574028 -3.48381C3.206215 -3.48381 2.893921 -3.150696 2.651026 -2.762064L2.345673 -2.290154H2.331793L2.054198 -3.45605L2.012559 -3.48381L0.867482 -3.136817L0.888302 -2.97026L1.408792 -3.004959C1.533709 -3.011899 1.582288 -2.9772 1.658626 -2.657966L1.908461 -1.651687L1.714145 -1.318573C1.415731 -0.811964 1.158957 -0.451091 0.9924 -0.451091C0.923001 -0.451091 0.853603 -0.47885 0.791144 -0.51355C0.721745 -0.548249 0.631527 -0.617648 0.51355 -0.617648C0.333113 -0.617648 0.215136 -0.437211 0.215136 -0.270655C0.215136 -0.069399 0.381692 0.083278 0.659287 0.083278C1.131197 0.083278 1.401852 -0.388632 1.686386 -0.853603L1.96398 -1.311633H1.97786L2.102777 -0.770324C2.234635 -0.215136 2.366492 0.083278 2.720425 0.083278C3.227035 0.083278 3.490749 -0.437211 3.698945 -0.860543L3.518509 -0.957701Z'/>
<use id='g13-121' xlink:href='#g8-121' transform='scale(1.315788)'/>
<use id='g18-105' xlink:href='#g3-105'/>
<use id='g18-110' xlink:href='#g3-110'/>
<use id='g18-115' xlink:href='#g3-115'/>
<use id='g15-49' xlink:href='#g3-49' transform='scale(0.760001)'/>
</defs>
<g id='page1'>
<use x='0.498134' y='0' xlink:href='#g13-121'/>
<use x='7.333545' y='0' xlink:href='#g1-198'/>
<use x='15.633477' y='0' xlink:href='#g18-115'/>
<use x='19.628496' y='0' xlink:href='#g18-105'/>
<use x='22.29848' y='0' xlink:href='#g18-110'/>
<use x='30.405611' y='-3.925267' xlink:href='#g15-49'/>
<rect x='30.136829' y='-2.846123' height='0.531591' width='4.232511'/>
<use x='30.288264' y='3.437093' xlink:href='#g8-120'/>
</g>
</svg>

This svg renders as follows in every program that I've tested (Firefox, Inkscape, Emacs):

Looking at the svg code, we see for example that the first <use> tag in the 'page1' group, which is supposed to be the “y” character, refers to the element g13-121, which a few lines earlier is referring to the element g8-121, which isn't defined anywhere, so it's no wonder there is only whitespace instead of a “y” in the picture.

Contrast this with the same example using the default Computer Modern font, which works as expected:

\documentclass[preview]{standalone}
\begin{document}
$y = \sin \frac{1}{x}$
\end{document}

latex svgtest.tex; dvisvgm svgtest.dvi -n -o svgtest.svg

<?xml version='1.0' encoding='UTF-8'?>
<!-- This file was generated by dvisvgm 2.6.3 -->
<svg version='1.1' xmlns='http://www.w3.org/2000/svg' xmlns:xlink='http://www.w3.org/1999/xlink' width='38.131002pt' height='11.852377pt' viewBox='0 -8.416865 38.131002 11.852377'>
<defs>
<path id='g0-121' d='M4.841843 -3.795766C4.881694 -3.935243 4.881694 -3.955168 4.881694 -4.024907C4.881694 -4.204234 4.742217 -4.293898 4.592777 -4.293898C4.493151 -4.293898 4.333748 -4.234122 4.244085 -4.084682C4.224159 -4.034869 4.144458 -3.726027 4.104608 -3.5467C4.034869 -3.287671 3.965131 -3.01868 3.905355 -2.749689L3.457036 -0.956413C3.417186 -0.806974 2.988792 -0.109589 2.331258 -0.109589C1.823163 -0.109589 1.713574 -0.547945 1.713574 -0.916563C1.713574 -1.374844 1.882939 -1.992528 2.221669 -2.86924C2.381071 -3.277709 2.420922 -3.387298 2.420922 -3.58655C2.420922 -4.034869 2.102117 -4.403487 1.603985 -4.403487C0.657534 -4.403487 0.288917 -2.958904 0.288917 -2.86924C0.288917 -2.769614 0.388543 -2.769614 0.408468 -2.769614C0.508095 -2.769614 0.518057 -2.789539 0.56787 -2.948941C0.836862 -3.88543 1.235367 -4.184309 1.574097 -4.184309C1.653798 -4.184309 1.823163 -4.184309 1.823163 -3.865504C1.823163 -3.616438 1.723537 -3.35741 1.653798 -3.16812C1.255293 -2.11208 1.075965 -1.544209 1.075965 -1.075965C1.075965 -0.18929 1.703611 0.109589 2.291407 0.109589C2.67995 0.109589 3.01868 -0.059776 3.297634 -0.33873C3.16812 0.179328 3.048568 0.667497 2.650062 1.195517C2.391034 1.534247 2.012453 1.823163 1.554172 1.823163C1.414695 1.823163 0.966376 1.793275 0.797011 1.404732C0.956413 1.404732 1.085928 1.404732 1.225405 1.285181C1.325031 1.195517 1.424658 1.066002 1.424658 0.876712C1.424658 0.56787 1.155666 0.52802 1.05604 0.52802C0.826899 0.52802 0.498132 0.687422 0.498132 1.175592C0.498132 1.673724 0.936488 2.042341 1.554172 2.042341C2.580324 2.042341 3.606476 1.135741 3.88543 0.009963L4.841843 -3.795766Z'/>
<path id='g1-120' d='M1.736488 -0.739228C1.66675 -0.502117 1.436613 -0.125529 1.080946 -0.125529C1.060025 -0.125529 0.850809 -0.125529 0.704359 -0.223163C0.990286 -0.313823 1.011208 -0.564882 1.011208 -0.606725C1.011208 -0.760149 0.892653 -0.864757 0.732254 -0.864757C0.536986 -0.864757 0.334745 -0.697385 0.334745 -0.439352C0.334745 -0.09066 0.72528 0.069738 1.066999 0.069738C1.387796 0.069738 1.673724 -0.132503 1.84807 -0.425405C2.015442 -0.055791 2.399004 0.069738 2.677958 0.069738C3.47995 0.069738 3.905355 -0.801993 3.905355 -0.99726C3.905355 -1.08792 3.814695 -1.08792 3.793773 -1.08792C3.696139 -1.08792 3.689166 -1.053051 3.66127 -0.969365C3.514819 -0.488169 3.096389 -0.125529 2.705853 -0.125529C2.426899 -0.125529 2.280448 -0.313823 2.280448 -0.578829C2.280448 -0.760149 2.447821 -1.39477 2.643088 -2.168867C2.782565 -2.705853 3.096389 -2.880199 3.326526 -2.880199C3.340473 -2.880199 3.556663 -2.880199 3.703113 -2.782565C3.47995 -2.719801 3.396264 -2.524533 3.396264 -2.399004C3.396264 -2.245579 3.514819 -2.140971 3.675218 -2.140971S4.065753 -2.273474 4.065753 -2.566376C4.065753 -2.956912 3.619427 -3.075467 3.340473 -3.075467C2.991781 -3.075467 2.712827 -2.84533 2.559402 -2.580324C2.433873 -2.866252 2.113076 -3.075467 1.72254 -3.075467C0.941469 -3.075467 0.495143 -2.217684 0.495143 -2.008468C0.495143 -1.917808 0.592777 -1.917808 0.613699 -1.917808C0.704359 -1.917808 0.711333 -1.945704 0.746202 -2.036364C0.920548 -2.580324 1.3599 -2.880199 1.701619 -2.880199C1.931756 -2.880199 2.12005 -2.75467 2.12005 -2.419925C2.12005 -2.280448 2.036364 -1.931756 1.973599 -1.694645L1.736488 -0.739228Z'/>
<path id='g3-49' d='M2.336239 -4.435367C2.336239 -4.623661 2.322291 -4.630635 2.127024 -4.630635C1.680697 -4.191283 1.046077 -4.184309 0.760149 -4.184309V-3.93325C0.927522 -3.93325 1.387796 -3.93325 1.771357 -4.128518V-0.571856C1.771357 -0.341719 1.771357 -0.251059 1.073973 -0.251059H0.808966V0C0.934496 -0.006974 1.792279 -0.027895 2.050311 -0.027895C2.266501 -0.027895 3.145205 -0.006974 3.29863 0V-0.251059H3.033624C2.336239 -0.251059 2.336239 -0.341719 2.336239 -0.571856V-4.435367Z'/>
<path id='g2-61' d='M6.844334 -3.257783C6.993773 -3.257783 7.183064 -3.257783 7.183064 -3.457036S6.993773 -3.656289 6.854296 -3.656289H0.886675C0.747198 -3.656289 0.557908 -3.656289 0.557908 -3.457036S0.747198 -3.257783 0.896638 -3.257783H6.844334ZM6.854296 -1.325031C6.993773 -1.325031 7.183064 -1.325031 7.183064 -1.524284S6.993773 -1.723537 6.844334 -1.723537H0.896638C0.747198 -1.723537 0.557908 -1.723537 0.557908 -1.524284S0.747198 -1.325031 0.886675 -1.325031H6.854296Z'/>
<path id='g2-105' d='M1.763387 -4.403487L0.368618 -4.293898V-3.985056C1.016189 -3.985056 1.105853 -3.92528 1.105853 -3.437111V-0.757161C1.105853 -0.308842 0.996264 -0.308842 0.328767 -0.308842V0C0.647572 -0.009963 1.185554 -0.029888 1.424658 -0.029888C1.77335 -0.029888 2.122042 -0.009963 2.460772 0V-0.308842C1.803238 -0.308842 1.763387 -0.358655 1.763387 -0.747198V-4.403487ZM1.803238 -6.136986C1.803238 -6.455791 1.554172 -6.665006 1.275218 -6.665006C0.966376 -6.665006 0.747198 -6.396015 0.747198 -6.136986C0.747198 -5.867995 0.966376 -5.608966 1.275218 -5.608966C1.554172 -5.608966 1.803238 -5.818182 1.803238 -6.136986Z'/>
<path id='g2-110' d='M1.09589 -3.427148V-0.757161C1.09589 -0.308842 0.986301 -0.308842 0.318804 -0.308842V0C0.667497 -0.009963 1.175592 -0.029888 1.444583 -0.029888C1.703611 -0.029888 2.221669 -0.009963 2.560399 0V-0.308842C1.892902 -0.308842 1.783313 -0.308842 1.783313 -0.757161V-2.590286C1.783313 -3.626401 2.49066 -4.184309 3.128269 -4.184309C3.755915 -4.184309 3.865504 -3.646326 3.865504 -3.078456V-0.757161C3.865504 -0.308842 3.755915 -0.308842 3.088418 -0.308842V0C3.437111 -0.009963 3.945205 -0.029888 4.214197 -0.029888C4.473225 -0.029888 4.991283 -0.009963 5.330012 0V-0.308842C4.811955 -0.308842 4.562889 -0.308842 4.552927 -0.607721V-2.510585C4.552927 -3.367372 4.552927 -3.676214 4.244085 -4.034869C4.104608 -4.204234 3.775841 -4.403487 3.198007 -4.403487C2.470735 -4.403487 2.002491 -3.975093 1.723537 -3.35741V-4.403487L0.318804 -4.293898V-3.985056C1.016189 -3.985056 1.09589 -3.915318 1.09589 -3.427148Z'/>
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</defs>
<g id='page1'>
<use x='0' y='0' xlink:href='#g0-121'/>
<use x='8.009276' y='0' xlink:href='#g2-61'/>
<use x='18.525356' y='0' xlink:href='#g2-115'/>
<use x='22.455073' y='0' xlink:href='#g2-105'/>
<use x='25.222482' y='0' xlink:href='#g2-110'/>
<use x='33.886483' y='-3.922607' xlink:href='#g3-49'/>
<rect x='33.613201' y='-2.68991' height='0.398484' width='4.517787'/>
<use x='33.613201' y='3.435512' xlink:href='#g1-120'/>
</g>
</svg>

But here's comes the really weird part. Going back to the Fourier font, all of the following formulas do work as expected (one at a time, although I've put them all in one example here for simplicity):

\documentclass[preview]{standalone}
\usepackage{fourier}
\begin{document}
$$y = \sin \frac{1}{x}$$
$y = \frac{1}{x}$
$y = \sin \frac{z}{x}$
$y = \sin \left( \frac{1}{x} \right)$
\end{document}

But this one doesn't work (the result looks pretty much like the first example above):

\documentclass[preview]{standalone}
\usepackage{fourier}
\begin{document}
$y = \sin ( \frac{1}{x} )$
\end{document}

With

\usepackage{mathpazo}

the results are the same as with Fourier. On the other hand, with

\usepackage{newtxmath}

everything works as it should, just like for Computer Modern.

All the dvi files look fine with xdvi, dvips, dvipng, etc., but apparently there's something in the dvi files for

$y = \sin \frac{1}{x}$

and

$y = \sin ( \frac{1}{x} )$

with the fourier and mathpazo packages that differs from all the other examples in a way that triggers this strange behaviour (bug?) in dvisvgm. Since dvi is a binary format, I haven't been able to decipher exactly what this mysterious property is, but maybe some TeX guru out there can help?

Answer 1

After some further investigations I finally found the cause for the missing characters. Contrary to what I mentioned in the comments, I was now able to reproduce the issue with dvisvgm 2.6.3 and could bisect the changesets. It's actually a bug in dvisvgm that was already reported but only occurred sporadically and only with multi-page DVI files – until now. I fixed it a couple of months ago in version 2.7.3. So, unfortunately, there isn't much you can do except waiting for a more recent release.