xcoffins and fractional factors to \CoffinWidth fail

Question

The following code comes from this SE query: Can I centre my oversized coffin of unknown width?. It was posted by @cfr and accompanied by output, so, presumably, it worked at one time:

\begin{filecontents}{pic.tex}
\documentclass{standalone}
\begin{document}
\begin{tikzpicture}
  \shade [left color=Cerulean, right color=Cerulean, middle color=MidnightBlue] (0,0) rectangle (200mm,10mm);
\end{tikzpicture}
\end{document}
\end{filecontents}
\documentclass[a4paper,x11names,svgnames,dvipsnames]{article}
\usepackage{geometry,standalone,tikz,xcoffins,calc}
\begin{document}
\NewCoffin\TestCoffin
\SetHorizontalCoffin\TestCoffin{\input{pic}}

\MarkCoffinHandle\TestCoffin[hc,vc]{magenta}
\MarkCoffinHandle\TestCoffin[l,vc]{magenta}

Can I centre the coffin \emph{without} knowing the width of the picture?

This works:\par
\noindent
\TypesetCoffin\TestCoffin[l,vc](-.5\CoffinWidth\TestCoffin+.5\textwidth,0pt)
\bigskip

This doesn't:\par
{\centering
  \TypesetCoffin\TestCoffin[hc,vc]\par}


\end{document}

When I run that code now, it fails with the following error:

./Coffin_mwe_001.tex:22: Illegal unit of measure (pt inserted).
<to be read again> 
                   .
l.22 ...5\CoffinWidth\TestCoffin+.5\textwidth,0pt)

If I delete the fractional factor .5 from -.5\CoffinWidth\TestCoffin to make it -\CoffinWidth\TestCoffin/2 then the code works, though division by non-integers, not surprisingly, does not work. Note also that integer factors to \CoffinWidth also work correctly.

I cannot seem to find discussion of this on SE. Is there a workaround?

Answer 1

Explanation why it doesn't work:

\CoffinWidth is defined as \dim_eval:n { \coffin_wd:N #1 }. \dim_eval:n expands to \dim_use:N \__dim_eval:w #1\__dim_eval_end: so the above becomes \dim_use:N \__dim_eval:w \coffin_wd:N #1\__dim_eval_end:. That is (removing the l3 nomenclature) the same as \the\dimexpr\wd#1\relax.

You're using that inside of the optional argument of \TypesetCoffin which is again a \dimexpr so something like \dimexpr\the\dimexpr\wd\TestCoffin\relax\relax. In the following I'll drop the outer \dimexpr occasionally, I hope it is still understandable.

\the\dimexpr\wd#1\relax would expand to something like <float>pt. In the following let us assume that <float> is the (rather unspectacular) float 1.0. Note that \the\<dimen> always returns a period even if it won't make a different (so if the only decimal digit is a 0). Now with our example we take a look at what happens if you precede it with a factor:

Choosing an integer factor (let's take 2) what you get is 21.0pt, which is valid but certainly not the double of 1.0pt. Using another float as the factor (let's take 0.5) you get 0.51.0pt which is not a valid length, hence the error.

What do we do now? One way would be to use a division with an integer of 2 (putting back the outer \dimexpr and replacing the inner one with our exemplary 1.0pt): \dimexpr 1.0pt/2\relax is valid input. Another way would be to use multiplication, again only with integers: \dimexpr1.0pt*2\relax is still valid. But what if we want to use a float as the factor? We have to turn the thing back into something TeX considers a dimen, which is why we use \dimexpr around the inner expression: .5\dimexpr 1.0pt\relax is again valid.

Turning it into a working bit of code that does the intended thing:

With the wrapping of \dimexpr we considered a viable solution above your code becomes:

\TypesetCoffin\TestCoffin[l,vc](-.5\dimexpr\CoffinWidth\TestCoffin\relax+.5\textwidth,0pt)

And we're done. Complete MWE:

\begin{filecontents}{pic.tex}
\documentclass{standalone}
\begin{document}
\begin{tikzpicture}
  \shade [left color=Cerulean, right color=Cerulean, middle color=MidnightBlue] (0,0) rectangle (200mm,10mm);
\end{tikzpicture}
\end{document}
\end{filecontents}
\documentclass[a4paper,x11names,svgnames,dvipsnames]{article}
\usepackage{geometry,standalone,tikz,xcoffins,calc}
\begin{document}
\NewCoffin\TestCoffin
\SetHorizontalCoffin\TestCoffin{\input{pic}}

\MarkCoffinHandle\TestCoffin[hc,vc]{magenta}
\MarkCoffinHandle\TestCoffin[l,vc]{magenta}

Can I centre the coffin \emph{without} knowing the width of the picture?

This works:\par
\noindent
\TypesetCoffin\TestCoffin[l,vc](-.5\dimexpr\CoffinWidth\TestCoffin\relax+.5\textwidth,0pt)
\bigskip

This doesn't:\par
{\centering
  \TypesetCoffin\TestCoffin[hc,vc]\par}


\end{document}