Setting image size using em units instead of pt

Question

I have found this question very helpful in my configuration of images. Inspired by tex4ht's capabilities (it's so much more powerful than I thought!) I'm wondering if I could have image width configured in em units instead of pt units.

Here are examples of my tex and cfg files:

mwe.tex:

\documentclass{article}
\usepackage[]{graphicx}
\usepackage[]{amsmath}
\begin{document}
  EPS file (0.3 textwidth)
\begin{center}
  \includegraphics[width=0.3\textwidth]{plot.eps}
\end{center}
  PNG file: (0.5 textwidth)
\begin{center}
  \includegraphics[width=0.5\textwidth]{plot.png}
\end{center}
\end{document}

my_mwe.cfg:

\Preamble{html}
\Configure{graphics*}
  {eps}
   {%  
    \Picture[pict]{\csname Gin@base\endcsname\PictExt
     \space width="\the\dimexpr \expandafter\the\csname Gin@req@width\endcsname *3/2"
   }%  
  }
\Configure{graphics*}
  {png}
   {%  
    \Picture[pict]{\csname Gin@base\endcsname .png
     \space width="\the\dimexpr \expandafter\the\csname Gin@req@width\endcsname *3/2"
   }%
  }
\begin{document}
\EndPreamble

Running

htlatex mwe.tex "my_mwe.cfg" 

produces images with widths of 155pt and 258pt, respectively, which look great in my pc browser but not a tablet or phone browser: I'd much rather have the width in em units or percentage. (As a follow up to a comment on the linked question, I found that "...\endcsname *1.5" did not work, but "...\endcsname *3/2" does.)

Being originally drawn to tex4ht in order to produce both html and pdf documents, I'd like to be able to take advantage of the responsiveness of html.

Is there hope that this is configurable? Thanks.

Answer 1

I've created simple macro for calculating em value, you just need to divide the dimension by fontsize:

% default font size
\let\emwidth\f@size  
% convert pt to rem
\newcommand\CalcRem[1]{\strip@pt\dimexpr(#1)/\emwidth}

\emwidth is set to be the fontsize, parameter for \CalcRem is dimension, but it may be also an expression. \strip@pt is used to output the value and strip pt suffix, which would be added if we used \the to output the value.

Now you can change configurations for images:

\Preamble{html}
\begin{document}
\makeatletter
% Various helper functions
% default font size
\newcommand\emwidth{16}
\let\emwidth\f@size
% convert pt to rem
\newcommand\CalcRem[1]{\strip@pt\dimexpr(#1)/\emwidth}

\Configure{graphics*}
  {png}
   {% we must add the image to the list of output files
   \special{t4ht+@File: \csname Gin@base\endcsname.png}
    \Picture[pict]{\csname Gin@base\endcsname\PictExt
     \space style="width:\CalcRem{\Gin@req@width}em;"
   }%  
  }
\Configure{graphics*}
  {eps}
   {% you need to coonvert eps to png first
   \Needs{"convert \csname Gin@base\endcsname.eps  
                           \csname Gin@base\endcsname.png"}% 
    \special{t4ht+@File: \csname Gin@base\endcsname.png}
    \Picture[pict]{\csname Gin@base\endcsname .png
     \space style="width:\CalcRem{\Gin@req@width}em;"
   }%
  }
    \makeatother
\EndPreamble

as you can see, css value width is used instead of width attribute, because it support only values in pixels.

The result (I used some dummy picture I found in my computer):

<!--l. 7--><p class="noindent" ><img 
src="demo_carenv.png" alt="pict"  
style="width:10.35011em;" ></div>
<!--l. 9--><p class="noindent" >PNG file: (0.5 textwidth)
<div class="center" 
>
<!--l. 10--><p class="noindent" >
<!--l. 11--><p class="noindent" ><img 
src="demo_carenv.png" alt="pict"  
style="width:17.25em;" ></div>