SVG px conversion in Perl

TL;DR

Converting from px via SVG::DOM.

In SVG viewBox and px we took a look at a way to convert from the px inside a SVG file to the pt units, i.e. in $\frac{1}{72}$-ths of an inch.

Assuming that we have a SVG::DOM element of type (tag) svg, we can use it to extract all relevant values:

sub parse_conversion ($el) {
   state $inch_to = {    # conversion table
      mm => 25.4,
      cm => 2.54,
      dm => 0.254,
      m  => 0.0254,
      in => 1,
      pt => 72
   };

   my ($x, $y, $S_W, $S_H) = split m{\s+}mxs,
     ($el->getAttribute('viewBox') =~ s{\A\s+|\s+\z}{}rgmxs);
   my ($W_U, $U) = $el->getAttribute('width') =~ m{
      \A\s*
         (.*?)
         ([a-zA-Z]+)
      \s*\z
   }mxs;
   my $C_U    = $inch_to->{$U} // die "cannot converto inches to $U\n";
   my $factor = (72 * $W_U) / ($C_U * $S_W);
   return {
      X_offset => $x,
      X_span   => $S_W,
      Y_offset => $y,
      Y_span   => $S_H,
      factor   => $factor,
   };
} ## end sub parse_conversion

We are assuming that there the px is the same along both axes; I think this is a good assumption in practical cases, and that InkScape enforces this too.

The calculated $factor allows turning px into pt, so it’s put as the inverse of what we calculated in SVG viewBox and px. Makes sense, right?

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