PWC165 - Scalable Vector Graphics (SVG)


Here we are with TASK #1 from The Weekly Challenge #165. Enjoy!

The challenge

Scalable Vector Graphics (SVG) are not made of pixels, but lines, ellipses, and curves, that can be scaled to any size without any loss of quality. If you have ever tried to resize a small JPG or PNG, you know what I mean by “loss of quality”! What many people do not know about SVG files is, they are simply XML files, so they can easily be generated programmatically.

For this task, you may use external library, such as Perl’s SVG library, maintained in recent years by our very own Mohammad S Anwar. You can instead generate the XML yourself; it’s actually quite simple. The source for the example image for Task #2 might be instructive.

Your task is to accept a series of points and lines in the following format, one per line, in arbitrary order:

Point: x,y

Line: x1,y1,x2,y2



Then, generate an SVG file plotting all points, and all lines. If done correctly, you can view the output .svg file in your browser.

The questions

No specific question, although maybe…

  • should we put points consistently above or below lines?
  • is there a preferred choice of colors/sizes?
  • what size should the final image be advertised as?

The solution

This solution is entirely how not to solve the problem, because we should be relying upon modules to do this (like the SVG module in the very challenge text). Let’s take this as a small exercise in reinventing wheels badly.

We go Raku first, as it’s become customary. We have three lower-level functions to build upon, to handle generation of taggish struff, plus two drawing primitives for point and line.

#!/usr/bin/env raku
use v6;
sub MAIN {
   put svg-for("53,10\n53,10,23,30\n23,30")

sub svg-for ($input) {
      gather {
         take open-tag('svg', width => 400, height => 400);
         for $input.lines -> $line {
            my @nums = $line.split(/ \, /);
            take @nums == 2 ?? point(@nums) !! line(@nums);
         take close-tag('svg');

sub open-tag ($tag, *%args) {
   ("<$tag",> $k, $v {qq<$k="$v">}), '>').join(' ');

sub oneshot-tag ($tag, *%args) {
   ("<$tag",> $k, $v {qq<$k="$v">}), '/>').join(' ');

sub close-tag ($tag) { return "</$tag>" }

sub point (@p, *%args) {
   my %pargs =
      'cx', @p[0],
      'cy', @p[1],
      'r', 4,
      'stroke-width', 0,
      'fill', '#000000'
   oneshot-tag('circle', |%pargs, |%args);

sub line (@ps, *%args) {
   my %pargs =
      'points', @ps.join(' '),
      'stroke-width', 6,
      'stroke', '#ff0000';
   oneshot-tag('polyline', |%pargs, |%args);

I still have to get the hang of passing parameters properly in Raku and, to be honest, I much prefer the way it’s done in Perl, at least from a day-to-day perspective. It’s either the variable or a reference to it there, one rule to learn and we’re up to speed; here… we have to think so much more.


For the Perl alternative, I did it even more badly if possible. I’m using my own Template::Perlish module to define a template which implements exactly what’s requested. So in a sense I’m using CPAN, but not how I’m supposed to do it in real life.

#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';

use Template::Perlish;

say svg_for("53,10\n53,10,23,30\n23,30");

sub svg_for ($text) {
   state $tp2 = Template::Perlish->new->compile_as_sub(
q{<svg height="400" width="400">
[% for my $item (A 'lines') {
      if ($item->@* == 2) {
%] <circle r="4" cx="[%= $item->[0] %]" cy="[%= $item->[1] %]" stroke-width="0" fill="#000000" />
[%    } else {
%] <polyline points="[%= join ' ', $item->@* %]" stroke="#ff0000" stroke-width="6" />
[%    }
   $tp2->({lines => [map {[ split m{,+}mxs ]} split m{\n+}mxs, $text]});

All in all, though, I find it more readable.

Stay safe!

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