Starting a bit early around Mar Vista, even though the nights are cold.

People often ask me “How do you put together those paper shadow theater things you make?”^*. Here’s a quick visual tour of the process. You can click on the pictures to see more detail.

^*(Nobody has ever asked me this.)

As mentioned in Part 1, the purpose of this code is to help automate file format conversions and presentation for posting vector designs to Etsy. One kind of design I make is “shadow theaters,” which are essentially little layered dioramas that look nice when backlit.

The individual layers of these designs comprise an outline, scoring lines for folding the paper, and the design itself. However, creating a thumbnail to represent the design has a unique challenge: the layers are negatives. The design file fills the areas that will be cut out, but this is where the greatest illumination will come through. I want the thumbnail to look like the photograph above. Furthermore, I can’t simply composite all the layers, as they are opaque, and the result would only show the top layer.

So the task becomes to invert the layers. But even that isn’t so simple. In each of the layers, I want to take the filled area and make it transparent, and take the unfilled area and fill it. To really make it work, the filled areas should not be a uniform color, but should vary by how far “down” in the stack the layer is.

However, this isn’t as simple as removing layers like I did in Part 1. What I have to do is take the outermost layer, fill it, and subtract the geometry of the inner design from it. Fortunately, that’s not so difficult to do in SVG. If you take a path in SVG and close it, you can continue to add sub-paths. When those sub-paths are outside the original path, they are added to it. If the sub-paths are inside the original path, they are subtracted from it. If they overlap, they’re both added and subtracted. This is probably better explained with pictures.

Disclaimer: for the purpose of this discussion, I’m assuming two SVG settings: fill-rule and clip-rule are set to “evenodd”. There are other options, but they aren’t helpful for what I want to do.

What do these paths and sub-paths look like? If you look at the example from Part 1, you can see that a path has a d attribute, which is a string of commands and coordinates. Rather than go through those here, I’ll just link the relevant Mozilla SVG tutorial page which I found helpful. The key thing is that a complete, closed path ends with a Z command. Any path details after that are a subpath.

Looking at an SVG file of one of my shadow gallery layers, what I want to do is convert the “Design” layer to a fully-contained sub-path of the “PaperOutline” layer. Based on what we know of SVG paths, that means I can just append the design path to the outline path – the design path will become a sub-path which will be subtracted from the outline path! I then can set the fill color of this composite path, and I will effectively have filled the outline and subtracted my design.

I again take advantage of the fact that the layer names are following my standard. I search through the file for the “Design” layer, and copy its path into the $subtract_path variable. Then I loop through the groups again, find the “PaperOutline” layer, and append the path. While I’m at it, I set the fill color with a shade I calculate based on the layer number.

<?php
$total_layer_count = 1;
for ($this_layer_index = 1; $this_layer_index <= $total_layer_count; $this_layer_index++)
{
    $doc = new DOMDocument();
    $doc->load("layer-{$this_layer_index}.svg");
    $shade = 255 * ($total_layer_count - $this_layer_index / 1.5) / $total_layer_count;
    $subtract_path = '';
    $rm = [];
    $groups = $doc->getElementsByTagName('g');
    foreach ($groups as $this_group)
    {
        $layer_name = $this_group->getAttribute('id');
        if (preg_match('/design/i', $layer_name))
        {
            foreach ($this_group->childNodes as $node)
            {
                if (in_array(get_class($node), ['DOMNode', 'DOMElement']))
                    $subtract_path .= $node->getAttribute('d');
            }
            array_push($rm, $this_group);
        }
    }
    foreach ($groups as $this_group)
    {
        $layer_name = $this_group->getAttribute('id');
        if (preg_match('/outlinepaper/i', $layer_name))
        {
            foreach ($this_group->childNodes as $node)
            {
                if (in_array(get_class($node), ['DOMNode', 'DOMElement']))
                {
                    $opath = $node->getAttribute('d');
                    $node->setAttribute('d', $opath . $subtract_path);
                    $style = $node->getAttribute('style');
                    if (preg_match_all('/fill:([^;]+)/', $style, $matches))
                    {
                        foreach ($matches[1] as $tm)
                        {
                            $style = str_replace($tm, "rgb($shade,$shade,$shade)", $style);
                        }
                    }
                    $node->setAttribute('style', $style);
                }
            }
        }
    }
    foreach ($rm as $trm)
    {
        $trm->parentNode->removeChild($trm);
    }
    $res = $doc->saveXML();
    $out = fopen("layer-{$this_layer_index}-inverted.svg", 'w');
    fwrite($out, $res);
    fclose($out);
}

Not the most elegant code in the world, and will certainly fail on SVG files that don’t match my naming and layout conventions, but illustrates the process.

Once I’ve created these inverted SVGs, I convert them into PNGs, composite the stack into a single image, and come up with what I consider a handsome representation for my design thumbnail.

Once I have the ability to take layers and change the transparent areas and the fill color, I can do all sorts of other manipulations. For example, I can create striking negative images, or feed the the layers into other scripts to create “exploded” views of the shadow theaters.

Now, before I even start, I have to admit this is a weird idea. I’m sure there are better languages and libraries for this stuff. But I’ll explain the rationale and then go into the details.

I’ve been creating a lot of designs for cards and other things which I cut out of paper with a Silhouette Cameo or out of other materials using the laser cutter at CrashSpace. I sell some of these designs at Etsy.com. To streamline the process of making files available in a variety of formats and posting them for sale at Etsy, I’ve written myself some PHP scripts that manipulate files, build thumbnails, and bundle stuff together and submit them for sale via Etsy’s API. Why PHP? The short answer is that it’s the language I use most on a day-to-day basis for work, and I have a lot of experience using it to manipulate images and interact with APIs. Are there languages that would be better suited? Probably, but the point of these scripts is not to be elegant. They just have to be something I can maintain, extend, and use to get the job done.

Anyway, I don’t know exactly how people will use the design files or what software they’ll use, so to make things flexible for them, I export the different layers of a file separately and in combination, e.g., a file with the outline, a file with the scoring lines, a file with the design, and a file combining all of the elements. To do this, I have a naming convention for the layers that I create in Affinity Designer, and use those names to manipulate the layers.

So, for example, start with a card with three layers: Outline, Score, and Design. I export it as an SVG file from Designer. There are a few settings that make the easier to manipulate later: especially “flatten transforms” and “add line breaks”.

SVG files are just fancy text files. Consider the following sample image:

If you open up this SVG file in a text editor, you’ll see the source. It’s got a header with some file details like viewport size and defaults for rendering lines and shapes. Each layer from Designer is represented in the file as a group (the <g> tag), and each group contains paths with the actual geometry:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="3300px" height="2550px" version="1.1" xmlns="http://www.w3.org/2000/svg"
     xmlns:xlink="http://www.w3.org/1999/xlink" xml:space="preserve"
     xmlns:serif="http://www.serif.com/"
     style="fill-rule:evenodd;clip-rule:evenodd;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:1.5;">
    <g id="Score">
        <path d="M1650.09,152.011L1650.09,2246.78" style="fill:none;stroke:black;stroke-
width:0.42px;stroke-dasharray:1.25,4.17,0,0;"/>
    </g>
    <g id="Outline">
        <path d="M150,150L3150,150" style="fill:none;stroke:black;stroke-width:4.63px;"/>
        <path d="M3150,150L3150,2250" style="fill:none;stroke:black;stroke-width:4.63px;"/>
        <path d="M150,150L150,2250" style="fill:none;stroke:black;stroke-width:4.63px;"/>
        <path d="M150.124,2250.07L3150.12,2250.07" style="fill:none;stroke:black;stroke-width:4.63px;"/>
    </g>
    <g id="Design">
        <path d="M2594.13,912.644L2594.13,524.494L2087.86,524.494L2087.86,1030.77L2387.83,1030.77L2387.83,
1362.85L2838.03,1362.85L2838.03,912.644L2594.13,912.644Z" style="fill:rgb(128,128,128);stroke:black;
stroke-width:1.46px;"/>
    </g>
</svg>

One of the things my scripts do is create “hairlined” versions of the file. This makes sure all shapes are unfilled and have very thin border strokes. Since the SVG file is just text, I can manipulate it with same kinds of tools I’d use to manipulate any text file. If you’re as demented as I am, this includes regular expressions, which is what I use for the hairlining:

$svg = file('original.svg');
foreach ($svg as $tlidx => $tl)
{
   if (preg_match_all('/fill:((?!none)[^";]+)/', $svg[$tlidx], $matches))
   {
      foreach ($matches[1] as $tm)
         $svg[$tlidx] = str_replace($tm, 'none', $svg[$tlidx]);
   }
   if (preg_match_all('/stroke-width:([^";]+)/', $svg[$tlidx], $matches))
   {
      foreach ($matches[1] as $tm)
         $svg[$tlidx] = str_replace('stroke-width:' . $tm, 'stroke-width:1px', $svg[$tlidx]);
   }
}
file_put_contents('converted.svg',implode("\n",$svg));

This is crude but effective. It replaces every fill style with “none,” and converts every stroke-width to a single pixel width. Using regular expressions is generally a fool’s errand, but this particular set seems to work pretty reliably.

One of the other things that my scripts do is take the source file with all three layers, and save versions with fewer layers. For example, one file contains only layers that will be cut rather than scored. This is where the specifics of the SVG format gets interesting. It turns out that the SVG file format is based on XML, thus we have a wealth of tools at our disposal to process them. In this case, we will use the power of PHP’s dreaded DOMDocument to manipulate the SVG file.

Here’s how to go through and remove the “Score” layer from the file.

$remove = [];
$dom = new DOMDocument();
$dom->load('original.svg');
$groups = $dom->getElementsByTagName('g');
foreach ($groups as $this_group)
{
    $layer_name = $this_group->getAttribute('id');
    if (!strcasecmp($layer_name, 'Score'))
    {
        array_push($remove, $this_group);
    }
}
foreach ($remove as $this_removal)
{
    $this_removal->parentNode->removeChild($this_removal);
}
$res = $dom->saveXML();
$out = fopen("scoreless.svg", 'w');
fwrite($out, $res);
fclose($out);

In the SVG file, the layer name is stored in the group’s id attribute, so the code iterates through the groups in the and uses that attribute to identify the layer it wishes to remove.

In the next post on this subject, I’ll discuss combining DOMDocument manipulations and string manipulation to do other fancy stuff like subtracting shapes from one another.

This monarch caterpillar takes advantage of the hot weather to make a late-November start.