Merge duplicate defs

[drewnoakes]

An Inkscape document I created used mid-markers on paths.

After running SVGO I see:

<defs>
  <marker id="a" orient="auto" overflow="visible">
    <path fill="#727272" fill-rule="evenodd" stroke-width="1pt" d="M-2.5-1c0 2.76-2.24 5-5 5s-5-2.24-5-5 2.24-5 5-5 5 2.24 5 5z" transform="matrix(.2 0 0 .2 1.48 .2)"/>
  </marker>
  <marker id="b" orient="auto" overflow="visible">
    <path fill="#727272" fill-rule="evenodd" stroke-width="1pt" d="M-2.5-1c0 2.76-2.24 5-5 5s-5-2.24-5-5 2.24-5 5-5 5 2.24 5 5z" transform="matrix(.2 0 0 .2 1.48 .2)"/>
  </marker>
  ...
</defs>

<path marker-mid="url(#a)" ... > ... </path>
<path marker-mid="url(#b)" ... > ... </path>
...

This particular document has 15 duplicate marker definitions. Merging them reduces the file size by 30%.

Would merging these duplicate markers be a safe optimisation for SVGO? Are there similar examples I can look at to help build a PR?

As I said elsewhere, I'm interested in making some contributions to this project. I'd like some feedback on ideas before putting together PRs.

[GreLI]

I guess, it can be. However, I think it's a rare case, so it will not give much profit in general. There is no same plugin. mergePaths concatenates sibling paths with same params, but you'll have to track IDs.

[drewnoakes]

@GreLI thanks for your responses.

I suppose there are two general scenarios that could be looked at here:

1. Merge duplicate defs
2. Remove unused defs

Both will require the ability to track IDs, as you say.

My questions are:

• Is the removal of unused defs already implemented? (I didn't find it when I looked)
• Is there any guidance on how defs can be referenced/discovered elsewhere in the document? (e.g. specific attributes, specific syntax, whole/partial attribute value matching on url\(#[^)\s]+\) or similar)

[drewnoakes]

cleanupIDs.js discovers IDs and can serve as a reference for this.

[GreLI]

Yes, useless defs are being removed by removeUselessDefs. cleanupIDs removes unused IDs, so it removes defs without IDs.
No guidance for that for now. Only cleanupIDs saves IDs for its internal need.s

[jakwings]

I want this feature. These duplicates are usually generated by copy&paste in the SVG editor.

[flipflop97]

Yes please, Inkscape creates lots of duplicate defs.

[guignonv]

+1. I do a lot of copy/paste and generate a huge lot of duplicate defs. My document takes a lot of time to load! :-S Would be nice to optimize that.

[PRR24]

My +1, very useful to remove duplicate raster images (for exmaple created by inkscape or poppler), significantly reducting some svg-s.

[nschloe]

I also find SVGs in the wild with excessive use of duplicate <def>s. Most of the time those are simple red lines like

<defs>
  <image ns1:href="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAABCAIAAABR8BlyAAAABmJLR0QA/wD/AP+gvaeTAAAADklEQVR42mO4lZZGNgIA9wYg+cZ4RWoAAAAASUVORK5CYII=" id="f" width="20" height="1" x="0" y="0" />
...
</defs>

with hundreds of repetitions.