Add basic <symbol> support in SVG

[Theo1335]

Add basic support in SVG parsing

This PR adds support for SVG <symbol> elements.

• Reuse existing group parsing logic for symbols.
• Add a unit test covering symbol parsing and cross-reference registration.

Checklist:

• A unit test is covering the code added / modified by this PR

• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder (N/A)

• A mention of the change is present in CHANGELOG.md

• This PR is ready to be merged

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[andersonhc]

Thank you for this PR @Theo1335 , it's looking very promising!

I found one problem testing this SVG

The symbols are rendered but the scale is wrong.

Can you please check how to fix this?

[Theo1335]

Thank you for your feedback @andersonhc !

I think I've finally solved the scaling issue with <symbol> elements: the viewBox is now extracted and stored when <symbol> is parsed, and when a <use> directive references a symbol with a width and height, the appropriate scaling is applied based on the `viewBox's dimensions.

Please let me know what you think.

[andersonhc]

Right now the test is failing:

    @force_nodocument
    def build_symbol(self, symbol: "Element") -> GraphicsContext:
        """Parse <symbol> as reusable content, not rendered directly."""
        group = self.build_group(symbol)
        viewbox = symbol.attrib.get("viewBox")
        if viewbox:
            parts = viewbox.replace(",", " ").split()
            if len(parts) >= 4:
>               setattr(group, "_vw", float(parts[2]))
E               AttributeError: 'GraphicsContext' object has no attribute '_vw'

I don't think adding the viewbox attribute to the GraphicsContext is the best approach.
I recommend looking into using the GaphicsContext's transform to apply the viewbox.

Apply translate(-vx, -vy) @ scale(1 / vw, 1 / vh) to the symbol group should make it normalized to unit coordinates. In build_xref(), apply scale(width, height) @ translate(x, y).

[Theo1335]

Hi @andersonhc , I've implemented your suggestion using transforms. The viewBox is now applied in build_symbol with scale(1/vw, 1/vh) @ translate(-vx, -vy), and build_xref applies scale(width, height) when specified. I also ignore percentage values for width/height. I tested it and it should be working now. Let me know what you think !

[andersonhc]

Please see suggested changes.

I would also add you to add a new item on test/svg/parameters.py on the test_svg_sources set:

pytest.param(svgfile("symbol.svg"), id="symbol reused in different placements"),

then create the file test/svg/svg_sources/symbol.svg with the content below:

<svg viewBox="0 0 80 20" xmlns="http://www.w3.org/2000/svg">
  <!-- Our symbol in its own coordinate system -->
  <symbol id="myDot" width="10" height="10" viewBox="0 0 2 2">
    <circle cx="1" cy="1" r="1" />
  </symbol>

  <!-- A grid to materialize our symbol positioning -->
  <path
    d="M0,10 h80 M10,0 v20 M25,0 v20 M40,0 v20 M55,0 v20 M70,0 v20"
    fill="none"
    stroke="pink" />

  <!-- All instances of our symbol -->
  <use href="#myDot" x="5" y="5" opacity="1.0" />
  <use href="#myDot" x="20" y="5" opacity="0.8" />
  <use href="#myDot" x="35" y="5" opacity="0.6" />
  <use href="#myDot" x="50" y="5" opacity="0.4" />
  <use href="#myDot" x="65" y="5" opacity="0.2" />
</svg>

After that you will need to execute test_svg_conversion() once adding generate=True to assert_pdf_equal so it can generate the new PDF reference file.

[Theo1335]

Thanks for the detailed feedback! I will implement your suggestions and get back to you once it's done, or if I don't understand some details.

[Theo1335]

Hi @andersonhc, thank you again! I've implemented all your suggestions, let me know what you think.

[andersonhc]

@allcontributors please add @Theo1335 for code

[allcontributors]

@andersonhc

I've put up a pull request to add @Theo1335! 🎉

[andersonhc]

Thank you for the contribution @Theo1335
The PR has been merged