Fix bare-number/px unit inconsistency and coordinate system (2.0b1)

[deeplook]

Fixes #439.

Summary

SVG spec §5.9.2 defines 1 user unit = 1 px = 0.75 pt (at 96 dpi). Previous
releases treated 1 user unit as 1 pt, causing font-size="13" and
font-size="13px" to render at different sizes, and all output to be 33%
larger than a browser would produce.

Changes

• convertLength now returns user units (px) instead of ReportLab points.
• New convertLengthToPt helper multiplies by PX_TO_PT = 0.75 for the few
  places that need absolute points (font sizes, canvas dimensions).
• get_box parses viewBox as raw floats — never applies unit conversion
  (viewBox defines a unitless coordinate space, SVG spec §8.1).
• renderSvg viewport scale = canvas_pts / viewBox_user_units, which
  naturally incorporates ×0.75 for all shape coordinates via the group
  transform.
• SVGs with a viewBox but no explicit width/height now fall back to
  viewBox dimensions for the scale, fixing cropping of such files.
• DEFAULT_FONT_SIZE restored to 12 (pt, the CSS default); em bases use
  DEFAULT_FONT_SIZE / PX_TO_PT = 16 user units.
• Regression tests added for bare-number == px consistency at both the
  convertLength level and the rendered fontSize level.

⚠️ Breaking change

Output PDF sizes will differ from 1.x: any SVG whose dimensions are expressed
in user units or px will produce a document that is 75% of its previous
linear size (correct physical size per spec).

Migration — to restore 1.x dimensions:

drawing = svg2rlg("file.svg")
factor = 4 / 3   # 1 / 0.75
drawing.width  *= factor
drawing.height *= factor
drawing.scale(factor, factor)

See CHANGELOG.rst for full details.

Related issues filed

• preserveAspectRatio not implemented — non-uniform scaling distorts SVGs with mismatched viewBox/canvas aspect ratios #448 — preserveAspectRatio not implemented (pre-existing, unrelated)
• Add support for SVG 2 viewport and root-relative units: rem, vw, vh, vmin, vmax, q #449 — SVG 2 viewport units rem/vw/vh not yet supported (future work)

[mosc9575]

I tried this changes and they solve the problem I had with the font size. The transformation from SVG to PDF works well.

But the new factor PX_TO_PT = 0.75 is not reverted in reportlab.

The function setFont of class SVGCanvas in reportlab.renderSVG looks like

  def setFont(self, font, fontSize):
      if self._font != font or self._fontSize != fontSize:
          self._font = font
          self._fontSize = fontSize
          style = self.style
          for k in TEXT_STYLES:
              if k in style:
                  del style[k]
          svgAttrs = self.fontHacks[font] if font in self.fontHacks else {}
          if isinstance(font,RLString):
              svgAttrs.update(iter(font.svgAttrs.items()))
          if svgAttrs:
              for k,v in svgAttrs.items():
                  a = 'font-'+k
                  if a in TEXT_STYLES:
                      style[a] = v
          if 'font-family' not in style:
              style['font-family'] = font
          style['font-size'] = '%spx' % fontSize

In the last line the point to pixel ratio is 1:1. This should be reported.

My initial example in #433 shows this behavior. The initial font size is 13px when calling svglib.svg2rlg and after calling svg.asString(format="svg") the font size shrinks to 9.75px which is odd. I know, that this is an issue in reportlab.

It is great the the one direction now works as expected.

Once again I want to thank you for your work.

[deeplook]

Re: font size shrinkage in SVG round-trip

This is a known limitation caused by a bug in ReportLab's renderSVG module. When ReportLab renders a drawing back to SVG, SVGCanvas.setFont writes font sizes with px units even though the value is already in points:

# reportlab/graphics/renderSVG.py
style['font-size'] = '%spx' % fontSize  # should be 'pt', not 'px'

Minimal reproduction with ReportLab 4.4.3:

from reportlab.graphics.shapes import Drawing, String
from reportlab.graphics import renderSVG

# A string at 9.75pt (equivalent to 13px at 0.75 scale)
d = Drawing(200, 100)
d.add(String(10, 50, "Hello", fontSize=9.75))

svg_out = renderSVG.drawToString(d)
print(svg_out.decode())
# Output contains: font-size: 9.75px
# But 9.75 is in points — should be font-size: 9.75pt (or font-size: 13px)

The fix in ReportLab would be to change the last line of SVGCanvas.setFont to:

style['font-size'] = '%spt' % fontSize

This is outside svglib's control. The svg2rlg direction (SVG→RLG) now works correctly — the round-trip shrinkage is a pre-existing ReportLab issue.

[mosc9575]

@deeplook Thanks for you summary. I will send a mail to a reportlab author and link your comment to fix also this issue.

[claudep]

@deeplook, I'm sorry, but I will clearly be less active on this module, so don't count on me for reviews in the future. Thanks a LOT for all you are doing 🏅

[deeplook]

@deeplook, I'm sorry, but I will clearly be less active on this module, so don't count on me for reviews in the future. Thanks a LOT for all you are doing 🏅

@claudep Thank you for your many contributions in the past! Wishing you all the best!