Font-size in px results in incorrect displayed size

[taminomara]

This is new in 1.6.0.

This is the code I used to test the issue:

import io
from reportlab.graphics import renderPM
from svglib import svglib

render = """
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="500" height="100" viewBox="0 0 500 100">
  <text x="250" y="14" font-family="Courier New" font-size="14" text-anchor="middle" stroke="#000">The quick brown fox jumps over the lazy dog</text>
  <text x="250" y="28" font-family="Courier New" font-size="14px" text-anchor="middle" stroke="#000">The quick brown fox jumps over the lazy dog</text>
</svg>
"""
stream = io.StringIO(render)
drawing = svglib.svg2rlg(stream)
renderPM.drawToFile(drawing, "output.gif", dpi=72)

In 1.5.1, both lines have the same size:

In 1.6.0, the <text> with font-size given in px, the text is significantly smaller:

[github-actions]

Thank you for raising your first issue! Your help to improve svglib is much appreciated!

[claudep]

This change is caused by that commit: f207876 (see #386). So I'll let @deeplook explained why this was changed.
Also note that the spec is not allowing a non-0 value without an explicit unit.

[deeplook]

I can reproduce the effect (for 1.5.1 and 1.6.0), but from taking a quick look I see that #387 referenced indirectly be @claudep was before 1.6.0, and I don't see immediately how 1.6.0 has changed this.

[deeplook]

PR #387 wasn't merged to main for a long while until 1.6.0. It was intended to make unit conversions between pt and px more compliant with standards (specifically, a 96 DPI standard where 1px = 0.75pt) and fix the reported issue. But it seems like it introduced an inconsistency between how values with px units and unitless values are handled.

The SVG spec says unitless length values have "user units" which in the non-print world is effectively equal to pixels (a virtual length). But the base unit in reportlab is points (a physical length), and if you treat all unitless lengths as pixels things start to break. I want to add a few more tests, but I consider reverting #387, and maybe add an option to interpret unitless values as pixels.

[taminomara]

Thank you! Just to avoid any confusion, in the examples that I've sent, the unitless text looks correct, while the one with px is smaller. I.e. right now, px unit is too small.