[Texmacs-dev] Qt and SVG images

[Philippe Joyez]

Hi all,

Best wishes for 2022.

Recently Joris and Darcy made changes in the handling of SVG images :
they are now entirely managed by Qt, which seems a nice and consistent
move. However, if a user tries for instance to insert
https://commons.wikimedia.org/wiki/File:Subduction-en.svg (a valid svg
file as per W3C), they will find a barely recognizable image and
probably blame TeXmacs for that. This is not an isolated issue; most of
my images produced with Inkscape also have rendering issues with Qt,
although they look perfect in web browsers.

The main reason for these problems is that Qt only supports the
restrictive specification SVG 1.2 Tiny, while browsers support SVG 1.1
Full, although it is far from obvious in Qt documentation (to have an
idea of the differences see
https://en.wikipedia.org/wiki/Comparison_of_layout_engines_(Scalable_Vector_Graphics)#SVG_1.1_support).
Beyond this limitation, there are also a lot of open Qt bugs around SVG
(QTBUG-96013, 95193, 96013, 44863, 94326, 90803, 42196, 87712, 87327,
88494, 25896 (open since 2012!) , 73776, 72997, 69508, 27611 ...). Note
also that in QTBUG-87327 I found the comment "Currently the SVG
component is unmaintained" (Oct 2020) and looking at the commits at
https://github.com/qt/qtsvg/graphs/contributors confirmed that.

So, sadly, SVG in Qt is a mess that will not resolve anytime soon. For
improving the situation I suggest TeXmacs has the following behavior :

- When an external converter is available (Inkscape or rsvg-convert),
use it unconditionally for all svg images (converting them to bitmap for
screen display or pdf for printing).

- If no converter is available, when inserting/displaying an SVG image,
check the svg version, and if it is not 1.2 Tiny, display a message to
the user advising them to convert the image to pdf, or to install a
converter.

Philippe