bug#42240: Interpretation of 'size' Property in Fontconfig Pattern Strings

[Sebastian Miele <sebastian.miele@gmail.com>]

Robert Pluim <rpluim@gmail.com>:

> Cairo implies --without-xft in any case, so no harm done.

Good to know. Thank you.

> Looks like Iʼll have to boot up my GNU/Linux box (although Iʼm hazy on
> the difference between 'size' and 'pixelsize')

The main Fontconfig manual page describing all properties is called 'man
5 fonts-conf' (which can be hard to find, because neither 'fc' nor
'fontconfig' are substrings of 'fonts-conf').

The 'size' is the point size. In general, there are many different
definitions of point size. But in practice only one is used nowadays
(except by TeX which uses a slightly different but very close definition
that works better with its fixed point arithmetics). Usually a point is
1/72 inch (https://en.wikipedia.org/wiki/Point_(typography)).

Together with 'dpi' (dots/pixels per inch) that defines the relation
between point size and pixel size.

X has at least two different places where a dpi may be defined. One for
the entire X display, although that does not really make sense with an X
display having monitors of different pixel densities. Apart from that at
least the Xrandr extension has a notion of distinct dpis per monitor.

In general, Fontconfig has an even separate dpi. I do not really know
what dpi Fontconfig uses, if it is not explicitly set in the Fontconfig
configuration, but, e.g.,

  fc-match :family=Hack:size=11.1 size pixelsize dpi

or

  fc-match -v :family=Hack:size=11.

display the dpi used.