Font DPI and screen scaling

[test]

On Fri, 2020-06-12 at 01:32 +0200, Frank Steinmetzger wrote:
> On Tue, Jun 09, 2020 at 12:24:25AM +0200, test wrote:
> > On Sun, 2020-06-07 at 22:39 +0200, Frank Steinmetzger wrote:
> > > [...]
> > > In your valued experience/opinion, what is the appropriate way of
> > > ensuring
> > > that a 10 pt font has the same physical size on all screens if I use
> > > display
> > > scaling exclusively? Is a screen scale of 100 % assuming 96 dpi?
> > 
> > The only way to do that is to use displays that have the same amount of
> > dots per square(!) inch.
> 
> Or tell the system how many DPI my screens have, so fonts have the proper
> physical size.

How would that make the font have the same size?  For example, when a
character occupies 10x10 dots in size and the display has 192x192 dots per
inch instead of 96x96, the character would need to be somehow converted
from 10x10 into 20x20 dots.  Where is the additional information required
for this conversation supposed to come from?

Are all common fonts like, for example, Liberation Sans Mono vector fonts?
 I really don't know how that works other than that it's really bad when
you have a 4k display.  You can get useful results when you mess around
with it, yet it all goes totally out of whack, and some things are
unfixable, like the font size of the message list in Evolution being way
too small and with no way to change that.

> > What do you mean by "screen scaling"?
> 
> System settings → Screens → Global Scale
> It uses a percent scale just like MS Winblows. But a percentage is always
> a
> comparison to a base figure. So what is that here? This is the same (IMHO
> stupid) thing that MS did in their settings; scaling with a percentage
> with
> no physical meaning behind it.

Cool, I didn't know that there is such a setting in KDE.  I guess 100% is
the base figure, so it should look the same when you have twice the dots
and set it to 200%.

But what when the aspect ratio is different, or when I turn the display to
make use of the width as height?  Why aren't there two percentages?

Should I use this screen scaling and make the fonts smaller again such that
the ones which are too small finally become large enough?

Can I use the screen scaling per display when multiple displays are
connected?