Hello Tor Arne and all,

I'll try to give a reply to this, but keep in mind I'm not a core
developer; my response is mostly guided by my experience with working
with Xorg in mixed-DPI environment, and as much insight as I've
managed to gather from the experience, experience that has matured
mostly in:

* the xdpi debug tool: https://github.com/Oblomov/xdpi
* a write-up about the reality of mixed-DPI in X11 as of a couple of
years ago: http://wok.oblomov.eu/tecnologia/mixed-dpi-x11/ (not that
much has changed; also, if there's any feedback about the content of
this article, suggestions are welcome)
* a tentative patchset to include mixed-DPI support in awesome WM,
https://github.com/awesomeWM/awesome/pull/2053 (currently without too
much chance of going forward, and not only because I don't have the
time to work on it as would be appropriate);
* some discussion on IRC with keithp concerning his proposed
window-scaling extension https://keithp.com/blogs/window-scaling/

Before going forward, I'd like to clarify that I may have a somewhat
different idea about DPI and scaling. To make sure we understand each
other, I'd like to clarify some  terminology (independently from the
window system being used).

For each device, there are three (at least; possibly four)
display-related values that are relevant to the discussion.

One is the physical pixel density, represented by the number of
physical pixels spanning an inch of physical media. Ajax has written
at length about the issues concerning the retrieval of correct
information about this value, and I'm quite convinced that any
possible solution for the issues related to this cannot come from
within the display server itself, although the server may provide
features to override any detected values (still, I think these would
be better handled at a lower level, e.g. by the kernel). This is
particularly true for cases (such as projects) where the physical
density is much more dependent on the user setup than on a particular
hardware characteristic.

The second value is the “visual” pixel density, which depends on the
physical pixel density as well as on the distance of the observer to
the viewing surface. A high-resolution display held very close to the
eyes (e.g. VR headset) may have a “visual” pixel density which is the
same or lower than that of a coarse-resolution display which is much
farther away (e.g. a standard-resolution projector seen from several
meters away).

The third value is the user preference for UI scaling, which is (or
rather should be, see below) completely independent from the display
resolution. A possible fourth value is the “reference” pixel density
(for which we can consider the CSS “reference” of 96dpi), which is the
one with respect to which the UI scaling _should_ be defined. And one
of the biggest issues with the correct handling of DPI is that almost
everywhere the UI scaling preference is “squashed together” with the
physical-to-reference DPI setting, which ultimately causes a bit (or a
lot) of confusion at both the display server and toolkit/application
level.

The fact that the UI scaling and DPI handling should be separate
becomes particularly important in mixed-DPI setup. Consider for
example the (relatively common) case of two monitors (a 192-DPI and a
96-DPI one) attached to the same display server and viewed from the
same distance. Then, for an image to appear to be at the same size, it
should be scaled 2x when on the 192-DPI monitor compared to when
displayed on the 96-DPI monitor, because the high-DPI monitor needs a
2x “DPI scaling” to reach the “reference” pixel density. This is
_independent_ of any user preference for UI scaling, so that if the
end user opts for a 150% UI scaling (e.g. to compensate for their poor
eyesight) this ends up using a 3x _overall_ scaling for the high-DPI
monitor vis-a-vis a 1.5 scaling on the standard-DPI monitor. Ideally,
the user would only have to choose the UI scaling, with the DPI
scaling managed automatically (as far as possible i.e. within the
limits of the autodetection of the device DPI).

My understanding from reading
https://keithp.com/~keithp/talks/xtc2001/paper/ is that the intent of
the Xft.scale resource was to manage the “user UI preference”
(hopefully keithp can confirm), but my understanding is that
“everybody” has settled on using Xft.dpi for this instead —which is
quite a bother, if you ask me.

I'm not entirely sure how the Qt concept of logical DPI fits into
these. I'm guessing it's somewhere between the reference DPI and the
UI scaling configuration?

Now onto your question:

> Now, for X, there's at least four different things to consider, as far as I 
> can tell:
>
>     1) The resolution and size of the X Screen
>     2) The resolution and size of the individual outputs
>     3) The resolution and size of the RandR 1.5 monitors
>     4) The Xft.DPI setting.
>
> (For all the things exposed through RandR (1-3), as far as I can tell they 
> are all stored as resolution and size (in mm), so all DPI-numbers going in or 
> out of X are effectively converted to a width and height in mm to represent 
> that DPI with the current resolution taken into account.)

You may want to add to these the XSETTINGS, whose (dynamically
adjustable) Xft/DPI value works in pretty much the same way as the
Xft.dpi resource (and overriding it if both are present). This has the
same limitation as Xft.dpi concerning globality, though.

> The last one is the easy one, it's clearly a logical DPI, and we reflect that 
> in Qt if set. Unfortunately it's a global DPI.

Arguably, the biggest issue is that  Xft.dpi is being used beyond its
original intentions (defining the DPI for point size to pixel count
conversion used by Xft). Since Xft isn't compatible with RANDR (in the
sense that its API isn't output- or monitor-aware) the fact that it
deals only in global value would be acceptable. The unfortunateness of
it is that the value is otherwise used to set the UI scaling (where
Xft.scale would have been a better choice).

Given the current usage, though, Xft.dpi is one of the ways in which
users can override the global scaling (conflating UI scaling and
physical-to-reference DPI scaling).

> Now, I'm guessing that #1, as set by Xorg -dpi, xorg.conf DisplaySize, or 
> xrandr --dpi, originally was meant as a physical DPI override, for cases 
> where the detection and heuristics in X would fail? But nowadays, especially 
> with a single X Screen representing multiple physical displays, with 
> potentially different physical DPIs, it feels like it's effectively a logical 
> DPI setting on an X level, with the same limitation as Xft.DPI in that it's a 
> global setting. What is your take on this?
>
> If it's the former — a physical DPI override (however little that makes sense 
> when reflecting multiple displays) — we don't want to reflect it per QScreen, 
> as that would not be specific enough in a multi monitor setup. Nor do we want 
> to reflect it for a QScreen's logicalDpi, if it's strictly defined as a 
> physical property, not to be used for adjusting logical DPI.
>
> But if it's in practice the latter — a logical DPI override — then we should 
> reflect it through a QScreen's logicalDpi, if Xft.DPI hasn't been set to 
> override it.

AFAIK, the DPI of the X Screen has no physical meaning today, which is
why it's normally set to 96 rather than trying to second-guess the
value from the RANDR setup. Legacy applications continue using it as a
fallback if Xft.dpi is not defined (following the Xft.dpi
specification), so it can be used to control their rendering through
it.

Note however that its value is actively ignored by GTK3 (see also
https://bugzilla.gnome.org/show_bug.cgi?id=757142 and associated
issues such as https://gitlab.xfce.org/xfce/xfce4-settings/-/issues/34
and https://bugs.mageia.org/show_bug.cgi?id=21201 for example).
Personally, I disagree with the choice of the GTK3 developers, since
ignoring the value is an unnecessary regression that also breaks the
Xft.dpi fallback, and as a frequent user of mixed DPI configurations
I'd rather see it used for the logical DPI override.

> Now, for #2, as far as I can tell there isn't any option in xrandr to 
> override this, nor does tweaking DisplaySize in xorg.conf affect it (even for 
> multiple Monitor sections), so I'm guessing it's strictly a physical size 
> picked up from EDID? If that's not the case, and it's possible to override it 
> for the user, then the same questions as for #1 apply: Does that make it a 
> logical DPI?

According to the spec, RANDR reports the physical size (if known), and
there is no way to change it via API (it's not user-settable), so from
it you get a physical DPI.

By the way, considering the globality of Xft.dpi, I think toolkits
should agree on using a user-settable per-output property to define
the physical-to-reference scaling of that output (_NETWM_SCALE or
whatever). This could even be used by the server (with keithp's
window-scaling extension) to automatically scale legacy apps (e.g.
clients that do not have a specific hint saying that they can do the
scaling themselves). At the very least Qt could start using this as a
more flexible alternative to the environment variables currently used
to set per-output scaling.

> Finally, for #3, this is where it gets interesting. From reading the RandR 
> spec [3] about the new Monitors introduced in 1.5, this seems like a defined 
> logical DPI:
>
>     "This new object separates the physical configuration of the hardware
>     from the logical subsets of the screen that applications should
>     consider as single viewable areas."
>
> It's possible to combine two outputs into one monitor, to split a single 
> output into multiple monitors,

Correct me if I'm wrong, but I don't think it's possible to split a
single output into multiple monitors, since adding an output to a
monitor will remove it from the other monitors.

> or even to override the auto-generated monitor for a an output. And all these 
> allow you to pass a width and height, effectively setting the DPI. E.g.
>
>   xrandr --setmonitor DUMMY0-DPIOVERRIDE 1600/200x1200/200+0+0 DUMMY0
>
> This seems like the definition of logical DPI, where the desktop environment 
> can give the user a nice control panel on how to adjust these things, either 
> directly by adding/removing/moving monitors, or by setting a DPI or scale 
> (200% e.g.) on an individual monitor, and then reflect that as RandR updates.

Now this is an interesting side effect. I believe the original intent
of the Monitor concept was to improve support for video walls and
physical monitors that require two streams because of how large they
are, but the possibility to override the physical size definitely
allows for user-selection of the presented DPI. Would you then go look
for the physical DPI as reported by the corresponding output(s)?

> Based on all of this, it seems Qt should do the following:
>
>   1. If Xft.DPI has been set, respect that as a global override, and reflect 
> that as the logical DPI for all QScreens
>   2. If not, reflect the resolution and size of individual RandR 1.5 monitors 
> as logical DPI per QScreen
>   3. If 1.5 is not available, reflect the resolution and size of the X Screen 
> as a global logical DPI for all QScreens
>   4. Reflect the resolution and size of the individual outputs as physical 
> DPI, or read EDID ourselves
>
> As far as I can tell this should cover DEs like Ubuntu 20.04 that sets a 
> global 192 Xft.DPI to represent 200% scaling (and fractional scales in 
> between 100% and 200%), as well as DEs that (in the future) allow per-monitor 
> DPI/scale control via the 1.5 monitors.

I suspect this might not be future-proof: DEs that allow per-monitor
DPI/scale control via RANDR 1.5 may still want to use Xft.DPI for
legacy applications. I don't think there's a way out of this without
adding some kind of side-channel setting (_NET_PER_MONITOR_DPI boolean
property on the root window). So the idea could be:

1. If _NET_PER_MONITOR_DPI is set, and RANDR 1.5 is present, use
Monitor info for logical DPI per QScreen;
2. if _NET_PER_MONITOR_DPI is set, and RANDR 1.5 is not present use
to-be-determined user-controllable per-output property for logical DPI
per QScreen; (assuming we want to support this kind of configuration,
with new DE/WM on pre-RANDR1.5 server);
3. fall back to Xft/DPI => Xft.dpi => X Screen dpi as global logical
DPI for all QScreens; (note that the X Screen dpi can change with RR,
and clients can get a notification when it happens; if possible, do
keep this into consideration);

Honestly while we're at it I would appreciate if Qt spearheaded the
separation of DPI scaling from UI scaling (with a separate root window
property or XSETTING or whatever), but I understand if this is
considered being “too much” (especially since AFAIK other OSes/display
servers don't have the concept either, but feel free to correct me if
I'm wrong).

Cheers,

Giuseppe Bilotta
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