Re-thinking DPI and scaling (Re: Physical vs logical DPI on X)

Tor Arne Vestbø Tor.arne.Vestbo at
Mon Oct 5 12:37:59 UTC 2020

Hey hey,

Thanks for forwarding the thread Pekka! I absolutely agree that this is relevant for Wayland too.

I think it makes sense to initially clarify the Xorg situation, with what’s there today (randr 1.5), before we go into new extensions, or the Wayland situation, so I will focus my attention to that thread first. Hopefully that gives us a baseline to compare against when brainstorming about Wayland too.

Tor Arne 

> On 5 Oct 2020, at 10:35, Pekka Paalanen <ppaalanen at> wrote:
> Hi all,
> The below email was sent to xorg-devel, but I think it is such a good
> discussion of the topic that I want to CC it to wayland-devel as well.
> While the email does go into X11 specifics, the fundamental ideas are
> well applicable to Wayland as well. There was an IRC discussion of the
> very same topic recently on #wayland.
> The difference in Wayland is that one believes the compositor should do
> some kind of fallback scaling to make windows legible in case the
> application did not or is not capable of doing it itself.
> Though, even that idea in the below is mentioned to have been
> investigated for Xorg as well.
> Currently in Wayland we have buffer scale, output scale, and the
> fractional scaling implementations in some compositors. We also *had*
> monitor physical size exposed via wl_output, however that is a
> controversial feature too IIRC. Maybe this discussion could
> eventually produce better ideas for Wayland as well.
> Due to my work on Wayland color management and HDR extension, my
> personal opinion on the Wayland buffer/output scale system has started
> to shift as well. AFAIK, the output/buffer scale design chose integers
> as the scaling factors because scaling raster images by non-integer
> factors necessarily makes them blurry. This stems from the graphics
> designs using pixels not as point samples but as colored "tiles",
> relying on the edge between two tiles to be crisp and visible (for
> low-DPI monitors), e.g. one pixel wide lines. Obviously, implementing
> fractional scaling makes the image quality degradation unavoidable
> under the current Wayland design when monitors are not high-DPI enough
> to hide the effect.
> The discussion below also criticises the choice of using a single
> number to conflate both UI scale and output resolution (DPI). If these
> two concepts need to be clearly separate, then I'm not sure the integer
> scale factor design is sufficient going forward.
> One more thing I'd like to have considered is the viewing distance. The
> write-up below does mention it: monitor physical size, pixel resolution
> and viewing distance all affect how "big" graphics appears to the user.
> But when we talk about DPI, it does not include the viewing distance,
> hence it is only a partial description of the output "apparent
> size" (in lack of a better term). DPI sets the lower limit of how small
> graphical patterns can theoretically be observable on an output, but
> viewing distance determines what sizes are actually legible, combined
> with variation in people's eye sight.
> An example of what I mean above: if you have a projector, it does not
> make sense to attempt to display 12pt (physical size! e.g. printed on
> A4 paper) font on it if 12pt would be two pixels high. The DPI
> of the projector is too low. You have to use at least some many pixels
> to make a font legible, so either you pick something much bigger than
> 12pt or there is another scaling factor involved with the projector.
> I intended this email as an opening for discussion, where I do not plan
> to participate actively, since CM&HDR is currently my priority.
> Thanks,
> pq
> On Sun, 4 Oct 2020 17:42:43 +0200
> Giuseppe Bilotta <giuseppe.bilotta at> wrote:
>> 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:
>> * a write-up about the reality of mixed-DPI in X11 as of a couple of
>> years ago: (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,
>> (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
>> 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
>> 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
>> and associated
>> issues such as
>> and 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
>> _______________________________________________
>> xorg-devel at X.Org development
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