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I found some information in unity-settings-
/* As we cannot rely on the X server giving us good DPI information, and
* that we don't want multi-monitor screens to have different DPIs (thus
* different text sizes), we'll hard-code the value of the DPI
*
* See also:
* https:/
* https:/
*
* http://
* Why EDID is not trustworthy for DPI
* Adam Jackson ajax at redhat.com
* Tue Oct 4 17:54:57 UTC 2011
*
* Previous message: GNOME 3 - font point sizes now scaled?
* Next message: Why EDID is not trustworthy for DPI
* Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
*
* On Tue, 2011-10-04 at 11:46 -0400, Kaleb S. KEITHLEY wrote:
*
* > Grovelling around in the F15 xorg-server sources and reviewing the Xorg
* > log file on my F15 box, I see, with _modern hardware_ at least, that we
* > do have the monitor geometry available from DDC or EDIC, and obviously
* > it is trivial to compute the actual, correct DPI for each screen.
*
* I am clearly going to have to explain this one more time, forever.
* Let's see if I can't write it authoritatively once and simply answer
* with a URL from here out. (As always, use of the second person "you"
* herein is plural, not singular.)
*
* EDID does not reliably give you the size of the display.
*
* Base EDID has at least two different places where you can give a
* physical size (before considering extensions that aren't widely deployed
* so whatever). The first is a global property, measured in centimeters,
* of the physical size of the glass. The second is attached to your (zero
* or more) detailed timing specifications, and reflects the size of the
* mode, in millimeters.
*
* So, how does this screw you?
*
* a) Glass size is too coarse. On a large display that cm roundoff isn't
* a big deal, but on subnotebooks it's a different game. The 11" MBA is
* 25.68x14.44 cm, so that gives you a range of 52.54-54.64 dpcm horizontal
* and 51.20-54.86 dpcm vertical (133.4-138.8 dpi h and 130.0-139.3 dpi v).
* Which is optimistic, because that's doing the math forward from knowing
* the actual size, and you as the EDID parser can't know which way the
* manufacturer rounded.
*
* b) Glass size need not be non-zero. This is in fact the usual case for
* projectors, which don't have a fixed display size since it's a function
* of how far away the wall is from the lens.
*
* c) Glass size could be partially non-zero. Yes, really. EDID 1.4
* defines a method of using these two bytes to encode aspect ratio, where
* if vertical size is 0 then the aspect ratio is computed as (horizontal
* value + 99) / 100 in portrait mode (and the obvious reverse thing if
* horizontal is zero). Admittedly, unlike every other item in this list,
* I've never seen this in the wild. But it's legal.
*
* d) Glass size could be a direct encoding of the aspect ratio. Base EDID
* doesn't condone this behaviour, but the CEA spec (to which all HDMI
* monitors must conform) does allow-but-
* 1920x1080 TV could claim to be 16 "cm" by 9 "cm". So of course that's
* what TV manufacturers do because that way they don't have to modify the
* EDID info when physical construction changes, and that's cheaper.
*
* e) You could use mode size to get size in millimeters, but you might not
* have any detailed timings.
*
* f) You could use mode size, but mode size is explicitly _not_ glass
* size. It's the size that the display chooses to present that mode.
* Sometimes those are the same, and sometimes they're not. You could be
* scaled or {letter,
* can control from the host side.
*
* g) You could use mode size, but it could be an encoded aspect ratio, as
* in case d above, because CEA says that's okay.
*
* h) You could use mode size, but it could be the aspect ratio from case d
* multiplied by 10 in each direction (because, of course, you gave size in
* centimeters and so your authoring tool just multiplied it up).
*
* i) Any or all of the above could be complete and utter garbage, because
* - and I really, really need you to understand this - there is no
* requirements program for any commercial OS or industry standard that
* requires honesty here, as far as I'm aware. There is every incentive
* for there to _never_ be one, because it would make the manufacturing
* process more expensive.
*
* So from this point the suggestion is usually "well come up with some
* heuristic to make a good guess assuming there's some correlation between
* the various numbers you're given". I have in fact written heuristics
* for this, and they're in your kernel and your X server, and they still
* encounter a huge number of cases where we simply _cannot_ know from EDID
* anything like a physical size, because - to pick only one example - the
* consumer electronics industry are cheap bastards, because you the
* consumer demanded that they be cheap.
*
* And then your only recourse is to an external database, and now you're
* up the creek again because the identifying information here is a
* vendor/model/serial tuple, and the vendor can and does change physical
* construction without changing model number. Now you get to play the
* guessing game of how big the serial number range is for each subvariant,
* assuming they bothered to encode a serial number - and they didn't. Or,
* if they bothered to encode week/year of manufacturer correctly - and
* they didn't - which weeks meant which models. And then you still have
* to go out and buy one of every TV at Fry's, and that covers you for one
* market, for three months.
*
* If someone wants to write something better, please, by all means. If
* it's kernel code, send it to dri-devel at lists.freedeskt
* and I will happily review it. Likewise xorg-devel@ for X server
* changes.
*
* I gently suggest that doing so is a waste of time.
*
* But if there's one thing free software has taught me, it's that you can
* not tell people something is a bad idea and have any expectation they
* will believe you.
*
* > Obviously in a multi-screen set-up using Xinerama this has the potential
* > to be a Hard Problem if the monitors differ greatly in their DPI.
* >
* > If the major resistance is over what to do with older hardware that
* > doesn't have this data available, then yes, punt; use a hard-coded
* > default. Likewise, if the two monitors really differ greatly, then punt.
*
* I'm going to limit myself to observing that "greatly" is a matter of
* opinion, and that in order to be really useful you'd need some way of
* communicating "I punted" to the desktop.
*
* Beyond that, sure, pick a heuristic, accept that it's going to be
* insufficient for someone, and then sit back and wait to get
* second-guessed on it over and over.
*
* > And it wouldn't be so hard to to add something like -dpi:0, -dpi:1,
* > -dpi:2 command line options to specify per-screen dpi. I kinda thought I
* > did that a long, long time ago, but maybe I only thought about doing it
* > and never actually got around to it.
*
* The RANDR extension as of version 1.2 does allow you to override
* physical size on a per-output basis at runtime. We even try pretty hard
* to set them as honestly as we can up front. The 96dpi thing people
* complain about is from the per-screen info, which is simply a default
* because of all the tl;dr above; because you have N outputs per screen
* which means a single number is in general useless; and because there is
* no way to refresh the per-screen info at runtime, as it's only ever sent
* in the initial connection handshake.
*
* - ajax
*
*/