Why EDID is not trustworthy for DPI

Andreas Tunek andreas.tunek at gmail.com
Tue Oct 4 18:58:26 UTC 2011

Thanks for writing this up! It was good info.
On Oct 4, 2011 7:55 PM, "Adam Jackson" <ajax at redhat.com> wrote:
> 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-not-require it, which means your
> 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,pillar}boxed, and that's not necessarily something you
> 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.freedesktop.org and cc me
> 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
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