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Old 10-06-2011, 12:46 PM
Evandro Giovanini
 
Default Why EDID is not trustworthy for DPI

Em Qui, 2011-10-06 às 08:21 -0400, Simo Sorce escreveu:
> On Thu, 2011-10-06 at 13:06 +0200, Nicolas Mailhot wrote:
> > Le Mer 5 octobre 2011 21:44, Simo Sorce a écrit :
> >
> > > Are you saying fonts should change on the fly when I move an app between
> > > 2 monitors that have different DPIs ?
> >
> > Unfortunately, when you get into situations with more than 150% difference in
> > pixel densities between displays (as we've been creeping towards in the last
> > decade) that's the only way to display text the user will be able to read.
> >
> > You can check it now easily, just get a run-of-the-mill full-hd 15" laptop
> > (not even a tiny netbook), a run-of-the-mill 22" or more screen (nothing
> > especially uncommon either), create an extended desktop with both screens and
> > try to set a satisfying font size. I defy you to find a setting that won't
> > look way too small or way too big on one of the screens. And it won't matter
> > if the user likes small or big fonts.
>
> Nicolas I am aware of the issue, but I am also aware of the technical
> difficulties in doing something like that.
> It's not possible today and I am not sure it will be in the near future.
>
> So currently the only option is to tell the user that we do not support
> multiple displays where pixel density varies by moire than 10% between
> them.
>
> I would even go as far as saying that by default gnome should refuse to
> let you join together screens of so high difference in density except we
> cannot trust the HW info apparently, so all we are left with is a bad
> user experience.
>

Please don't. I extend my 11" laptop on a 32" TV and despite poor
readability of regular fonts it still works just fine for what I need -
movie playback, photo viewing, PDF presentations, etc.

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Old 10-06-2011, 01:37 PM
Matthew Garrett
 
Default Why EDID is not trustworthy for DPI

On Thu, Oct 06, 2011 at 01:13:21PM +0200, Nicolas Mailhot wrote:
>
> Le Mer 5 octobre 2011 23:35, Matthew Garrett a écrit :
>
> > This... works badly. Really. Open gimp and add some text. Now double the
> > size of the font. Save the image and open it in image viewer, and zoom
> > out so the text is half the size. It doesn't look the same as your
> > original text.
> >
> > Rendering fonts (and even SVGs) well requires you to know the scale that
> > you're rendering to. More pixels mean you can add more detail. If you
> > shrink that then the additional detail is still there, getting in the
> > way of the actually important information. Doing this properly requires
> > that the original object renderer be part of the scaling process, and
> > doing that on the fly with reasonable performance just isn't part of our
> > rendering stack at the moment.
>
> Which is exactly why forcing 96dpi on displays which have very different pixel
> densities *today* is not a good idea at all.

Knowing the number of pixels available means that the output will be
legible, even if you'd prefer it to be a different size. Rescaling after
rendering means that the output will be illegible, even if it's the
correct size. Given that we don't have the ability to dynamically
re-render everything the moment an application is moved between screens,
what's your proposed solution?

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Old 10-06-2011, 01:57 PM
Michel Alexandre Salim
 
Default Why EDID is not trustworthy for DPI

On 10/05/2011 07:49 PM, Matthew Garrett wrote:
> But what about the single monitor case? Let's go back to your Vaio. It's
> got a high DPI screen, so let's adjust to that. Now you're happy. Right
> up until you plug in an external monitor and now when you run any
> applications on the external display your fonts are twice the size they
> should be. WOOHOO GO TEAM of course that won't make us look like
> amateurs at all. So you need another heuristic to handle that, and of
> course "heuristic" is an ancient african word meaning "maybe bonghits
> will make this problem more tractable".
>
Heh, I don't know about Adam's Vaio, but mine (the now-discontinued 13"
Y, a.k.a. Vaio-S-with-ULV-without-optical-drive) has all sorts of
strange quirks (e.g. totally broken ACPI backlight interface; Matthew
would remember this) -- and it turns out that it did what ajax noted
earlier too: xrandr reports a 0x0 physical screen size. *sigh*. So much
for quality products.

But maybe a quick 'I know I have a 13.3" widescreen laptop, you know the
resolution, just make things work' should work for the single-screen
case (esp if we stick to certain target DPIs as Adam suggested). One
shouldn't ask the typical user for information that's too cumbersome to
use, oui? Like asking them to use a physical ruler to match up against.

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Old 10-06-2011, 02:15 PM
Matthew Garrett
 
Default Why EDID is not trustworthy for DPI

On Thu, Oct 06, 2011 at 03:57:38PM +0200, Michel Alexandre Salim wrote:

> But maybe a quick 'I know I have a 13.3" widescreen laptop, you know the
> resolution, just make things work' should work for the single-screen
> case (esp if we stick to certain target DPIs as Adam suggested). One
> shouldn't ask the typical user for information that's too cumbersome to
> use, oui? Like asking them to use a physical ruler to match up against.

Like I said, that works fine right up until the point where you plug in
a monitor with a different DPI. What do we do then?

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Old 10-06-2011, 02:22 PM
Robert Marcano
 
Default Why EDID is not trustworthy for DPI

On 10/06/2011 09:45 AM, Matthew Garrett wrote:
> On Thu, Oct 06, 2011 at 03:57:38PM +0200, Michel Alexandre Salim wrote:
>
>> But maybe a quick 'I know I have a 13.3" widescreen laptop, you know the
>> resolution, just make things work' should work for the single-screen
>> case (esp if we stick to certain target DPIs as Adam suggested). One
>> shouldn't ask the typical user for information that's too cumbersome to
>> use, oui? Like asking them to use a physical ruler to match up against.
>
> Like I said, that works fine right up until the point where you plug in
> a monitor with a different DPI. What do we do then?
>

Use the same DPI of the main monitor? what is the difference of using
the wrong DPI obtained from the main monitor on the external one and
using the hardcoded 96 value when the external monitor is not 96dpi.
both are wrong without solution, why not at least give the user the
option to set the correct DPI for the internal one, using 96 as default
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Old 10-06-2011, 02:30 PM
Chris Adams
 
Default Why EDID is not trustworthy for DPI

Once upon a time, Matthew Garrett <mjg59@srcf.ucam.org> said:
> Like I said, that works fine right up until the point where you plug in
> a monitor with a different DPI. What do we do then?

I would wager that the majority of Fedora systems are single monitor
(or, in the case of notebooks, single monitor much of the time); can't
we at least try to correct for that case first, and _then_ try to deal
with multi-monitor setups?
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Old 10-06-2011, 02:33 PM
Matthew Garrett
 
Default Why EDID is not trustworthy for DPI

On Thu, Oct 06, 2011 at 09:30:50AM -0500, Chris Adams wrote:
> Once upon a time, Matthew Garrett <mjg59@srcf.ucam.org> said:
> > Like I said, that works fine right up until the point where you plug in
> > a monitor with a different DPI. What do we do then?
>
> I would wager that the majority of Fedora systems are single monitor
> (or, in the case of notebooks, single monitor much of the time); can't
> we at least try to correct for that case first, and _then_ try to deal
> with multi-monitor setups?

Changing the current behaviour doesn't make the most common case
significantly better, but potentially makes a less common (but still
common) case significantly worse. What's the benefit?

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Old 10-06-2011, 03:05 PM
"Nicolas Mailhot"
 
Default Why EDID is not trustworthy for DPI

Le Jeu 6 octobre 2011 16:33, Matthew Garrett a écrit :
> On Thu, Oct 06, 2011 at 09:30:50AM -0500, Chris Adams wrote:
>> Once upon a time, Matthew Garrett <mjg59@srcf.ucam.org> said:
>> > Like I said, that works fine right up until the point where you plug in
>> > a monitor with a different DPI. What do we do then?
>>
>> I would wager that the majority of Fedora systems are single monitor
>> (or, in the case of notebooks, single monitor much of the time); can't
>> we at least try to correct for that case first, and _then_ try to deal
>> with multi-monitor setups?
>
> Changing the current behaviour doesn't make the most common case
> significantly better, but potentially makes a less common (but still
> common) case significantly worse. What's the benefit?

Have the same font size value mean the same thing in gnome and not-gnome apps?

Help people who use several computers calibrate them so they get the same font
sizes on all of them without having to remember size 14 means something on one
computer and another on others?

Support single-screen high-density screen setups by default?

Make network homes work as long as every client uses a display with working EDID?

Really, this focus on not letting people provide simple display sizes when the
EDID is broken¹ is ridiculous. Especially when *at the same time* GNOME 3.2
advertises support for colour correcting displays using ridiculously complex
procedures²

As you've already pointed out, the new heuristic does not solve the complex
cases. So it's no use enumerating them to justify it. It won't make them go
away and they will need handling sooner or later with the heuristic or without
it. In the meanwhile all the heuristic does is introduce inconsistencies in
font handling between GNOME and everyone else.

¹ Which BTW is not the general case, most EDIDs are fine and have been for years
² Which I support (and indeed have packaged argyll for Fedora in the past) but
it is hardly simple

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Old 10-06-2011, 03:14 PM
Jon Masters
 
Default Why EDID is not trustworthy for DPI

On Tue, 2011-10-04 at 13:54 -0400, Adam Jackson 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.

How about "EDID as it exists today". Since you're able to so beautifully
explain what the pitfalls are, I'd assume you've raised this with the
VESA and asked that they revisit this in the future to accurately
provide DPI information that Operating Systems can rely on?

Jon.


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Old 10-06-2011, 03:18 PM
Matthew Garrett
 
Default Why EDID is not trustworthy for DPI

The heuristic isn't the problem. The problem is that we have no
technology that allows us to handle the complicated case of multiple
displays, and solving it purely for the simple case makes the
complicated case *worse*. Adding additional complexity for what would
be, at best, a different set of problems doesn't seem like a worthwhile
way to spend time. The only people who are actively upset by the status
quo are the ones who have the ability to fix it for their case, anyway.

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