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Nikolaus Rath 12-18-2008 05:21 PM

The DPI problem
 
Hello,

I have tried to address the following issue by filing bug reports on
several individual applications. However, this remained by and large
unsuccessful, since there seems to be no consensus on where the
problem actually lies. For that reason, I'm raising the issue here.

The root of the problem is that there are currently two different
values of the screen resolution:

1. The X11 server's value
2. The value from the gnome font properties.

The X11 value is automatically calculated from the monitor resolution
and monitor dimensions. It distinguishes between horizontal and
vertical dpi. It can be set by manually specifying the monitor
dimensions in /etc/X11/xorg.conf or by using xrandr --dpi. There is no
graphical way to see or change it.

The gnome DPI is set to 96 by default and can be changed and viewed
graphically. There is no distinction between horizontal and vertical.


Problems now arise because different applications use these two values
for different things, e.g:

- The gnome value is used to render fonts in gnome applications.
According to gnome developers, the gnome value should be chosen to
get a good *angular* resolution, so the correct value depends on
the distance between monitor and user.

- Applications like OpenOffice however, also use the gnome value to
calculate the physical size of the display. So if the gnome value
does not match the physical dpi, an A4 page shown at 100% zoom does
not have the size of an A4 page.

- Applications like evince use both values. The physical dimensions
are calculated from X11 and the font sizes are based on the gnome
dpi.

- Non Gnome-Applications use the X11 value for both font rendering
and calculation of physical sizes.

- Applications like Gimp and Adobe Acrobat allow manual specification
of the dpi, because both X11 and gnome dpi are notoriously
unreliable in value and meaning.

- Applications like Mathematica effectively rely on the gnome dpi
value to be fixed at 96dpi (otherwise the fonts get ridiculously
large).


Personally, I think the following policy would best resolve the mess:

- The X11 dpi should correspond to the physical monitor dimensions.
In order to ensure this, the "Screen Resolution" preferences dialog
should also allow to specify the size of the monitor.

- The gnome dpi should be renamed to "font magnification" and be
given as a percentage of the X11 dpi.

- All font rendering should be done using the x11 dpi scaled by the
magnification factor.

- All physical sizes (e.g. "how many pixels do I have to use to show
something as 10x10cm?) should be calculated based on the X11 dpi.


This

- fixes the meaning of the different values.

- makes the difference between them more apparent by exposing both
values to the user and giving them meaningful names.



I hope that it will be possible to either find a better solution or to
get a general agreement on the above one. This will hopefully be
enough to remove the confusion around the dpi handling that apparently
prevents the related bugs from being fixed.



Best,


-Nikolaus

--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

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Vincent Untz 12-19-2008 08:01 AM

The DPI problem
 
Le jeudi 18 décembre 2008, à 13:21 -0500, Nikolaus Rath a écrit :
> The gnome DPI is set to 96 by default and can be changed and viewed
> graphically. There is no distinction between horizontal and vertical.

Unless Ubuntu changes things, GNOME uses the X dpi by default (unless
the user forces a dpi). Note that in openSUSE, we went back to 96 by
default, see: http://bugzilla.gnome.org/show_bug.cgi?id=553652

(but there's no distinction between horizontal and vertical, indeed)

Vincent

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Nikolaus Rath 12-19-2008 04:41 PM

The DPI problem
 
Vincent Untz <vuntz@ubuntu.com> writes:
> Le jeudi 18 décembre 2008, à 13:21 -0500, Nikolaus Rath a écrit :
>> The gnome DPI is set to 96 by default and can be changed and viewed
>> graphically. There is no distinction between horizontal and vertical.
>
> Unless Ubuntu changes things, GNOME uses the X dpi by default (unless
> the user forces a dpi). Note that in openSUSE, we went back to 96 by
> default, see: http://bugzilla.gnome.org/show_bug.cgi?id=553652

In Hardy, GNOME certainly ignores my X11 dpi.


Best,

-Nikolaus

--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

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Chris Cheney 12-19-2008 10:30 PM

The DPI problem
 
As I understand it Gnome was supposedly going to do the right thing with
respect to DPI soon, I don't know how long from now 'soon' is though.

However, as you say the current situation is definitely not good. Many
laptops in particular have screens where the DPI is much higher than 96
DPI and on the other end of the spectrum most HDTV's have much lower
than 96 DPI.

Chris


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