Solving the Linux DPI Puzzle
by Billy Biggs <email@example.com>, Sat Dec 4 2004
Linux applications use the DPI reported by the X server when
converting from font point size to pixels. "Sans 10" will be
a smaller number of pixels if your X server is configured at 75 DPI
than if it is at 100 DPI.
How the XFree86 and Xorg servers calculate DPI
The DPI of the X server is determined in the following
- The -dpi command line option has highest priority.
- If this is not used, the DisplaySize setting in the X config
file is used to derive the DPI, given the screen resolution.
- If no DisplaySize is given, the monitor size values from DDC
are used to derive the DPI, given the screen resolution.
- If DDC does not specify a size, 75x75 DPI is used by default.
You can check what DPI your X server is set to by running
xdpyinfo | grep resolution in a terminal window.
DPI is also defined in two other places:
- By Xft/fontconfig. fontconfig defines a "dpi" value independent
of X which you can define in your local.conf file. GTK+ 2 and Qt 3
applications will honour this value if it is set.
- By GNOME. If "gnome-settings-daemon" is running, this will
advertise the DPI value set in the gnome-font-properties
dialog. This also changes the Xft value to match, and so this will
affect Qt applications started after the daemon is running.
Problem: choosing default font sizes
Having a standardized DPI is important for choosing good default
Windows machines use the DPI value as a way of globally changing
font size. Windows XP defaults to 96 DPI. Changing to large font
mode increases the DPI to 120. Users can also specify a custom DPI
value. The default application font on Windows is "Tahoma 8".
MacOS X standardizes on 72 DPI, which means that fonts are
smaller on the Mac at the same point size as on Windows. The
default font on my MacOS X laptop is "Lucida Grande 13".
GTK+ uses a default application font of "Sans 10". This size
seems to be chosen assuming a screen DPI of 96x96.
DPI in practice
The DPI used on a Linux desktop is defined by the following:
- If gnome-settings-daemon is running, it defaults to 96 DPI, and
all GTK+/Qt applications will use this value. Your fonts will
appear as intended.
- Otherwise, some distributions launch X using "-dpi 100". Fonts
will appear as intended.
- If your monitor announces a size via DDC, X will derive its DPI
from these values. These values are unreliable, and regardless,
this is not a good way to determine font sizes. The result is fonts
which are usually either too big or too small.
- Your X server uses 75x75, and your fonts are all too small.
In one weekend supporting tvtime and Eclipse on IRC, I saw the
following DPI values from various users, all of whom were using the
default X setup from their distribution: 75x75, 85x80, 100x100,
117x115, and 133x133.
I strongly believe that fonts on Linux should pick a standard
default DPI value. Applications and desktop systems cannot reliably
choose default font sizes without this. My proposal is that we
clean up all of the rough edges by deciding on a default DPI, and
work towards making all of our abstractions track one global
The proposal is as follows:
- Decide on 96x96 DPI as the default, since this is already quite
popular and matches all default GNOME desktops.
- Distributions should go through all of their scripts: startx,
gdm's gdm.conf, and xdm/kdm's Xservers files. These should all
start X with the -dpi 96 command line option.
- Distributions should modify fontconfig's dpi setting in the
default /etc/fonts/local.conf file.
Here is the code for inclusion in the fontconfig local.conf file:
Why is a fixed default DPI better than an autodetected one?
An obvious criticism of this proposal is that it is proposing
a single and arbitrary DPI, which seems to go against the whole
concept of "dots per inch". Having a DPI value calculated based on
the monitor size seems like a better idea.
The reasons why I believe a fixed DPI is a good idea are as follows:
- The values obtained from monitors via DDC seem to give wildly
incorrect and variable results for users, and if it fails, the
fallback DPI is 75x75 which is too small for the default font
sizes of GTK+ applications.
- DPI applies well to printing, but not well to the screen. If I
project my laptop display on a screen for a presentation, the
theoretical DPI has clearly changed, but I do not want all of my
fonts to suddenly change with it. DPI values for computer
screens are simply convention and not meaningful.
- Other operating systems like Windows and MacOS choose arbitrary
DPI values rather than auto-calculate it.
- Having a standard default fixed DPI makes it easier for
application and desktop developers to choose default fonts.
- Font hints are specified for certain popular font sizes.
Changing the DPI can affect the appearance of text, not just its
- fontconfig bug 2014
- DPI on Windows systems