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<h2 class="title"><a name="dpi" id=
"dpi"></a>Appendix E. Dots Per Inch</h2>
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<p>DPI (Dots Per Inch), also known as PPI (Pixels Per Inch), is a
property of an X screen that describes the physical size of pixels.
Some X applications, such as xterm, can use the DPI of an X screen
to determine how large (in pixels) to draw an object in order for
that object to be displayed at the desired physical size on the
display device.</p>
<p>The DPI of an X screen is computed by dividing the size of the X
screen in pixels by the size of the X screen in inches:</p>
<pre class="screen">
DPI = SizeInPixels / SizeInInches
</pre>
<p>Since the X screen stores its physical size in millimeters
rather than inches (1 inch = 25.4 millimeters):</p>
<pre class="screen">
DPI = (SizeInPixels * 25.4) / SizeInMillimeters
</pre>
<p>The NVIDIA X driver reports the size of the X screen in pixels
and in millimeters. On X.Org 6.9 or newer, when the XRandR
extension resizes the X screen in pixels, the NVIDIA X driver
computes a new size in millimeters for the X screen, to maintain a
constant DPI (see the "Physical Size" column of the `xrandr -q`
output as an example). This is done because a changing DPI can
cause interaction problems for some applications. To disable this
behavior, and instead keep the same millimeter size for the X
screen (and therefore have a changing DPI), set the <a href=
"xconfigoptions.html#ConstantDPI">ConstantDPI</a> option to
FALSE.</p>
<p>You can query the DPI of your X screen by running:</p>
<pre class="screen">
% xdpyinfo | grep -B1 dot
</pre>
<p>which should generate output like this:</p>
<pre class="screen">
dimensions: 1280x1024 pixels (382x302 millimeters)
resolution: 85x86 dots per inch
</pre>
<p></p>
<p>The NVIDIA X driver performs several steps during X screen
initialization to determine the DPI of each X screen:</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>If the display device provides an EDID, and the EDID contains
information about the physical size of the display device, that is
used to compute the DPI, along with the size in pixels of the first
mode to be used on the display device.</p>
<p>Note that in some cases, the physical size information stored in
a display device's EDID may be unreliable. This could result in a
display device's DPI being computed incorrectly, potentially
leading to undesired consequences such as fonts that are scaled
larger or smaller than expected. These issues can be worked around
by manually setting a DPI using the "DPI" X configuration option,
or by disabling the use of the EDID's physical size information for
computing DPI by setting the <a href=
"xconfigoptions.html#UseEdidDpi">UseEdidDpi</a> X configuration
option to "FALSE"'.</p>
<p>If multiple display devices are used by this X screen, then the
NVIDIA X screen will choose which display device to use. You can
override this with the "UseEdidDpi" X configuration option: you can
specify a particular display device to use; e.g.:</p>
<pre class="screen">
Option "UseEdidDpi" "DFP-1"
</pre>
<p>or disable EDID-computed DPI by setting this option to
false:</p>
<pre class="screen">
Option "UseEdidDpi" "FALSE"
</pre>
<p>EDID-based DPI computation is enabled by default when an EDID is
available.</p>
</li>
<li>
<p>If the "-dpi" commandline option to the X server is specified,
that is used to set the DPI (see `X -h` for details). This will
override the "UseEdidDpi" option.</p>
</li>
<li>
<p>If the <a href="xconfigoptions.html#DPI">DPI</a> X configuration
option is specified, that will be used to set the DPI. This will
override the "UseEdidDpi" option.</p>
</li>
<li>
<p>If none of the above are available, then the "DisplaySize" X
config file Monitor section information will be used to determine
the DPI, if provided; see the xorg.conf or XF86Config man pages for
details.</p>
</li>
<li>
<p>If none of the above are available, the DPI defaults to
75x75.</p>
</li>
</ul>
</div>
<p>You can find how the NVIDIA X driver determined the DPI by
looking in your X log file. There will be a line that looks
something like the following:</p>
<pre class="screen">
(--) NVIDIA(0): DPI set to (101, 101); computed from "UseEdidDpi" X config option
</pre>
<p></p>
<p>Note that the physical size of the X screen, as reported through
`xdpyinfo` is computed based on the DPI and the size of the X
screen in pixels.</p>
<p>The DPI of an X screen can be poorly defined when multiple
display devices are enabled on the X screen: those display devices
might have different actual DPIs, yet DPI is advertised from the X
server to the X application with X screen granularity. Solutions
for this include:</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>Use separate X screens, with one display device on each X
screen; see <a href="configmultxscreens.html" title=
"Chapter 14. Configuring Multiple X Screens on One Card">Chapter 14,
<i>Configuring Multiple X Screens on One Card</i></a> for
details.</p>
</li>
<li>
<p>The RandR X extension version 1.2 and later reports the physical
size of each RandR Output, so applications could possibly choose to
render content at different sizes, depending on which portion of
the X screen is displayed on which display devices. Client
applications can also configure the reported per-RandR Output
physical size. See, e.g., the xrandr(1) '--fbmm' command line
option.</p>
</li>
<li>
<p>Experiment with different DPI settings to find a DPI that is
suitable for all display devices on the X screen.</p>
</li>
</ul>
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