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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 ConstantDPI
option to FALSE (see <a href="appendix-b.html" title=
"Appendix B. X Config Options">Appendix B, <i>X
Config Options</i></a> for details).</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">
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<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. 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 "DPI" X configuration option is specified (see <a href=
"appendix-b.html" title=
"Appendix B. X Config Options">Appendix B, <i>X
Config Options</i></a> for details), 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 confusing when TwinView is
enabled: with TwinView, multiple display devices (possibly with
different DPIs) display portions of the same X screen, yet DPI can
only be 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, rather than TwinView; see <a href=
"chapter-15.html" title=
"Chapter 15. Configuring Multiple X Screens on One Card">Chapter 15,
<i>Configuring Multiple X Screens on One Card</i></a> for
details.</p>
</li>
<li>
<p>Experiment with different DPI settings to find a DPI that is
suitable for both display devices.</p>
</li>
</ul>
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