Setting Correct DPI

In rare cases, the display of fonts by the system is incorrect. They are displayed either too big or too small. To fix this, you need to find out what the current Dots Per Inch (DPI) settings are, and what they should be.

Adjusting the DPI can be done if you have installed pure:dyne on your machine. You could also do it when using the Live CD, but the new settings will be lost after a reboot.

What is DPI?

Dots per inch (DPI) is a measure of spatial resolution, the number of individual dots or pixels within the span of one linear inch (2.54 cm). It refers to the physical size of an image when it is reproduced as a real physical entity, for example printed onto paper, or displayed on a monitor.

A correct DPI setting is important. It determines how fonts are rendered on your screen. 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. So if your DPI setting is too low, all fonts are displayed too small and vice versa.

Find out DPI settings

To see what DPI the X server is currently set to, open a terminal and type xdpyinfo | grep resolution:

   
   xdpyinfo | grep resolution
    resolution:    96x96 dots per inch

The best way to find out what the DPI setting should be, is to grab a tape measure or ruler and measure the size of your screen (in inches). To determine the proper DPI, divide the horizontal screen resolution by the horizontal dimension in inches and the same for the vertical resolution and dimension.

To convert millimeters to inches, calculate using 1 millimeter = 0.03937 inch

For instance, a 17 inch screen with a resolution of 1280x1024 should have a DPI of:

1280 / 13.3 = 96 DPI

1024 / 10.6 = 96 DPI

DPI = 96 x 96

If this is the same as the result xdpyinfo gave you, your system is correctly configured. If the results are different, you need to find out what dimensions the system "thinks" your screen has.

To find out if the X server has the same ideas about the size of your screen use xdpyinfo again, this time with the argument dimension. Note that this time the size is in millimeters, just to make it more confusing ;):

   
   xdpyinfo | grep dimension
     dimensions:    1280x1024 pixels (338x270 millimeters)

To convert inches to millimeters, calculate using 1 inch = 25.4 millimeters

If the real size of the screen differs from the results xpdyinfo gives you, you need to tell the X server what size the screen really is.

Changing display size in xorg.conf

As root, edit /etc/X11/xorg.conf and add the display size in millimeters, to Section "Monitor" (replacing the values with your own):

   DisplaySize	338      270