# Tools Options

The Tools options panel controls how Regina works with third-party software.

The settings here are:

Use default PDF viewer

  If checked, Regina will open PDF packets using the default PDF viewer on your computer (for instance, this might be Evince under GNOME, or Preview under MacOS X). If unchecked, Regina will use the custom PDF viewer specified in the text box below.

Custom PDF viewer

  If you have unchecked Use default PDF viewer (as described above), here you can specify the command to start your preferred PDF viewer (such as `xpdf` under GNU/Linux, or `open` under MacOS X). Regina will look for this command on the default search path. If your viewer is installed somewhere unusual then you will need to provide the full path to the executable (such as `/usr/local/bin/xpdf`).

  You may include additional command-line arguments here (so, for instance, MacOS X users might enter `/usr/bin/open -a Preview`). Regina will automatically add the location of the PDF document to the end of this command, and the entire command will be passed to a shell for execution.

GAP executable

  Tells Regina where to find GAP (Groups, Algorithms and Programming). GAP is not required by Regina, but if it is installed then it can help simplify group presentations.

  This setting should be the name of the command that starts GAP (the default is simply `gap`). Regina will look for this command on the default search path. If you have installed GAP somewhere unusual then you will need to provide the full path to the executable (such as `/usr/local/bin/gap`).

Graphviz executable

  Tells Regina where to find Graphviz. Graphviz is not required by Regina, but is needed if you wish to view face pairing graphs.

  This setting should be the name of the command that Graphviz uses to draw undirected graphs. The default is `neato`, but you may use one of Graphviz' other drawing commands if you prefer (but note that the command is not called `graphviz`).

  Again, Regina will look for this command on the default search path. If you have installed Graphviz somewhere unusual then you will need to provide the full path to the executable (such as `/usr/local/bin/neato`).

  ### Tip

  Regina works best with Graphviz version ≥ 2.0. If you have an ancient version (Graphviz 1.x) then you will need to set your Graphviz command to dot, not neato. This is because ancient versions of neato could not display multiple edges. Regina will check the version and warn you if you seem to be using the wrong command.