/******************************************************************************
 *
 * 
 *
 * Copyright (C) 1997-2013 by Dimitri van Heesch.
 *
 * Permission to use, copy, modify, and distribute this software and its
 * documentation under the terms of the GNU General Public License is hereby 
 * granted. No representations are made about the suitability of this software 
 * for any purpose. It is provided "as is" without express or implied warranty.
 * See the GNU General Public License for more details.
 *
 * Documents produced by Doxygen are derivative works derived from the
 * input used in their production; they are not affected by this license.
 *
 */
/*! \page external Linking to external documentation

If your project depends on external libraries or tools, there are several 
reasons to not include all sources for these with every run of doxygen:

<dl>
<dt>Disk space:<dd> Some documentation may be available outside of the output 
    directory of doxygen already, for instance somewhere on the web. 
    You may want to link to these pages instead of generating the documentation 
    in your local output directory.
<dt>Compilation speed:<dd> External projects typically have a different update 
    frequency from your own project. It does not make much sense to let doxygen 
    parse the sources for these external project over and over again, even if 
    nothing has changed.
<dt>Memory:<dd> For very large source trees, letting doxygen parse all sources 
    may simply take too much of your system's memory. By dividing the sources 
    into several "packages", the sources of one package can be parsed by 
    doxygen, while all other packages that this package depends on, are 
    linked in externally. This saves a lot of memory.
<dt>Availability:<dd> For some projects that are documented with doxygen,
    the sources may just not be available.
<dt>Copyright issues:<dd>If the external
    package and its documentation are copyright someone else, it may be
    better - or even necessary - to reference it rather than include a
    copy of it with your project's documentation.  When the author forbids
    redistribution, this is necessary.  If the author requires compliance
    with some license condition as a precondition of redistribution, and
    you do not want to be bound by those conditions, referring to their
    copy of their documentation is preferable to including a copy.

</dl>

If any of the above apply, you can use doxygen's tag file mechanism.
A tag file is basically a compact representation of the entities found in the
external sources. Doxygen can both generate and read tag files.

To generate a tag file for your project, simply put the name of the
tag file after the \ref cfg_generate_tagfile "GENERATE_TAGFILE" option in 
the configuration file. 

To combine the output of one or more external projects with your own project 
you should specify the name of the tag files after 
the \ref cfg_tagfiles "TAGFILES" option in the configuration file.

A tag file typically only contains a relative location of the documentation from the
point where doxygen was run. So when you include a tag file in other project 
you have to specify where the external documentation is located in relation this project.
You can do this in the configuration file by assigning the (relative) location to the
tag files specified after the \ref cfg_tagfiles "TAGFILES" configuration
option. If you use a relative path it should be relative with respect to 
the directory where the HTML output of your project is generated; so a relative path
from the HTML output directory of a project to the HTML output of the other project that 
is linked to.

\par Example: 
Suppose you have a project \c proj that uses two external 
projects called \c ext1 and \c ext2.
The directory structure looks as follows:

\par
\verbatim
<root>
  +- proj
  |   +- html               HTML output directory for proj
  |   +- src                sources for proj
  |   |- proj.cpp
  +- ext1
  |   +- html               HTML output directory for ext1
  |   |- ext1.tag           tag file for ext1
  +- ext2
  |   +- html               HTML output directory for ext2
  |   |- ext2.tag           tag file for ext2
  |- proj.cfg               doxygen configuration file for proj
  |- ext1.cfg               doxygen configuration file for ext1
  |- ext2.cfg               doxygen configuration file for ext2
\endverbatim

\par
Then the relevant parts of the configuration files look as follows:
\par
proj.cfg:
\verbatim
OUTPUT_DIRECTORY  = proj
INPUT             = proj/src
TAGFILES          = ext1/ext1.tag=../../ext1/html \
                    ext2/ext2.tag=../../ext2/html 
\endverbatim 
ext1.cfg:
\verbatim
OUTPUT_DIRECTORY  = ext1
GENERATE_TAGFILE  = ext1/ext1.tag 
\endverbatim 
ext2.cfg:
\verbatim
OUTPUT_DIRECTORY  = ext2
GENERATE_TAGFILE  = ext2/ext2.tag
\endverbatim

\htmlonly
Go to the <a href="faq.html">next</a> section or return to the
 <a href="index.html">index</a>.
\endhtmlonly

*/