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    <title>libigl developer tutorial</title>
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      <ul>
        <li>
          <a href=#header_library>Headers (.h) only library</a>
          <ul>
            <li><a href=#widget>IGL_HEADER_ONLY widget</a></li>
            <li><a href=#header_benefits_drawbacks>Benefits and drawbacks</a></li>
          </ul>
        </li>
        <li> 
          <a href=#static_library>Statically linked library (libigl.a)</a>
          <ul>
            <li><a href=#static_benefits_drawbacks>Benefits and drawbacks</a></li>
          </ul>
        </li>
        <li> 
          <a href=#compress>Compressed igl.h/igl.cpp pair</a>
          <ul>
            <li><a href=#compress_benefits_drawbacks>Benefits and drawbacks</a></li>
          </ul>
        </li>
        <li>
          <a href=#dependencies>Dependencies</a>
        </li>
        <li><a href=#matlab>Conversion from matlab</a>
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    <div class=article_outer>
    <div class=article_inner>
      <a href=.><img src=libigl-logo.jpg alt="igl logo" class=center></a>
      <h1>Using the libigl library</h1>
      <p>
        The <a href=.>libigl</a> library is a collection of useful/reusable/sharable C++ functions
        with very few external <a href="#dependencies">dependencies</a>. The
        library may be used as a <a href="#header_library">"headers only"
        library</a>, a <a href=#static_library>statically linked library</a>, or as a compressed <a href="#compress">.h/.cpp pair</a>.
      </p>

      <p>
        This duality between statically compiled and header-only is illustrated
        in the example <code>examples/example_fun</code>. When built, this example
        compiles two binaries, one using the <code>example_fun</code> routine of
        the igl library, either as a headers-only library or linking against the
        igl static library.
      </p>

      <h2 id=header_library>Headers (.h) only library</h2>
      <p>
        All classes and functions in the libigl library are written in a way in
        which the entire library may be compiled <i>just-in-time</i>,
        effectively behaiving as if it were a "headers only" library (like e.g.
        Eigen). This is achieved by careful organization of each pair of .h and
        .cpp files. To take advantage of this one must only include the path to
        <code>libigl</code> directory in one's project's include path and
        define the preprocessor macro <code>IGL_HEADER_ONLY</code>.
      </p>

      <p>
        Defining <code>IGL_HEADER_ONLY</code> may be done at the project level,
        prescribing that all included igl headers be treated as code that
        should be inlined. Consequently all templated functions will be derived
        at compile time if need be.
      </p>

      <p id=widget>
        One may also define <code>IGL_HEADER_ONLY</code> not only on a per-file
        basis, but a <i>per-include</i> basis. For example it may be useful for a
        project to use the static library for most functionality from IGL, but then
        include a certain IGL function as an inlined function. This may be achieved
        by surrounding the relevant include with a define and undefine of the
        <code>IGL_HEADER_ONLY</code> macro. Here's an example of the little
        widget:
      </p>
      <pre><code>
...
#include &lt;igl/some_other_igl_function.h&gt;
#ifndef IGL_HEADER_ONLY
#  define IGL_HEADER_ONLY
#  define IGL_HEADER_ONLY_WAS_NOT_DEFINED
#endif
#include &lt;igl/igl_function_to_inline.h&gt;
#ifdef IGL_HEADER_ONLY_WAS_NOT_DEFINED
#  undef IGL_HEADER_ONLY
#endif
#include &lt;igl/yet_another_igl_function.h&gt;
...
      </code></pre>
      <span class=todo>example <code>examples/XXX</code> also highlights this
      feature</span>

      <div class=note>This practice is not recommended outside of debugging
      purposes.</div>
      

      <h3 id=header_benefits_drawbacks>Benefits of headers-only library</h3>
      <ul>
        <li><strong>Easy templates:</strong> When using the libigl library as a
        headers-only library no special care need be taken when using templated
        functions.</li>
      </ul>
      <h3>Drawbacks of headers-only library</h3>
      <ul>
        <li><strong>Inlining not guaranteed:</strong> Most compilers do not
        guarantee that functions will get inlined even if explicitly told to do
        so. Though we have not yet encountered this problem, it is always a
        risk.</li>
        <li><strong>Long compile, large binary:</strong>As a headers-only
        library we depend on the compiler to properly inline each function
        call. This means compile time is high and binary size can be quite
        large.</li>
      </ul>

      <h2 id=static_library>Statically linked library</h2>
        <h3 id=explicit_specialization_of_templated_functions>Explicit
        specialization of templated functions</h3>
        <p>
          Special care must be taken by the developers of each function and
          class in the libigl library that uses C++ templates. If this function
          is intended to be compiled into the statically linked libigl library
          then function is only compiled for each <i>explicitly</i> specialized
          declaration. These should be added at the bottom of the corresponding
          .cpp file surrounded by a <code>#ifndef IGL_HEADER_ONLY</code>.
        </p>

        <p>
          Of course, a developer may not know ahead of time which
          specializations should be explicitly included in the igl static lib.
          One way to find out is to add one explicit specialization for each
          call in one's own project. This only ever needs to be done once for
          each template.
        </p>

        <p>
          The process is somewhat mechanical using a linker with reasonable error
          output.
        </p>

        <p>
          Supposed for example we have compiled the igl static lib, including the
          cat.h and cat.cpp functions, without any explicit instanciation. Say
          using the makefile in the <code>libigl</code> directory:
        </p>
        <pre><code>
cd $LIBIGL
make
        </code></pre>
        <p>
          Now if we try to compile a project and link against it we may get
          an error like:
        </p>
        <pre><code>
Undefined symbols for architecture x86_64:
  "<span class=highlight>Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; igl::cat&lt;Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; &gt;(int, Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; const&, Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; const&)</span>", referenced from:
      uniform_sample(Eigen::Matrix&lt;double, -1, -1, 0, -1, -1&gt; const&, Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; const&, int, double, Eigen::Matrix&lt;double, -1, -1, 0, -1, -1&gt;&)in Skinning.o
  "Eigen::SparseMatrix&lt;double, 0, int&gt; igl::cat&lt;Eigen::SparseMatrix&lt;double, 0, int&gt; &gt;(int, Eigen::SparseMatrix&lt;double, 0, int&gt; const&, Eigen::SparseMatrix&lt;double, 0, int&gt; const&)", referenced from:
      covariance_scatter_matrix(Eigen::Matrix&lt;double, -1, -1, 0, -1, -1&gt; const&, Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; const&, ArapEnergy, Eigen::SparseMatrix&lt;double, 0, int&gt;&)in arap_dof.o
      arap_rhs(Eigen::Matrix&lt;double, -1, -1, 0, -1, -1&gt; const&, Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; const&, ArapEnergy, Eigen::SparseMatrix&lt;double, 0, int&gt;&)in arap_dof.o
        </code></pre>
        <p>
          This looks like a mess, but luckily we don't really need to read it
          all. Just copy the first highlighted part in quotes, then append it
          to the list of explicit templat specializations at the end of
          <code>cat.cpp</code> after the word
          <strong><code>template</code></strong> and followed by a semi-colon.
          Like this:
        </p>
        <pre><code>
...
#ifndef IGL_HEADER_ONLY
  // Explicit template specialization
  <strong>template</strong> <span class=highlight>Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; igl::cat&lt;Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; &gt;(int, Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; const&, Eigen::Matrix&lt;int, -1, -1, 0, -1, -1&gt; const&)</span>;
#endif
        </code></pre>
        <p>
          Then you must recompile the IGL static library.
        </p>
        <pre><code>
cd $LIBIGL
make
        </code></pre>
        <p>
          And try to compile your project again, potentially repeating this
          process until no more symbols are undefined.
        </p>

        <div class=note>It may be useful to check that you code compiles with
        no errors first using the <a href=#header_library>headers-only
        version</a> to be sure that all errors are from missing template
        specializations.</div>

        <p>
          If you're using make then the following command will
          reveal each missing symbol on its own line:
        </p>
        <pre><code>
make 2&gt;&1 | grep "referenced from" | sed -e "s/, referenced from.*//"
        </code></pre>

        <p>
          Alternatively you can use the <code>autoexplicit.sh</code> function
          which (for well organized .h/.cpp pairs in libigl) automatically
          create explicit instanciations from your compiler's error messages.
          Repeat this process until convergence:
        </p>
        <pre><code>
cd /to/your/project
make 2&gt;$LIBIGL/make.err
cd $LIBIGL
cat make.err | ./autoexplicit.sh
make clean 
make
        </code></pre>

        <h3 id=static_benefits_drawbacks>Benefits of static library</h3>
        <ul>
          <li><strong>Faster compile time:</strong> Because the libigl library
          is already compiled, only the new code in ones project must be
          compiled and then linked to IGL. This means compile times are
          generally faster.</li>
          <li><strong>Debug <i>or</i> optimized:</strong> The IGL static
          library may be compiled in debug mode or optimized release mode
          regardless of whether one's project is being optimized or
          debugged.</li>
        </ul>
        <h3>Drawbacks of static library</h3>
        <ul>
          <li><strong>Hard to use templates:</strong> <a
          href="#explicit_specialization_of_templated_functions">Special
          care</a> (by the developers of the library) needs to be taken when
          exposing templated functions.</li>
        </ul>

      <h2 id=compress>Compressed .h/.cpp pair</h2>
      <p>Calling the script:</p>
      <pre><code>scripts/compress.sh igl.h igl.cpp</code></pre>
      <p>will create a single header
      <code>igl.h</code> and a single cpp file <code>igl.cpp</code>. 

      <p>Alternatively, you can also compress everything into a single header file (analagous to IGL_HEADER_ONLY):</p>
      <pre><code>scripts/compress.sh igl.h</code></pre>
      
      <h3 id=compress_benefits_drawbacks>Benefits of compressed .h/.cpp pair</h3>
      <ul>
        <li><strong>Easy incorporation:</strong> This can be easily incorporated
        into external projects.</li>
      </ul>
      <h3>Drawbacks of compressed .h/.cpp pair</h3>
      <ul>
        <li><strong>Hard to debug/edit:</strong> The compressed files are
        automatically generated. They're huge and should not be edited. Thus
        debugging and editting are near impossible.</li>
        <li><strong>Compounded dependencies:</strong>
        An immediate disadvantage of this
        seems to be that even to use a single function (e.g.
        <code>cotmatrix</code>), compiling and linking against
        <code>igl.cpp</code> will require linking to all of <code>libigl</code>'s
        dependencies (<code>OpenGL</code>, <code>GLUT</code>,
        <code>AntTweakBar</code>, <code>BLAS</code>). However, because all
        depencies other than Eigen should be encapsulated between
        <code>#ifndef</code> guards (e.g. <code>#ifndef IGL_NO_OPENGL</code>, it
        is possible to ignore certain functions that have such dependencies.</li>
        <li><strong>Long compile:</strong> Compiling <code>igl.cpp</code> takes a long time and isn't easily parallelized (no <code>make -j12</code> equivalent).</li>
      </ul>

      <p>Here's a tiny test example using <code>igl.h</code> and <code>igl.cpp</code>. Save the following in <code>test.cpp</code>:</p>
      <pre><code>
#include &lt;igl.h&gt;
#include &lt;Eigen/Core&gt;

int main(int argc, char * argv[])
{
  Eigen::MatrixXd V;
  Eigen::MatrixXi F;
  return (argc&gt;=2 &amp;&amp; igl::read(argv[1],V,F)?0:1);
}
      </code></pre>
      <p>Then compile <code>igl.cpp</code> with:
      <pre><code>
g++ -o igl.o -c igl.cpp -I/opt/local/include/eigen3 -DIGL_NO_OPENGL -DIGL_NO_ANTTWEAKBAR
      </code></pre>
      <p>Notice that we're using <code>-DIGL_NO_OPENGL -DIGL_NO_ANTTWEAKBAR</code> to disable any libigl dependencies on OpenGL and AntTweakBar.</p>

      <p>Now compile <code>test.cpp</code> with:
      <pre><code>
g++ -g -I/opt/local/include/eigen3/ -I/usr/local/igl/libigl/ -L/usr/local/igl/libigl/ -ligl -DIGL_NO_OPENGL -DIGL_NO_ANTTWEAKBAR -o test
      </code></pre>

      <p>Try running it with:</p>
      <pre><code>
./test path/to/mesh.obj
      </code></pre>

      <p>The following bash one-liner will find all source files that contain the string <code>OpenGL</code> but don't contain and <code>IGL_NO_OPENGL</code> guard:
      <pre><code>
grep OpenGL `grep -L IGL_NO_OPENGL include/igl/*`
      </code></pre>

      <h2 id="dependencies">Dependencies</h2>
      <p>
        By design the libigl library has very few external dependencies.
      </p>
        <h3>Mandatory dependencies</h3>
        <p> <a href=http://eigen.tuxfamily.org/>Eigen3</a> and the Standard
        Template Library (STL) are the only truly mandatory dependencies.
        Without them libigl will not compile or work properly.</p>
        <p> OpenGL is an <i>assumed</i> dependency, but is in fact also
        optional. All OpenGL-dependent functions may be disabled by defining
        the <code>IGL_NO_OPENGL</code> preprocessor macro.</p>
        </p>
        <p> Likewise, 
          <a href="http://www.antisphere.com/Wiki/tools:anttweakbar">AntTweakBar</a>
          is an 
        <i>assumed</i> dependency, similarly diabled by defining 
        the <code>IGL_NO_ANTTWEAKBAR</code> preprocessor macro.</p>

        <p><span class=todo>Each IGL_NO_XXX should just be replaced by a libiglXXX.a extra</span></p>
        <h3>Optional dependencies</h3>
        <p>
          Certain <i>extra</i> functions and classes included the libigl
          library have external dependencies by construction (e.g. the
          matlab_interface routines are only useful when matlab is present
          anyway). These are <strong>never</strong> compiled by default into
          the static igl library, rather they are compiled as "extras" (e.g.
          libiglmatlab.a contains the MATLAB-dependent functions).
        </p>
        <p>Currently these include:<p>
        <table>
          <tr class=header><th>Extra</th><th>Dependency</th></tr>
          <tr class=d0><td>libiglmatlab.a</td><td>MATLAB</td></tr>
          <tr class=d1><td>libiglmosek.a</td><td>Mosek</td></tr>
          <tr class=d0><td>libigltetgen.a</td><td>TetGen</td></tr>
          <tr class=d1><td>libiglxml.a</td><td>TinyXml2</td></tr>
          <tr class=d0><td>libiglpng.a</td><td>LibPNG</td></tr>
          <tr class=d1><td>libiglembree.a</td><td>Embree</td></tr>
        </table>
        <p>Some of our examples (<code>libigl/examples</code>) also depend on GLUT.</p>

      <h2 id=matlab>Converting matlab code to C++ using IGL and Eigen</h2>
      <p>
        Eigen's matrix API often makes it fairly to translate to and from
        matlab code (Eigen provides a <a
        href=http://eigen.tuxfamily.org/dox/AsciiQuickReference.txt>
        translation table</a>). We have implemented a few additional
        matlab-esque functions to make the translation even easier. <a
        href="matlab-to-eigen.html">Our own translation table</a> shows a list
        of common matlab functions and their igl-eigen equivalents.  
      </p>

      <h3>Including OpenGL</h3>
      <p>Just include the convenience header, which takes care of system dependent paths, <code>glew</code>, etc.:</p>
      <pre><code>
#include "OpenGL_convenience.h"
      </code></pre>
      <!--<p>Likewise for GLUT:</p>
      <pre><code>
#include "GLUT_convenience.h"
      </code></pre>-->
      <del>
      <p>
      A standard include for the OpenGL headers should be placed in the .cpp file if possible. To ensure compilability on Mac OS X, Windows and Linux, use:
      <pre><code>
#if __APPLE__
#  include &lt;OpenGL/gl.h&gt;
#elif defined(_WIN32)
#    define NOMINMAX
#    include &lt;Windows.h&gt;
#    undef NOMINMAX
#    include &lt;GL/glew.h&gt;
#    include &lt;GL/gl.h&gt;
#else
#  define GL_GLEXT_PROTOTYPES
#  include &lt;GL/gl.h&gt;
#  include &lt;GL/glext.h&gt;
#endif
      </code></pre>
      </p>
      </del>
      <h3>Including headers to other igl functions</h3>
      <p>
      Source files in the main igl directory should include other libigl headers using the name of the file in quotation marks:
      </p>
      <pre><code>
#include "other_function.h"
      </code></pre>

      <p>
      Source files in your project and in libigl <i>extras</i> should include libigl headers using the igl directory and name file in angle brackets:
      </p>
      <pre><code>
#include &lt;igl/some_function.h&gt;
      </code></pre>
      <p>
      libigl headers of extras can then be included using:
      </p>
      <pre><code>
#include &lt;igl/extra/some_extra_function.h&gt;
      </code></pre>


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