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    <title>libigl developer tutorial</title>
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    <div class=article>
    <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> or a <a href=#static_library>statically linked library</a>.
    </p>

    <p>
      This duality 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 IGL 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>igl_lib</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>
        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>Benefits of headers-only library</h3>
      <ul>
        <li><strong>Easy templates:</strong> When using the IGL 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 IGL library that uses C++ templates. If this function is
          intended to be compiled into the statically linked igl 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>igl_lib</code> directory:
        </p>
        <pre><code>
cd $IGL_LIB
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 $IGL_LIB
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 .cpp files in the igl_lib) 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;$IGL_LIB/make.err
cd $IGL_LIB
cat make.err | ./autoexplicit.sh
make clean 
make
        </code></pre>

        <h3>Benefits of static library</h3>
        <ul>
          <li><strong>Faster compile time:</strong> Because the igl 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="dependencies">Dependencies</h2>
      <p>
        By design the IGL library has very few external dependencies.
      </p>
        <h3>Mandatory dependencies</h3>
        <p>
          Besides the standard C++ library, there are a few dependencies,
          without which the main library will not compile or work properly.
          These are:
        </p>
        <ul>
          <li><a href=http://eigen.tuxfamily.org/>Eigen3</a></li>
          <li>OpenGL <span class=todo>implement IGL_NO_OPENGL compiler option</span></li>
          <li>GLUT <span class=todo>implement IGL_NO_GLUT compiler option</span></li>
        </ul>
        <h3>Optional dependencies</h3>
        <p>
          Certain functions and classes included the IGL 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 <span class=todo>and are only exposed through compiler
          options</span>. These are:
        </p>
        <ul>
          <li>MATLAB</li>
          <li><a href="http://www.antisphere.com/Wiki/tools:anttweakbar">AntTweakBar</a></li>
        </ul>
      <h1>Converting matlab code to C++ using IGL and Eigen</h1>
      <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>
      <p>See also: <a href=style_guidelines.html>style guidlines</a>, <a href=doc.html>auto-documentation</a></p>
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