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+<html>
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+ <head>
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+ <title>igl_lib tutorial</title>
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+ <style>
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+.note:before
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+{
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+ font-style: normal;
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+ content: "Note: ";
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+}
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+.note
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+{
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+ background-color: #ddd;
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+ border: 1px solid #bbb;
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+ font-style: italic;
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+ padding-top: 2px;
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+ padding-bottom: 2px;
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+}
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+span.highlight
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+{
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+ background-color: #4F5;
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+}
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+span.todo:before
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+{
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+ font-style: normal;
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+ content: "TODO: ";
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+}
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+span.todo
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+{
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+ color: #F54;
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+ font-style: italic;
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+}
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+pre
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+{
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+ background-color: #c3e0f0;
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+ overflow: auto;
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+ padding-left: 8px;
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+ padding-right: 8px;
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+ padding-top: 4px;
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+ padding-bottom: 4px;
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+ border: 1px solid #999;
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+}
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+ </style>
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+ </head>
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+ <body>
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+ <h1>Using the IGL library</h1>
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+ The igl lib is a collection of useful/reusable/sharable C++ functions with
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+ very few external <a href="#dependencies">dependencies</a>. The library may
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+ be used as a <a href="#header_library">"headers only" library</a> or a <a
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+ href=#static_library>statically linked library</a>.
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+
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+ <h2 id=header_library>Headers (.h) only library</h2>
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+ All classes and functions in the IGL library are written in a way in which
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+ the entire library may be compiled <i>just-in-time</i>, effectively
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+ behaiving as if it were a "headers only" library (like e.g. Eigen). This is
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+ achieved by careful organization of each pair of .h and .cpp files. To take
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+ advantage of this one must only include the path to <code>igl_lib</code>
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+ directory in one's project's include path and define the preprocessor
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+ macro <code>IGL_HEADER_ONLY</code>.
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+
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+ Defining <code>IGL_HEADER_ONLY</code> may be done at the project level,
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+ prescribing that all included igl headers be treated as code that should be
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+ inlined. Consequently all templated functions will be derived at compile
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+ time if need be.
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+
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+ One may also define <code>IGL_HEADER_ONLY</code> not only on a per-file
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+ basis, but a <i>per-include</i> basis. For example it may be useful for a
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+ project to use the static library for most functionality from IGL, but then
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+ include a certain IGL function as an inlined function. This may be achieved
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+ by surrounding the relevant include with a define and undefine of the
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+ <code>IGL_HEADER_ONLY</code> macro. Like so:
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+ <pre><code>
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+...
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+#include <some_other_igl_function.h>
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+#define IGL_HEADER_ONLY
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+#include <igl_function_to_inline.h>
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+#undef IGL_HEADER_ONLY
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+#include <yet_another_igl_function.h>
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+...
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+ </code></pre>
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+ <span class=todo>example <code>examples/XXX</code> also highlights this feature</span>
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+
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+ <div class=note>This practice is not recommended outside of debugging purposes.</div>
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+
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+ <h3>Benefits of headers-only library</h3>
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+ <strong>Easy templates:</strong> When using the IGL library as a
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+ headers-only library no special care need be taken when using templated
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+ functions.
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+ <h3>Drawbacks of headers-only library</h3>
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+ As a headers-only library we depend on the compiler to properly inline
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+ each function call. This means compile time is high and binary size can be
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+ quite large. Further, most compilers do not guarantee that functions will
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+ get inlined even if explicitly told to do so. Though we have not yet
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+ encountered this problem, it is always a risk.
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+
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+ <h2 id=static_library>Statically linked library</h2>
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+ <h3 id=explicit_specialization_of_templated_functions>Explicit
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+ specialization of templated functions</h3>
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+ Special care must be taken by the developers of each function and class
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+ in the IGL library that uses C++ templates. If this function is
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+ intended to be compiled into the statically linked igl library then
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+ function is only compiled for each <i>explicitly</i> specialized
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+ declaration. These should be added at the bottom of the corresponding
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+ .cpp file surrounded by a <code>#ifndef IGL_HEADER_ONLY</code>.
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+
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+ Of course, a developer may not know ahead of time which specializations
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+ should be explicitly included in the igl static lib. One way to find
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+ out is to add one explicit specialization for each call in one's own
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+ project. This only ever needs to be done once for each template.
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+
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+ The process is somewhat mechanical using a linker with reasonable error
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+ output.
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+
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+ Supposed for example we have compiled the igl static lib, including the
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+ cat.h and cat.cpp functions, without any explicit instanciation. Say
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+ using the makefile in the <code>igl_lib</code> directory:
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+ <pre><code>
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+cd $IGL
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+make
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+ </code></pre>
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+ Now we if we try to compile a project and link against it we may get
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+ an error like:
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+ <pre><code>
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+Undefined symbols for architecture x86_64:
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+ "<span class=highlight>Eigen::Matrix<int, -1, -1, 0, -1, -1> igl::cat<Eigen::Matrix<int, -1, -1, 0, -1, -1> >(int, Eigen::Matrix<int, -1, -1, 0, -1, -1> const&, Eigen::Matrix<int, -1, -1, 0, -1, -1> const&)</span>", referenced from:
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+ uniform_sample(Eigen::Matrix<double, -1, -1, 0, -1, -1> const&, Eigen::Matrix<int, -1, -1, 0, -1, -1> const&, int, double, Eigen::Matrix<double, -1, -1, 0, -1, -1>&)in Skinning.o
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+ "Eigen::SparseMatrix<double, 0, int> igl::cat<Eigen::SparseMatrix<double, 0, int> >(int, Eigen::SparseMatrix<double, 0, int> const&, Eigen::SparseMatrix<double, 0, int> const&)", referenced from:
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+ covariance_scatter_matrix(Eigen::Matrix<double, -1, -1, 0, -1, -1> const&, Eigen::Matrix<int, -1, -1, 0, -1, -1> const&, ArapEnergy, Eigen::SparseMatrix<double, 0, int>&)in arap_dof.o
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+ arap_rhs(Eigen::Matrix<double, -1, -1, 0, -1, -1> const&, Eigen::Matrix<int, -1, -1, 0, -1, -1> const&, ArapEnergy, Eigen::SparseMatrix<double, 0, int>&)in arap_dof.o
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+ </code></pre>
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+ This looks like a mess, but luckily we don't really need to read it
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+ all. Just copy the first highlighted part in quotes, then append it to
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+ the list of explicit templat specializations at the end of
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+ <code>cat.cpp</code> after the word <strong><code>template</code></strong> and followed by a semi-colon. Like this:
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+ <pre><code>
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+...
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+#ifndef IGL_HEADER_ONLY
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+ // Explicit template specialization
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+ <strong>template</strong> <span class=highlight>Eigen::Matrix<int, -1, -1, 0, -1, -1> igl::cat<Eigen::Matrix<int, -1, -1, 0, -1, -1> >(int, Eigen::Matrix<int, -1, -1, 0, -1, -1> const&, Eigen::Matrix<int, -1, -1, 0, -1, -1> const&)</span>;
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+#endif
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+ </code></pre>
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+
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+ Then you must recompile the IGL static library.
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+
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+ <pre><code>
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+cd $IGL
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+make
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+ </code></pre>
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+
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+ And try to compile your project again, potentially repeating this
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+ process until no more symbols are undefined.
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+
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+ <div class=note>It may be useful to check that you code compiles with
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+ no errors first using the <a href=#header_library>headers-only
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+ version</a> to be sure that all errors are from missing template
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+ specializations.</div>
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+
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+ <h3>Benefits of static library</h3>
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+ <ul>
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+ <li><strong>Faster compile time:</strong> Because the igl library is
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+ already compiled, only the new code in ones project must be compiled
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+ and then linked to IGL. This means compile times are generally
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+ faster.</li>
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+ <li><strong>Debug <i>or</i> optimized:</strong> The IGL static
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+ library may be compiled in debug mode or optimized release mode
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+ regardless of whether one's project is being optimized or
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+ debugged.</li>
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+ </ul>
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+ <h3>Drawbacks of static library</h3>
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+ <a href="#explicit_specialization_of_templated_functions">Special care</a>
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+ (by the developers of the library) needs to be taken when exposing
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+ templated functions.
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+
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+ <h2 id="dependencies">Dependencies</h2>
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+ By design the IGL library has very few external dependencies.
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+ <h3>Mandatory dependencies</h3>
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+ Besides the standard C++ library, there are a few dependencies, without
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+ which the main library will not compile or work properly. These are:
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+ <ul>
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+ <li>Eigen3</li>
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+ <li>OpenGL <span class=todo>implement IGL_NO_OPENGL compiler option</span></li>
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+ <li>GLUT <span class=todo>implement IGL_NO_GLUT compiler option</span></li>
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+ </ul>
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+ <h3>Optional dependencies</h3>
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+ Certain functions and classes included the IGL library have external
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+ dependencies by construction (e.g. the matlab_interface routines are only
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+ useful when matlab is present anyway). These are <strong>never</strong>
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+ compiled by default into the static igl library <span class=todo>and are only exposed
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+ through compiler options</span>. These are:
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+ <ul>
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+ <li>MATLAB</li>
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+ <li><a href="http://www.antisphere.com/Wiki/tools:anttweakbar">AntTweakBar</a></li>
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+ </ul>
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+ </body>
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+</html>
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jalec