info about header/static [ci skip]

.gitignore

@@ -48,3 +48,6 @@ LibiglOptions.cmake
 # macos debris
 .DS_Store
 *~
+dox/
+latex/
+scripts/

docs/index.md

@@ -16,6 +16,39 @@ resides in the [`igl::spectra::` namespace](./namespaceigl_1_1spectra.html).
 Functions which depend on external code under a copyleft license reside in the
 [`igl::copyleft::` namepsace](file:///Users/alecjacobson/Repos/libigl/dox/namespaceigl_1_1copyleft.html).
 
+
+Most libigl functions are templated over the Eigen matrix inputs and outputs.
+Callers can choose their own scalar types (e.g., `double`/`float`) and storage
+orders (`Eigen::ColMajor`/`Eigen::RowMajor`). Libigl can be used as a:
+
+ - **header only library** (via CMake, make sure
+   `LIBIGL_USE_STATIC_LIBRARY=OFF`) and insure that `IGL_STATIC_LIBRARY` is
+   _not_ defined_ when compiling --- easiest if you're new to libigl, or 
+ - **static library** (`LIBIGL_USE_STATIC_LIBRARY=ON` → `IGL_STATIC_LIBRARY` is
+  defined) --- speeds up repeated compilation.
+
+The libigl static library is filled with _explicit template instantiations_ for
+common Eigen inputs and outputs. If the library doesn't contain your types, you
+may get some form of linker error (e.g., `Undefined symbols for architecture`,
+`undefined reference to` or `unresolved external symbol`).
+
+You can fix this by:
+
+1. Switching to header only mode for your project,
+2. Making  a file in your project to compile the missing templates. E.g., `my_templates.cpp` 
+```cpp
+#ifdef IGL_STATIC_LIBRARY
+#undef IGL_STATIC_LIBRARY
+#endif
+#include <igl/per_vertex_normals.h>
+template void igl::per_vertex_normals<Eigen::Matrix<float, -1, -1, 0, -1, -1>, Eigen::Matrix<int, -1, -1, 0, -1, -1>, Eigen::Matrix<float, -1, -1, 0, -1, -1> >(Eigen::MatrixBase<Eigen::Matrix<float, -1, -1, 0, -1, -1> > const&, Eigen::MatrixBase<Eigen::Matrix<int, -1, -1, 0, -1, -1> > const&, Eigen::PlainObjectBase<Eigen::Matrix<float, -1, -1, 0, -1, -1> >&); 
+```
+3. [Submit a PR](https://github.com/libigl/libigl/pulls) containing your missing template to the development branch of libigl
+4. Change your input/output types to match existing templates (e.g., `Eigen::MatrixXd`).
+
+
+
+
 https://libigl.github.io/
 
 https://github.com/libigl/libigl/