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	.. _`Custom scales`:

	Custom scales
	=============

	Reversed HEJ comes with a small selection of built-in renormalisation
	and factorisation scales, as described in the :ref:`scales <scales>` setting. In
	addition to this, user-defined scales can be imported from custom
	libraries.

	Writing the library
	-------------------

	Custom scales are defined through C++ functions that take an event and
	compute the corresponding scale. As an example, let's consider a
	function returning the transverse momentum of the softest jet in an
	event. To make it accessible from reversed HEJ, we have to prevent C++
	name mangling with :code:`extern "C"`:

	.. code-block:: C++

	#include "RHEJ/Event.hh"

	extern "C"
	double softest_jet_pt(RHEJ::Event const & ev){
	const auto softest_jet = sorted_by_pt(ev.jets()).back();
	return softest_jet.perp();
	}

	After saving this code to some file :code:`myscales.cc`, we can compile
	it to a shared library. With the :code:`g++` compiler this can be done
	with the command

	.. code-block:: sh

	g++ -fPIC -shared -Wl,-soname,libmyscales.so -o libmyscales.so myscales.cc

	If :code:`g++` is used and the library also contains other definitions,
	it is recommended to add :code:`-fvisibility=hidden` to the compiler
	flags and :code:`__attribute__((visibility("default")))` after
	:code:`extern "C"` for each exported function in order to avoid possible
	name clashes.


	Importing the scale into reversed HEJ
	-------------------------------------

	Our custom scale can now be imported into reversed HEJ by adding the
	lines

	.. code-block:: YAML

	import scales:
	/path/to/libmyscales.so: softest_jet_pt

	It is also possible to import several scales from one or more libraries:

	.. code-block:: YAML

	import scales:
	/path/to/libmyscales1.so: [first_scale, second_scale]
	/path/to/libmyscales2.so: [another_scale, yet_another_scale]

	The custom scales can then be used as usual in the :ref:`scales <scales>`
	setting, for example

	.. code-block:: YAML

	scales: [H_T, softest_jet_pt, 2*softest_jet_pt]

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