Describe the process of creating editor icons globally and per module

Also created a new `Editor development` chapter/section to encompass
C++ editor development side specifically which is not necessarily
related to pure core development, generic types and patterns.

development/cpp/custom_modules_in_cpp.rst

@@ -410,6 +410,35 @@ the docs will become accessible within the engine's built-in documentation syste
 In order to keep documentation up-to-date, all you'll have to do is simply modify
 one of the ``ClassName.xml`` files and recompile the engine from now on.
 
+.. _doc_custom_module_icons:
+
+Adding custom editor icons
+--------------------------
+
+Similarly to how you can write self-contained documentation within a module,
+you can also create your own custom icons for classes to appear in the editor.
+
+For the actual process of creating editor icons to be integrated within the engine,
+please refer to :ref:`doc_editor_icons` first.
+
+Once you've created your icon(s), proceed with the following steps:
+
+1. Make a new directory in the root of the module named ``icons``. This is the
+   default path for the engine to look for module's editor icons.
+
+2. Move your newly created ``svg`` icons (optimized or not) into that folder.
+
+3. Recompile the engine and run the editor. Now the icon(s) will appear in
+   editor's interface where appropriate.
+
+If you'd like to store your icons somewhere else within your module,
+add the following code snippet to ``config.py`` to override the default path:
+
+   .. code:: python
+
+       def get_icons_path():
+           return "path/to/icons"
+
 Summing up
 ----------
 

development/editor/creating_icons.rst

@@ -0,0 +1,98 @@
+.. _doc_editor_icons:
+
+Editor icons
+============
+
+When a new class is created and exposed to scripting, the editor's interface
+will display it with a default icon representing the base class it inherits
+from. Yet in most cases it is recommended to create icons for new classes
+to improve the user experience.
+
+Creating icons
+~~~~~~~~~~~~~~
+
+In order to create new icons, you first need a vector graphics editor installed.
+For instance, you can use the open-source `Inkscape <https://inkscape.org/>`_ editor.
+
+Clone the ``godot-design`` repository containing all the original editor icons:
+
+   .. code:: bash
+
+       git clone https://github.com/godotengine/godot-design
+
+The icons must be created in a vector graphics editor in ``svg`` format. You
+can use ``engine/icons/inkscape_template.svg`` with default icon properties
+already set up.
+
+Once you're satisfied with the icon's design, save the icon in
+``engine/icons/svg/`` folder. But in order for the engine to automatically
+pick up the icons, each icon's filename:
+
+1. Must be prefixed with ``icon_``.
+
+2. ``PascalCase`` name should be converted to ``snake_case``, so words
+   are separated by ``_`` whenever case changes, and uppercase acronyms must
+   also have all letters, numbers, and special characters separated as distinct
+   words. Some examples:
+
+   +--------------------+----------------------------------+
+   | Name               | Filename                         |
+   +====================+==================================+
+   | ``Polygon2D``      | ``icon_polygon_2_d.svg``         |
+   +--------------------+----------------------------------+
+   | ``CSGPolygon``     | ``icon_c_s_g_polygon.svg``       |
+   +--------------------+----------------------------------+
+   | ``CPUParticles2D`` | ``icon_c_p_u_particles_2_d.svg`` |
+   +--------------------+----------------------------------+
+   | ``C#``             | ``icon_c_#.svg``                 |
+   +--------------------+----------------------------------+
+
+Icon optimization
+~~~~~~~~~~~~~~~~~
+
+Because the editor renders the ``svg``'s at runtime, they need to be small
+in size, so they can be efficiently parsed. Editor icons must be first
+optimized before being added to the engine, to do so:
+
+1. Add them to the ``engine/icons/svg`` folder.
+
+2. Run the ``optimize.py`` script. You must have the ``scour`` package installed:
+
+   .. code:: bash
+
+       pip install scour
+       cd godot-design/engine/icons && ./optimize.py
+
+The optimized icons will be generated in the ``engine/icons/optimized`` folder.
+
+Integrating and sharing the icons
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you're contributing to the engine itself, you should make a pull request to
+add optimized icons to ``godot/editor/icons`` in the main repository. Recompile
+the engine to make sure it does pick up new icons for classes. Once merged,
+don't forget to add the original version of the icons to the ``godot-design``
+repository so that the icon can be improved upon by other contributors.
+
+It's also possible to create custom icons within a module. If you're creating
+your own module and don't plan to integrate it with Godot, you don't need to
+make a separate pull request for your icons to be available within the editor
+as they can be self-contained.
+
+For specific instructions on how to create module icons, refer to
+:ref:`Creating custom module icons<doc_custom_module_icons>`.
+
+Troubleshooting
+~~~~~~~~~~~~~~~
+
+If icons don't appear in the editor make sure that:
+
+1. Each icon's filename matches the naming requirement as described previously.
+
+2. ``modules/svg`` is enabled (should be enabled by default). Without it, icons
+   won't appear in the editor at all.
+
+References:
+~~~~~~~~~~~
+
+-  `editor/icons <https://github.com/godotengine/godot/tree/master/editor/icons>`__

development/editor/index.rst

@@ -0,0 +1,9 @@
+Editor development
+==================
+
+.. toctree::
+   :maxdepth: 1
+   :name: toc-devel-editor
+
+   introduction
+   creating_icons

development/editor/introduction.rst

@@ -0,0 +1,9 @@
+.. _doc_editor_intro:
+
+Introduction
+============
+
+As the engine is constantly evolving and new features are being added, it is
+important to familiarize yourself with the editor and tools development in C++.
+Without the editor, some of the engine's internal functionality would become
+unaccessible and make the development not intuitive.

index.rst

@@ -111,6 +111,7 @@ The main documentation for the site is organized into the following sections:
 
    development/compiling/index
    development/cpp/index
+   development/editor/index
    development/file_formats/index