Add Meta Composition Layer tutorial doc

docs/manual/meta/composition_layers.rst

@@ -0,0 +1,63 @@
+Meta Composition Layers
+=======================
+
+This tutorial will walk through the Meta-specific OpenXR composition layer extensions that are supported
+by the Godot OpenXR Vendors plugin. For a general introduction to OpenXR composition layers in Godot,
+check out the `OpenXR Composition Layer Introduction <https://docs.godotengine.org/en/latest/tutorials/xr/openxr_composition_layers.html>`_ tutorial.
+
+The following extensions will expose new properties to the inspector for `OpenXRCompositionLayer <https://docs.godotengine.org/en/latest/classes/class_openxrcompositionlayer.html#class-openxrcompositionlayer>`_ nodes.
+
+Secure Content
+--------------
+
+.. image:: img/composition_layers/secure_content_properties.png
+
+The ``XR_FB_composition_layer_secure_content`` extension allows users to specify how they would like a
+composition layer to be rendered to external outputs. This means that this setting will have **no effect**
+on the composition layers when viewed from inside the headset, but will take effect when casting to another device.
+
+The following ``external_output`` options are available:
+
+* ``display``: Display the content without any changes to external outputs.
+* ``exclude``: Hide this composition layer from external outputs, revealing content behind it.
+* ``replace``: Replace the content on this composition layer with a black screen.
+
+.. figure:: img/composition_layers/secure_content_example.png
+    :align: center
+
+    Example of what a composition layer looks like to an external output with the three different output modes.
+
+Filter Settings
+---------------
+
+.. image:: img/composition_layers/filter_settings_properties.png
+
+The ``XR_FB_composition_layer_settings`` extension allows users to choose supersampling and sharpening
+processing options on a composition layer. The first section of `this Meta article <https://developer.oculus.com/documentation/native/android/mobile-openxr-composition-layer-filtering/>`_
+describes what supersampling/sharpening is and why you may want to utilize these processing options; it also provides some screenshots of their effects.
+
+Supersampling and sharpening both have ``normal`` and ``quality`` options. Users are able to set supersampling/sharpening options manually,
+or to let the OpenXR runtime toggle between options automatically.
+
+.. note::
+    If ``enable_auto_filter`` is set to ``true`` and at least one of the ``options`` flags is selected,
+    automatic layer filtering will always take precedence over manual settings.
+
+When automatic layer filtering is enabled, the runtime will only swap between disabled supersampling/sharpening, and the modes indicated by the ``options`` property.
+
+Alpha Blend
+-----------
+
+.. image:: img/composition_layers/alpha_blend_properties.png
+
+The ``XR_FB_composition_layer_alpha_blend`` extension provides improved control over the source and destination blend factors of composition layers,
+for both color and alpha. Simply turn on `enable_alpha_blend_extension` and the custom blend factors will take effect.
+
+The ``source_color_blend_factor`` and ``source_alpha_blend_factor`` properties will determine the source contributions to the blending, while
+``destination_color_blend_factor`` and ``destination_alpha_blend_factor`` will do the same for the destination. Each of these properties can be set to
+:ref:`one of six different blend factors <enum_openxrfbcompositionlayeralphablendextensionwrapper_blendfactor>`.
+
+.. figure:: img/composition_layers/alpha_blend_example.png
+    :align: center
+
+    Example of different blending results depending on alpha blend settings.

docs/manual/meta/index.rst

@@ -9,4 +9,5 @@ Meta
 
     scene_manager
     passthrough
+    composition_layers