MergeGroups initial docs

Author: lawnjelly

tutorials/3d/img/bake_merge_group.webp

@@ -21,4 +21,5 @@
    procedural_geometry/index
    occluders
    portals/index
+   merge_groups
    3d_text

tutorials/3d/index.rst

@@ -0,0 +1,271 @@
+.. _doc_merge_groups:
+
+MergeGroups
+===========
+
+The ``MergeGroup`` node allows the user to merge similar meshes so that they can
+be grouped into a smaller number of nodes, and render using a single draw call
+(per :ref:`Material <class_Material>`). This can greatly increase performance when
+used with care.
+
+Explanation
+-----------
+
+Usually individual meshes in Godot are represented by their own
+:ref:`VisualInstance <class_VisualInstance>` in the
+:ref:`SceneTree <class_SceneTree>`, such as :ref:`MeshInstance <class_MeshInstance>`.
+These:
+
+-  Move independently
+-  Cull independently
+-  Render individually
+
+In terms of performance however, each ``VisualInstance`` adds a cost, both in
+terms of management, and in terms of rendering. Each mesh (and each surface on a
+``MeshInstance`` is rendered as an individual mesh) will require an expensive
+draw call to the graphics API, and often incur expensive rendering state changes.
+
+If instead we can identify groups of meshes that could be potentially be
+rendered together, and merge them ahead of time (either at design time, or
+during level load), this can potentially lighten the load on the CPU and GPU,
+and increase frame rates.
+
+The merging trade-off
+^^^^^^^^^^^^^^^^^^^^^
+
+Lowering draw calls and reducing state changes tends to increase performance.
+However, merged meshes are culled as a unit, and therefore if any part of the
+merged mesh is visible in the view frustum, the cost of rendering all parts of
+the mesh are incurred. For this reason it is recommended that you evaluate
+performance both before and after merging, in order to find the best balance in
+each case.
+
+.. tip::
+    Remember that the bottlenecks may be different on different hardware.
+    Be sure to test on the lowest-end platforms you're targeting for your
+    project, especially mobile devices and integrated graphics.
+
+Requirements
+------------
+
+There are some requirements for any group of meshes to be merged.
+
+The most important requirement is that the source meshes must not move in
+relation to each other. They must be either be completely static (non-moving)
+or move together only as a single unit. This is because once several meshes are
+merged into one, the relative transforms of the source meshes cannot change,
+the transforms are baked into the new geometry.
+
+The simplest candidate for merging is static level geometry, such as walls,
+floors, tables etc. But groups of meshes that move "as one" are also good
+candidates. Examples might be a pirate ship, or spacecraft.
+
+Using MergeGroups
+-----------------
+
+After adding a ``MergeGroup`` node into your ``SceneTree``, move all the
+VisualInstances you want to be merged so that they are parented by the
+``MergeGroup`` (either as children, or grandchildren). By default, if you now
+run the scene, the engine will automatically merge suitable meshes as the
+``MergeGroup`` enters the ``SceneTree``.
+
+You can change the transform of the ``MergeGroup`` at runtime, and the whole
+group will move as a unit (``MergeGroup`` is derived from
+:ref:`Spatial <class_Spatial>`, so can be manipulated in all the regular ways,
+such as changing transform or visibility).
+
+Mesh similarity
+---------------
+
+It is important to note that not all meshes (or more accurately, mesh surfaces,
+because merging takes place at the level of mesh surfaces) can be merged
+together. There are strict limitations for the mesh surfaces to match.
+
+All the following properties must match for merging to take place:
+
+-  Material
+-  Shadow casting settings
+-  Visibility
+-  Layer mask
+-  Portal settings
+-  Cull margin
+-  Lightmap settings
+
+Usually this means only multiple instances of the same mesh will be merged (such
+as a group of walls), but similar variations of mesh (e.g. different sized
+tables) can sometimes be merged. Additionally, if you reuse materials (using
+e.g. a "wood" material in several different meshes), these can often be merged.
+
+Exceptions
+----------
+
+When you place a hierarchy of nodes underneath a ``MergeGroup``, such as a
+pirate ship, there may be exceptions - meshes that you *do not* want merged.
+This may be because, for instance, you may want them to move independently of
+the ``MergeGroup``. An example might be a steering wheel, or sail.
+
+Godot allows you fine control over exactly which nodes should be considered for
+merging, via the ``merging_mode`` property which is present in the inspector
+for every ``Spatial``, inside the ``Misc`` tab.
+
+Merging modes:
+
+-  ``Inherit`` - Inherit the setting from the parent. The default for the scene root node is ``On``.
+-  ``On`` - Change the mode of the node (and any inheriting children) to allow merging.
+-  ``Off`` - Change the mode of the node (and any inheriting children) to disallow merging.
+
+This means that if you, e.g. set the mode of a steering wheel to ``Off``, it
+will not be considered for merging, and neither will any children or
+grandchildren of the steering wheel (unless one of them explicitly reactivates
+merging with an ``On`` ``merge_mode``).
+
+
+Ways to use MergeGroups
+-----------------------
+
+There are three ways to use MergeGroups:
+
+AutoMerge
+^^^^^^^^^
+
+The ``MergeGroup`` will attempt to merge any descendent merges as it enters the
+``SceneTree``.
+This is the simplest method, and the best introduction to merging.
+
+Baking in the Editor
+^^^^^^^^^^^^^^^^^^^^
+
+When the ``MergeGroup`` is selected in the editor, a new ``bake`` button should
+appear in the toolbar. This allows you to bake the entire merged scene out to a
+file (.tscn (text scene) or .scn (binary scene)) at *design time* rather than at
+*runtime*. This can be used creatively to build sensible composite objects which
+are later used to compose a game level (this general approach is often known as
+"kitbashing").
+
+Additionally, baking in advance offers a great way to preview what will happen
+when merging at runtime. It allows you to open the merged scene, and see which
+meshes were successfully merged, and which were problematic.
+
+.. figure:: img/bake_merge_group.webp
+    :align: center
+
+Pros
+~~~~
+
+-  Allows you easily see the results of merging
+-  Allows you to further build levels using these merged scenes
+-  No time is taken by the merging process at runtime
+
+Cons
+~~~~
+
+-  Merged scenes typically require substantially more storage than the original meshes
+-  The size of your exported game will likely increase
+-  Larger merged scenes may take longer to load
+-  Larger merged scenes may take up more RAM at runtime, especially on the GPU
+
+If you merge, e.g. 10 boxes, the merged scene will have to store 10 times as
+much geometry data, as the polygons are duplicated. More storage means your
+exported game will be larger, the data will take longer to load, and consume
+more RAM at runtime. For this reason, baking in advance tends to be more
+practical with low poly meshes and art styles.
+
+.. tip::
+    Due to the increased storage requirements, it is recommended that, wherever possible,
+    you bake scenes in binary format (``.scn``) rather than text (``.tscn``).
+    This is because binary scenes are much more compact in terms of storage,
+    while also being faster to load and save.
+
+Manually at runtime
+^^^^^^^^^^^^^^^^^^^
+
+If the ``automerge`` property of a ``MergeGroup`` is disabled, then the node
+will do nothing at runtime until you call its ``merge_meshes()`` function.
+Triggering merging manually in this way has two major use cases:
+
+1. Procedural levels. If you place objects (e.g. trees, boxes) at runtime using
+   script, rather than placing them at design time, then you want a way to *delay*
+   merging until after your placement is complete. Manually calling
+   ``merge_meshes()`` allows this.
+2. Merging parameters. These can be set via the ``MergeGroup::set_param()`` and
+   ``MergeGroup::set_param_enabled()`` functions, prior to calling
+   ``merge_meshes()``.
+
+Merging parameters
+------------------
+
+Although the default operation of the ``MergeGroup`` works well in many
+circumstances, there are a number of parameters which can be altered prior to
+merging in order to access more advanced features. These can be set via the
+``MergeGroup::set_param()`` functions, however the easiest way to visualize them
+is via the ``bake`` method, which displays a dialog allowing you to modify
+parameters. These are described in detail in the documentation for
+``MergeGroup``.
+
+Merging by locality - grouping and splitting
+--------------------------------------------
+
+When merging large numbers of meshes across a large map, sometimes the merging
+goes too far, and results in a huge mesh that is too difficult to cull (as part
+is always in the view frustum).
+
+For example if you merge every tree in a forest, regardless of your viewpoint,
+the whole forest will be rendered. Although rendering each tree individually
+would be inefficient, rendering the whole forest in all cases is also
+inefficient, but in a different way.
+
+You may think instead to create several ``MergeGroups`` spread across the
+forest, and only merge the trees in the local area. This would create an ideal
+balance between reduced drawcalls, but still allowing broad scale culling to
+take effect. The downside is that this kind of thing could be extremely labour
+intensive in terms of scene design.
+
+For this reason, ``MergeGroup`` has built in functionality for helping deal with
+this problem automatically.
+
+.. note::
+    Grouping and splitting is considered advanced, so is only available via the
+    manual method (setting parameters and calling ``merge_meshes()`` explicitly),
+    or via the bake method.
+
+Grouping
+^^^^^^^^
+
+One of the simplest ways to get the advantages of merging without the
+disadvantages of loss of culling resolution is grouping. The ``group_size``
+parameter defaults to zero, which indicates that all suitable meshes be merged.
+But if you set ``group_size`` to e.g. 2, then only the closest pairs of meshes
+will be merged.
+
+If you have 10 trees, it will try to merge 5 pairs of trees, with each pair
+being as close together as possible (so that they can be culled efficiently).
+This same principle works even in a forest of 1,000 trees.
+
+Splitting
+^^^^^^^^^
+
+The alternative approach to grouping is splitting. Splitting takes place as a
+post-step, *after* all the meshes have been merged.
+
+Following our forest example, if we assume our 1,000 tree forest has been merged
+into 1 huge mesh, splitting allows us to specify a 3D grid (with a horizontal
+number of splits and vertical number of splits) and the ``MergeGroup`` will
+attempt to split the large mesh into smaller ones more suitable for culling.
+
+For example, with 5 horizontal splits and 1 vertical split we should get 25
+final meshes (5 x 5 x 1) which should give us enough resolution for some decent
+culling for our forest.
+
+.. note::
+    The reason for allowing separate horizontal and vertical splits is that
+    many games are based on flat ground, where horizontal splits may be more
+    important than vertical. Increasing vertical splits may be counter-productive.
+.. tip::
+    Splitting can make a lot of sense if you are building a world with voxels.
+
+Other functionality
+^^^^^^^^^^^^^^^^^^^
+
+Also via the ``MergeGroup`` parameters, ``CSG`` nodes (e.g. :ref:`CSGBox <class_CSGBox>`)
+and :ref:`GridMap <class_GridMap>` nodes can optionally be converted to a regular
+:ref:`MeshInstance <class_MeshInstance>`. This allows them to be merged like any other mesh.