Merge pull request #4040 from Calinou/add-collision-shapes-3d-page

Add a page about 3D collision shapes

tutorials/physics/collision_shapes_3d.rst

@@ -0,0 +1,145 @@
+.. _doc_collision_shapes_3d:
+
+Collision shapes (3D)
+=====================
+
+Godot provides many kinds of collision shapes, with different performance and
+accuracy tradeoffs.
+
+You can define the body form of a :ref:`class_PhysicsBody` by adding one or more
+:ref:`CollisionShapes <class_CollisionShape>` as child nodes. These nodes act as
+shape holders. In these child nodes, you must add a :ref:`class_Shape`
+*resource* that will define the actual shape of the collision.
+
+.. note::
+
+    When you add multiple collision shapes to a single PhysicsBody, you don't
+    have to worry about overlapping shapes since these won't "collide" with each
+    other.
+
+Primitive collision shapes
+--------------------------
+
+Godot provides the following primitive collision shape types:
+
+- :ref:`class_BoxShape`
+- :ref:`class_SphereShape`
+- :ref:`class_CapsuleShape`
+- :ref:`class_CylinderShape` (only when using the Bullet physics engine)
+
+Using one or several primitive collision shapes, you can represent the collision
+of most smaller objects well. However, for more complex objects such as a large
+ship or a whole level, you may want to use convex shapes or concave shapes
+instead (see below).
+
+Primitive shapes are recommended for dynamic objects such as RigidBodies and
+KinematicBodies, as their behavior usually is the most reliable and predictable.
+On top of that, they often provide better performance as well.
+
+Convex collision shapes
+-----------------------
+
+:ref:`Convex collision shapes <class_ConvexPolygonShape>` are a compromise
+between primitive collision shapes and concave collision shapes. They can
+represent shapes of any complexity, but with an important caveat. As their name
+implies, an individual shape can only represent a *convex* shape. For instance,
+a pyramid is *convex* but a hollow box is *concave*. To represent a concave
+object with a single collision shape, you need to use a concave collision shape
+(see below).
+
+However, depending on the object's complexity, you may get better performance by
+using multiple convex shapes instead of a concave collision shape. This is why
+Godot lets you make use of *convex decomposition* to generate convex shapes that
+roughly match a concave object. That said, this performance advantage no longer
+applies after a certain amount of convex shapes. Due to this, for large and
+complex objects such as a whole level, it's recommended to use concave shapes
+instead.
+
+You can generate one or several convex collision shapes from the editor by
+selecting a MeshInstance and using the **Mesh** menu at the top of the 3D
+viewport. The editor exposes two generation modes:
+
+- **Create Single Convex Collision Sibling** uses the Quickhull algorithm.
+  It creates one CollisionShape node with an automatically generated convex
+  collision shape. Since it only generates a single shape, it provides better
+  performance and is recommended for small objects.
+
+- **Create Multiple Convex Collision Siblings** uses the V-HACD algorithm.
+  It creates several CollisionShape nodes, each with their own convex collision
+  shape. Since it generates multiple shapes, it is more accurate for concave
+  objects at the cost of performance. For objects with medium complexity, it
+  will likely be faster than using a single concave collision shape.
+
+Concave (trimesh) collision shapes
+----------------------------------
+
+:ref:`Concave collision shapes <class_ConcavePolygonShape>` (also called trimesh
+collision shapes) can take any form, from a few triangles to thousands of
+triangles. Concave shapes are the slowest option, but are also the most accurate
+collision shapes available in Godot. **Concave shapes can only be used within
+StaticBodies.** They will not work with KinematicBodies or RigidBodies unless
+the RigidBody's mode is set to Static.
+
+.. note::
+
+    Even though concave shapes offer the most accurate *collision*, contact
+    reporting can be less accurate compared to primitive shapes.
+
+When not using GridMaps for level design, concave collision shapes are usually
+the best approach for level collision. That said, if your level has small
+details, you will probably want to exclude those from collision for better
+performance and reliability. To do so, you can build a simplified collision mesh
+in a 3D modeler and have Godot generate a collision shape for it automatically
+(see below).
+
+Note that unlike primitive and convex shapes, a concave collision shape doesn't
+have an actual "volume". Objects can be placed both *outside* of the shape as
+well as *inside*.
+
+You can generate a concave collision shape from the editor by selecting a
+MeshInstance and using the **Mesh** menu at the top of the 3D viewport.
+The editor exposes two generation options:
+
+- **Create Trimesh Static Body** is a convenience option. It will create a
+  StaticBody containing a concave shape matching the mesh's geometry.
+
+- **Create Trimesh Collision Sibling** will create a CollisionShape node
+  containing the a concave shape matching the mesh's geometry.
+
+.. note::
+
+    If you need to make a RigidBody *slide* on a concave collision shape, you
+    may notice that sometimes, the RigidBody will bump upwards. To solve this,
+    open **Project > Project Settings** and enable
+    **Physics > 3d > Smooth Trimesh Collision**.
+
+    Once you've enabled smooth trimesh collision, make sure the concave shape is
+    the only shape of your StaticBody and that it's at located at its origin
+    without any rotation. This way, the RigidBody should slide perfectly on the
+    StaticBody.
+
+.. seealso::
+
+    Godot can generate collision shapes for your imported 3D scenes
+    automatically. See :ref:`doc_importing_scenes_import_hints` in the
+    documentation for more information.
+
+Performance caveats
+-------------------
+
+While you aren't limited to a single collision shape per PhysicsBody, it's
+recommended to keep the number of shapes as low as possible to improve
+performance. This is especially true for dynamic objects such as RigidBodies and
+KinematicBodies. On top of that, avoid translating, rotating or scaling
+CollisionShapes to benefit from the physics engine's internal optimizations.
+
+When a single non-transformed collision shape is used in a StaticBody, the
+engine's *broad phase* algorithm can discard inactive PhysicsBodies. The *narrow
+phase* will then only have to take into account the active bodies's shapes. If a
+StaticBody has many collision shapes, the broad phase would fail and the narrow
+phase (which is slower) must perform a collision check against each shape.
+
+If you run into performance issues, you may have to make tradeoffs in terms of
+accuracy. Most games out there don't actually have 100% accurate collision. They
+just find creative ways to hide it or otherwise make it unnoticeable during
+normal gameplay :)

tutorials/physics/index.rst

@@ -13,3 +13,4 @@ Physics
    ragdoll_system
    kinematic_character_2d
    soft_body
+   collision_shapes_3d