OGRE 2.1 includes changes to objects, scenes, and nodes including: 1) IDs are now optional and uniqueness is determined by IdObject::getId() instead of names. Entities and movable objects will not share the same ID. 2) Debug object data is stored in a structure of arrays format and MovableObject::mObjectData stores the SoA data including packed index and parent pointers. 3) Movable objects like entities and lights now require being attached to scene nodes, and scene node static/dynamic flags impact transform and bounds updating for performance.

1. OGRE (v2.1)
Changes: Objects, Scene &
Nodes
王聖川
2017/9/21

2. ID Uniqueness
Names are now optional (no need to be unique).
Identify uniqueness via IdObject::getId().
Entities/MovableObjects will not share the same ID.
Entity may share the same ID with scene node.

3. Debug Object Data
Data is stored in SoA (structure of arrays).
How 4 vectors are packed:
OGRE_USE_SIMD = 0 for C version array (debug purpose).
MovableObject::mObjectData for SoA data.
mIndex (0~3), packed index in SoA.
mParents: parents for packed 4 MovableObejcts.

4. Interpret ArrayVector3
mIndex order in m_chunkBase is right to left.
ArrayVector3 packs 4 vectors (ARRAY_PACKED_REALS = 4)
m_chunkBase: {mIndex=3, mIndex=2, mIndex=1, mIndex=0}
If 4 nodes created A, B, C, D.
Pack dummy pointer instead NULL.
MovableObject::mObjectData::mParents, Transform::mParents, …
Avoid null pointer checking.

5. Attachment and Visibility
OGRE 1.x:
Scene hierarchy traversal is expensive.
Objects were in the scene when being attached to scene
nodes.
OGRE 2.x:
Objects always in the scene, being attached to nodes to
hold their positions.
Detach object (entity) from node, will call setVisible(false)
implicitly. Previous visibility is lost. Call setVisible(true) while
detached will crash!
Optimized scene updates and iterations. Attach/detach
becomes more expensive. Call setVisible() for
hiding/showing objects.

6. MovableObject Require
SceneNode
MovableObjects (Entity, Light, Camera) require
being attached to SceneNodes.
No nodeless lights anymore. Light::setDirection() redirect to
SceneNode (fail if not attached).
Camera however have its own position and rotation (avoid
breaking old coder). Default camera is attached to the root
scene node. Need call Camera::detachFromParent() before
attaching camera to SceneNode manually.

7. Derived Transform
In 1.x, call SceneNode::_getDerivedPosition(), OGRE
keep flags to check if derived transform is dirty.
In 2.x, flags are removed for performance sake,
return cached derived transform.
The following won’t work.
SceneManager::updateAllTransforms() efficiently
update all, then cached derived transforms are
updated.
APIs force updating derived transform (including
all parents, slow!).

8. SCENE_STATIC and
SCENE_DYNAMIC
MovableObject/Node creation as static or dynamic.
Static means no move, rotation, scaling (or infrequently); save CPU
performance.
Default = dynamic.
SCENE_STATIC Node: no transform update.
SceneManager::notifyStaticDirty( scenenode ) to issue static node
update (recursively).
Dynamic node as child of static node (fit OGRE design); static node as
child of dynamic node may be a bug.
SCENE_STATIC Entity: no AABB bounds update (for culling).
SceneManager::notifyStaticDirty(entity) will update AABB, or when
attached Node is flagged as dirty.

9. Scene General Case
Static entity for static node only; dynamic entity for
dynamic node only.
Entity, Node can switch dynamic/static
(InstancedEntity cannot).
Destroy and recreate a new InstancedEntity instead.
SceneNode::setStatic() force all attached MovableObjects to
change concurrently.
Call setStatic(), change false to true, will call
notifyStaticDirty() implicitly.
Change static = true, can gain performance bump (no
transform & AABB update, and culling is much faster than
v1.x); batch counts remain the same.

10. Debug Mode Assertion
When AABB/transform is out of date, and
attempted to be used, OGRE issue assertion.
Solutions:
Static objects, do changes with notifyStaticDirty().
Avoid changes after SceneManager::updateAllTransforms()
or SceneManager::updateAllBounds().
Call Node::_getFullTransformUpdated() or
MovableObject::getWorldAabbUpdate(), be careful about
performance drop.
OGRE may also have bugs. Σ(;ﾟдﾟ), fire bug issues to them.

11. Renderable,
MovableObject subclass
In OGRE 1.x, allows add only Renderable objects
into render queues.
In OGRE 2.x, one Renderable MUST have one
MovableObject.
Visitor pattern for iterating MovableObject from Renderable
is removed, due to performance issue.
MovableObject subclasses must populate member,
MovableObject::mRenderables.

12. Vertex from V2 Mesh
Mesh  SubMesh  VertexArrayObjectArray 
VertexBufferPacked/IndexBufferPacked 
VertexElement2Vec.

13. Vertex Element
Element offset, index, and total size can be
calculated.
VaoManager::calculateVertexSize(const
VertexElement2Vec&).

14. Too Dark?
PBS (physically based shading) expects gamma
correction.
Materials for PBS need serious adjustment. High Fresnel,
roughness can affect diffuse a lot. Textures also need to be
tweaked.
Color (1.0, 1.0, 1.0) means incredibly bright.
PBS works best with HDR (high dynamic range rendering).
Light::setPowerScale( 3.14) to compensate in LDR case.
External GUI texture may need gamma correction manually.
Activate gamma correction (default).