Hi Martin,
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>Date: Tue, 25 Apr 2000 20:58:24 +0100
>From: Martin Baker <[EMAIL PROTECTED]>
>Subject: [JAVA3D] Collision Behaviors
>To: [EMAIL PROTECTED]
>
>Could anyone help me confirm my understanding of Collision Behaviors as
>below, or could you tell me about any tutorials that cover these issues)?
>
>* By default the bounds used in collision testing are internally computed
>each frame (unless overridden by Shape3D.setCollisionBound() or
>Node.setBounds()) ?
Yes, setCollisonBounds() will overwrite setBounds() which in
turn overwrite the internally computed bounds.
> even if the Transform3D is modified?
The bounds is relative to the local coordinate system that the Shape
is defined. As long as user set setCollisonBounds() or setBounds()
enclosed the entire Shape (it is the user responsibility to gaurantee
that if they choose to set their own bounds),
it should work fine. Java3D will transform the bounds user provided
(if any) to world coordinate and use it for collision/picking.
>
>* These bounds are in global coordinates. (does Node.getBounds() or
>Node.setBounds() get/set global coordinates as opposed to local
>coordinates?)
No, bounds coordinates are local to the Shape. Node.getBounds()/setBounds
use local coordinates.
>
>* If Group.setCollidable(true) is set then all its children will be liable
>to generate a collision so
Yes, on the other hand if Group.setCollidable(false) then all of its
children will not consider as a collision target.
By default Group node is not consider as collision target
(only leaf node does)
However, if Group.setAlternateCollisionTarget(true) then this Group
node will consider as a collision target. This behavior
is implement correctly in v1.2.
(if I want to get the triggering group I will
>have to set Group.ENABLE_COLLISION_REPORTING and find it in the returned
>path)?
>
Yes, this capability bit determine whether a specific group node
along the path from the collision target leaf to the root need to
report or not.
>* WakeupOnCollisionEntry.getTriggeringBounds() returns the global
>coordinates with the values set as they are at the time of the collision, or
>at the time when new WakeupOnCollisionEntry(armingPath,speedHint) was
>called?
The triggering bounds return is the Shape's local bounds
(again it is either setCollisionBounds() or setBounds() or
internally computed bounds if user did not define one)
that trigger this collision (i.e. at the time of the collision).
>
>* does speedHint flag make any difference at release 1.2beta2?
Yes, it makes a big different in speed since USE_BOUNDS
only use the bound (from setCollisionBounds() or setBounds() or
internally computed bounds) for collision which is much faster
than USE_GEOMETRY which need to compute triangle/triangle
intersection for every triangles in the shape using a naive method
in both v1.1 and v1.2.
Using USE_BOUNDS is much faster because in v1.2 we maintain a special
tree structure internally using Bounds of Shape for
quick view culling/picking/collision detection.
>* does it make any difference where the Behavior node is in the sceneGraph,
>relative to either armingPath or triggeringPath?
No, but the position of Behavior node determine the transform matrix
above it. So in the case like
public WakeupOnCollisionEntry(Bounds armingBounds)
The armingBounds is local to the Behavior node and it will be transformed
by the transformation matrix above it for collision.
>
>* If wakeupOn(WakeupOnCollisionEntry) is called when the armingPath is
>already overlapping another node (setCollidable(true)) will this mask any
>further collisions with a third node?
>
Yes, only after the shape( or bounds/boundingLeaf/Group/Morph)
in WakeupOnCollisionEntry() did not collide with any other shape will
this condition be triggered again. i.e. If you also defined
WakeupOnCollisionExit(), you would expect to see them triggered
alternatively.
>* Does the above apply if the already overlapping nodes are parent/child?
>
Yes.
>* If wakeupOn(WakeupOnCollisionMovement) is called when the armingPath is
>not already overlapping another node will it get called on collision entry?
>
The expected sequence Java3D invoke Collision wakeupCondition is (
if you ORed together WakeupOnCollisionEntry/Exit/Movement)
entry
movement
movement
..
exit
entry
movement
...
exit
As a result if WakeupOnCollisionMovement is invoked, it means
wakeupOnCollisionEntry is already invoked before. As mention
above in this case wakeupOnCollisionEntry will not invoke
if a third object collide with it.
Note that collision detection is rewritten in v1.2 to provide
faster and more consistent behaviors.
>Thanks for any help,
>
You're welcome.
- Kelvin
--------------------
Java 3D Team
Sun Microsystems Inc.
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