Detecting Specific child shape in CompoundShape Collision

[slavik262E]

In a combat flight sim I'm making, I obviously want the entire plane to be a rigid body, so it colliding with something / something colliding with it would behave properly. However, I also want to have regional hit detection, where shooting the tail will damage the tail, shooting the wing will damage the wing, etc. (similar to how FPS games differentiate between head, torso, and leg shots). Since the rigid body of the plane is already a collision object, what is the standard approach to do this with Bullet? If you use a btCompoundShape, will the contact manifold getBody0/getBody1 calls hold the individual child shape that collided, or just the parent compound shape?

[mi076]

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[slavik262E]

Thank you very much for the help. I was starting to worry that nobody would ever get back to me.

[gennoevus]

Thanks mi076, this is useful! I'm definitely going to dissect your code...

[slavik262E]

Sorry to revive this thread so long, but I finally got a chance to review the sample code. From what I can understand of it (the complete lack of comments doesn't really help), the program gets an intersecting triangle form a ray cast. That's not really what I'm looking for.

Allow me to clarify. If two RigidBodies made of compound collision shpes collide, how can I get the specific child collision shape of each body that collided with the other? Is this possible?

[slavik262E]

Another game developer suggested the solution of recursively checking if each collision shape in one of the compound shapes collides with the other compound shape. Is there a way to check if two collision shapes (not collision objects; I've found plenty of functions for that) with given transforms are colliding?

[mi076]

However, I also want to have regional hit detection, where shooting the tail will damage the tail, shooting the wing will damage the wing, etc.

this is what u have asked about.... before you have changed the title of your post (was something about "regional hit detection")... i said you can also detect subparts, not only index... It is in fact not applicable now and am going to delete my post now...

[slavik262E]

I'm sorry. I should have been more clear. Your example was useful, just not what I was looking for.

[mi076]

I'm sorry. I should have been more clear. Your example was useful, just not what I was looking for.

No problem..

For compound shapes the index returned by results/callbacks should be the index of child shape..

BTW, the custom ClosestRayResultCallback from the example does it. I have tested it just now. Looks like compound shape also has LocalShapeInfo, and if shape is compound - "index" is index of the child shape.

Here is another example, from ConvexDecompositionDemo, shows custom contact callback..

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///MyContactCallback is just an example to show how to get access to the child shape that collided
bool MyContactCallback (
    btManifoldPoint& cp,
    const btCollisionObject* colObj0,
    int partId0,
    int index0,
    const btCollisionObject* colObj1,
    int partId1,
    int index1)
{

        if (colObj0->getRootCollisionShape()->getShapeType()==COMPOUND_SHAPE_PROXYTYPE)
        {
                btCompoundShape* compound = (btCompoundShape*)colObj0->getRootCollisionShape();
                btCollisionShape* childShape;
                childShape = compound->getChildShape(index0);
        }

        if (colObj1->getRootCollisionShape()->getShapeType()==COMPOUND_SHAPE_PROXYTYPE)
        {
                btCompoundShape* compound = (btCompoundShape*)colObj1->getRootCollisionShape();
                btCollisionShape* childShape;
                childShape = compound->getChildShape(index1);
        }

        return true;
}

P.S. getRootCollisionShape() is not 100% clear for me, but is off-topic here...

[slavik262E]

Thanks! The example in the ConvexDecompositionDemo looks extremely useful, but I'm trying to figure out how it's used. Is gContactAddedCallback just a global variable that's automatically employed by Bullet? I see the extern in btManifoldResult.h. From what I can gather from the Bullet source, it's called when a collision pair is added, but then how would you determine when the pair is no longer colliding? I'm confused by how btManifoldResult is used. Also, what if the child of a compound shape is a compound shape? Will this method properly return the correct child shape of the child compound shape?

For compound shapes the index returned by results/callbacks should be the index of child shape..

Could you elaborate on which callbacks these would be and how to set them up?

Thanks once again for all the help.

[mi076]

Could you elaborate on which callbacks these would be and how to set them up?

OK, "MyContactCallback" above is an example. Set

Code: Select all

gContactAddedCallback = MyContactCallback

and

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body->setCollisionFlags(body->getCollisionFlags() |   btCollisionObject::CF_CUSTOM_MATERIAL_CALLBACK);

it's called when a collision pair is added, but then how would you determine when the pair is no longer colliding?

Look into btCollisionWorld class reference, Demos.. choose what is better for your purpose,
Just for example, look at contactPairTest / ContactResultCallback.

Code: Select all

void 	contactPairTest (btCollisionObject *colObjA, btCollisionObject *colObjB, ContactResultCallback &)

contactPairTest performs a discrete collision test between two collision objects and calls the resultCallback if overlap if detected.
And last, sometimes iterate over, for example, contact pairs may be not wrong too, it depends..

[slavik262E]

I meant besides gContactAddedCallback, but ContactResultCallback looks very useful. Based on the information you've given, my procedure to generate a list of colliding collision shapes each frame would be to iterate through all contact manifolds, as suggested in the wiki, then do a contactPairTest on each pair to get the specific child shape that's colliding. Even though it performs redundant collision tests, I don't see another way to get the child shape that's involved in the collision. I'll also explore alternatively using gContactAddedCallback and gContactDestroyedCallback. Thanks for the patience and help.

[Erwin Coumans]

The gContactAddedCallback provides the index of the child shape of a btCompoundShape.

See also the fracture demo work-in-progress, it has some code: http://code.google.com/p/bullet/issues/detail?id=90#c12
Thanks,
Erwin

[slavik262E]

Is there a reason gContactAddedCallback is a global variable and not the public member of a class?
Also, is there a reason why gContactDestroyedCallback doesn't pass a reference to the point, just like the other two functions?

[almatea]

P.S. getRootCollisionShape() is not 100% clear for me, but is off-topic here...

for me too!
it is strange: if I use a getRootCollisionShape() inside my gContactAddedCallback function - all is ok; if i use a getCollisionShape() instead getRootCollisionShape() - my program have segmentation fault after first getChildShape() use.