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mayobutter
PostPosted: Sat Aug 14, 2010 1:19 pm 
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Joined: Fri Dec 14, 2007 8:07 pm
Posts: 913
I'm hacking the contact solver to change "relative velocity at contact" to provide conveyor belt functionality, and need help figuring out why a few objects wont go in the right direction of the conveyor belt / suddenly change direction for no reason. Here's a demo (if it's not the "conveyor belt" demo then cache is preventing reloading):

http://www.sideroller.com/wck/?f=11

Every once in a while one of the triangles will go in the wrong direction.

And here's the hack:

Code:
// Relative velocity at contact
b2Vec2 dv = vB + b2Cross(wB, ccp->rB) - vA - b2Cross(wA, ccp->rA);

int flip = (c->manifold->type == b2Manifold::e_faceB ? -1 : 1);
dv.x += c->fixtureA->m_conveyorBeltSpeed * tangent.x * flip;
dv.y += c->fixtureA->m_conveyorBeltSpeed * tangent.y * flip;
dv.x -= c->fixtureB->m_conveyorBeltSpeed * tangent.x * flip;
dv.y -= c->fixtureB->m_conveyorBeltSpeed * tangent.y * flip;


Is there something else I should take into account to flip the effect? I figured taking into account the manifold type (faceA, faceB) would be allI needed.
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Erin Catto
PostPosted: Sun Aug 15, 2010 8:58 pm 
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Joined: Thu Sep 06, 2007 12:34 am
Posts: 2931
You should add a bias to vt instead.
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mayobutter
PostPosted: Wed Aug 18, 2010 11:31 am 
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Posts: 913
Ok, below is what I ended up with. I still get triangles that randomly change direction though. Obviously under certain circumstances I need to flip the direction of "vt" but I cannot fathom what that condition could be. And why only the triangles?!?

Code:
// Compute tangent force
float32 vt = b2Dot(dv, tangent);
         
int flip = (c->manifold->type == b2Manifold::e_faceB ? -1 : 1);
vt += c->fixtureA->m_conveyorBeltSpeed * flip;
vt -= c->fixtureB->m_conveyorBeltSpeed * flip;


Anyway thanks, close enough I guess.
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pozirk
PostPosted: Tue Nov 22, 2011 12:17 pm 
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Joined: Mon Sep 05, 2011 11:33 am
Posts: 7
I just wonder, was this issue solved?
Cause I have boxes, that randomly change directions...
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Erin Catto
PostPosted: Tue Nov 22, 2011 8:38 pm 
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Joined: Thu Sep 06, 2007 12:34 am
Posts: 2931
The SVN version has built in support for conveyor belts and there is an example in the testbed.
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pozirk
PostPosted: Fri Nov 25, 2011 10:17 am 
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Joined: Mon Sep 05, 2011 11:33 am
Posts: 7
I'm using this conveyor belt and objects are moving randomly.
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Erin Catto
PostPosted: Sat Nov 26, 2011 12:29 am 
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Joined: Thu Sep 06, 2007 12:34 am
Posts: 2931
Did you use the same logic as this code from ConveyorBelt.h?

Code:
void PreSolve(b2Contact* contact, const b2Manifold* oldManifold)
{
   Test::PreSolve(contact, oldManifold);

   b2Fixture* fixtureA = contact->GetFixtureA();
   b2Fixture* fixtureB = contact->GetFixtureB();

   if (fixtureA == m_platform)
   {
      contact->SetTangentSpeed(5.0f);
   }

   if (fixtureB == m_platform)
   {
      contact->SetTangentSpeed(-5.0f);
   }
}
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pozirk
PostPosted: Sat Nov 26, 2011 8:29 pm 
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Joined: Mon Sep 05, 2011 11:33 am
Posts: 7
Looks like we are talking about different things. :)
I'm using WCK with Box2D Flash Alchemy Port, so objects are moving randomly over there.
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AdamAdept
PostPosted: Wed May 09, 2012 9:41 am 
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Joined: Thu Nov 04, 2010 9:22 am
Posts: 13
I think I got the reversing direction issue fixed, by getting rid of flip altogether:

Code:
   vt += c->fixtureA->m_conveyorBeltSpeed;
   vt += c->fixtureB->m_conveyorBeltSpeed;
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