MaterialID and ContactProcess.

[ledahut]

1/
I would like to know if the id returned by NewtonMaterialCreateGroupID is always incremented:

Code: Select all

id[1]:=NewtonMaterialCreateGroupID (world);
id[2]:=NewtonMaterialCreateGroupID (world);
...
id[275]:=NewtonMaterialCreateGroupID (world);

Is id[275] will be always equals to 275 or it can be different?

---
2/
I would like to create conveyor belt [I already looked into the forum] but I want a constant speed for each contactJoint.
I tried:

Code: Select all

speed:=NewtonMaterialGetContactTangentSpeed(material,0);
if speed<2 then
NewtonMaterialSetContactTangentAcceleration(material,1,0);

But my body shake and I can't get a really good conveyor behavior [even by playing with contact friction].

Compare to ODE http://opende.sourceforge.net/wiki/inde ... 29#Contact [see dContactMotion] where we can directly set velocity to the contact point.
A function NewtonMaterialSetContactTangentSpeed would be realy good for my application.
---
3/
I tried to set velocity to body in ContactProcessCallback:

Code: Select all

  if Convoyeur then
  begin
    Dir := VectorMake(0, 0, -speed / 2, 0); // -Z Direction
    Body := NewtonJointGetBody1(contact);
    NewtonBodySetVelocity(Body, @Dir);
  end
  else
  begin
    Dir := VectorMake(0, 0, 0, 0); // 0
    Body := NewtonJointGetBody1(contact);
    NewtonBodySetVelocity(Body, @Dir);
  end;

It work good but I need to put a zero velocity to stop my conveyor [In ForceAndTorqueCallback the fact of not calling NewtonBodySetVelocity will make the body only move by its inertia].

[Julio Jerez]

Newton does not really work that way.
You cannot really set a velocity at a contact point; you can however set the contact tangent acceleration

Basically you can read the tangent speed and subtracted for the velocity at the of the floor of the conveyor and divide it by the time step.

The set this as the contact tangent acceleration, you also need to alight the contact tangent to the direction of motion of the conveyor

If you want to use a Material system the is more like the one you mention in you can just using the default material call back and instead use the Collision shape ID to implement you own contact callback system.

Check Tutorial_201_AbvanceMaterial in the wiki tutorial, is use a system just like that.
That material system is more like normal commercial physics engine use, and you have more control.

[ledahut]

Thank you, now I use timestep which is better.
Of course I have aligned my tangent direction before set acceleration.
Here a little exemple:

Code: Select all

ContactSpeed := NewtonMaterialGetContactTangentSpeed(material, 0);
NewtonMaterialSetContactTangentAcceleration(material,(DesiredVelocity - ContactSpeed ) / timestep, 0); //Velocity divided by time equal acceleration

---
In my breakpoint I discover something strange:

Code: Select all

ThisContact := NewtonContactJointGetFirstContact(contact);
material := NewtonContactGetMaterial(ThisContact);

material* and ThisContact* point the same address

[Julio Jerez]

I do not nothing though. a contact and a materail contain different information.