How to create an arm that can stretch and grab things

[IvanXuff]

My character's arm is a physical entity. I want to achieve:

• It can stretch freely.
• It can grab another physical entity when hand touches it
• It can stretch freely while grabbing something
• stretch is driving by a limited force
  

[jrouwe]

Jolt simulates rigid bodies, so in principle an arm cannot 'stretch'. That said, you can have two rigid bodies inside each other and connect them using a SliderConstraint so that they can slide out. You can drive these to a relative position using motors (SetMotorState(EMotorState::Position)) while limiting the force (MotorSettings::SetForceLimits). Ensure that the arm parts don't collide with each other by using a GroupFilterTable. Fingers can be made using HingeConstraints that can also be driven using motors. You could make the upper arm EMotionType::Kinematic and move it around using BodyInterface::MoveKinematic.

Search for the code references I gave in the Samples folder and you will find examples on how to use each of these.

[BenzzzX]

Understood. So if we want the arm to stretch beyond its original length by a factor of N, the naive implementation would require N connected rigid bodies through slider constraints. However, this approach might introduce unwanted compliance, making the arm appear too soft.
The design compromise would be: Remove arm segments and directly attach the hand to the character via a powered 'SliderConstraint'.
Thanks for the answer.

[jrouwe]

I would indeed avoid multiple arm segments as that will make things soft. It is possible to replace the shape of a rigid body on the fly, but maybe you can just cheat and have a difference between the visuals and the physics and leave a gap between the arm segments if they stretch too far.