Help Scripting rod forces

[Qriva0]

Hi, I have got several quesions about scripting rods.

1. Is there valid way to calculate rod tension? For example I need to know if rod is pressed against the wall and it's being compressed. I tried to compare compressed distances, but with chain constraint it does not work.
   
2. Is there way to calculate pressure against softbody or get it from softbody directly?
   
3. Is it possible to make different parts of rod have different elasticity? There is rotational mass, but it's hard to balance and there can't be huge difference in mass (1:10). Sometimes even if difference is not huge, the rod becomes wiggly and can't stop.
   
4. Is there way to damp rod without touching solver main parameter (damping)? I guess I could write script just for rod to reduce velocities every step, but it also affects other forces.
   
5. What is difference between changing particle velocity and externalForces?
   
6. Is it possible to change rod default (initial) shape during runtime? There are various properties like startPositions, endPositions, restPositions, any many other, but some of them has no effect and others are not what would be expected. I would expect particle delta to show something, but I think they are reset during simulation.
   
7. Is there way to "pin" my burst job to main job queue? I wish I could just add my job handle to be executed with all other stuff without need to complete job instantly.
   

[chenji]

Hi, I have got several quesions about scripting rods.

1. Is there valid way to calculate rod tension? For example I need to know if rod is pressed against the wall and it's being compressed. I tried to compare compressed distances, but with chain constraint it does not work.
   
2. Is there way to calculate pressure against softbody or get it from softbody directly?
   
3. Is it possible to make different parts of rod have different elasticity? There is rotational mass, but it's hard to balance and there can't be huge difference in mass (1:10). Sometimes even if difference is not huge, the rod becomes wiggly and can't stop.
   
4. Is there way to damp rod without touching solver main parameter (damping)? I guess I could write script just for rod to reduce velocities every step, but it also affects other forces.
   
5. What is difference between changing particle velocity and externalForces?
   
6. Is it possible to change rod default (initial) shape during runtime? There are various properties like startPositions, endPositions, restPositions, any many other, but some of them has no effect and others are not what would be expected. I would expect particle delta to show something, but I think they are reset during simulation.
   
7. Is there way to "pin" my burst job to main job queue? I wish I could just add my job handle to be executed with all other stuff without need to complete job instantly.
   

1. You can refer to the tear method of the rope. It shows how to read the "force" value of the rope/rod. But please note the value you read is from Obi engine, which is different from unity physics engine. As an example, if a rod is connected a cube with a unity joint, the force read from joint is most likely different from the value you read from Obi constraints. The difference depends on  the amount of constraints , the mass ratios between objects , and the frequency ratio 
2. I think yes but as I have no experience in softbody so leave this question to Obi dev.
3. Adjust rotational mass should be the way to go. Why it's hard to balance? There probably be something else wrongly configured when you see weird behavior.
4. No but you can reduce particle's velocity by yourself to get a customized damper effect.
5. Same as unity addForce (Velocity v.s Force)
6. Rest shape is defined by blueprint I don't think you can do that. If you mean change the shape as soon as rod/rope is initialized I think it's doable. Or you may create new blueprint every time do do that.
7. What's your "burst job" actually do? The solver has some event handlers in its lifecycle that you may hook so maybe you don't need that?

[josemendez]

Hi, I have got several quesions about scripting rods.
Is there valid way to calculate rod tension? For example I need to know if rod is pressed against the wall and it's being compressed. I tried to compare compressed distances, but with chain constraint it does not work.

Hi!

Most constraint types have a "lambda" value associated to each of them. These are lagrange multipliers, outputted by the simulation, that can be converted to forces dividing by the timestep squared. These lambda values are stored in an array, in each constraint batch. In ObiRope.cs you have an example of this in the ApplyTear() method: it iterates trough all lagrange multipliers for distance constraints, then breaks those constraints whose force exceeds a threshold.

Bend/Twist and Stretch/Shear constraints used in rods also have lambda values, that you can use to calculate deformation. Chain constraints however don't, since they're not really physically based - more of a geometrical constraint.

Is there a way to calculate pressure against softbody or get it from softbody directly?

You can calculate softbody deformation gradient/magnitude using the current linear transform matrix for each shape matching constraint. Using its Frobenius norm usually works well if you just want to measure amount of deformation. See the "DeformationGradient" softbody sample scene as well as the "DeformationToColors.cs" sample script.

Note however that pressure is force per area unit, mostly decoupled from deformation (pressure may or may not lead to deformation). That's a lot more complex to calculate accurately and would require processing individual contacts.

Is it possible to make different parts of rod have different elasticity? There is rotational mass, but it's hard to balance and there can't be huge difference in mass (1:10). Sometimes even if difference is not huge, the rod becomes wiggly and can't stop.

Not currently. We want to expose bend/twist/shear compliances in the path editor, as a per-control point property just like mass and rotational mass. This should arrive in Obi 7.2.

Is there way to damp rod without touching solver main parameter (damping)? I guess I could write script just for rod to reduce velocities every step, but it also affects other forces.

Not currently. You can do this yourself for each separate actor, the formula used for an individual particle is simply: velocity *= pow(saturate(1-damping), deltaTime).

Note that damping affects particle velocities after all constraints have been applied, at the end of each simulation step. If you are hoping for a per-constraint damping parameter, the situation is complicated: XPBD has built-in damping, but only works for compliant constraints. Infinitely stiff constraints (compliance = 0) apply the maximum positional correction possible, and this means the simulation will gain energy if the correction is too large.

What is difference between changing particle velocity and externalForces?

Force and Velocity are not the same thing. Force results in a mass-dependent acceleration (F=ma), so the same force results in smaller acceleration (less change in velocity) when applied to a heavier object.

Is it possible to change rod default (initial) shape during runtime?

Yes, by taking it out of the solver, modifying its blueprint and then bringing it back in. But this of course means re-loading the rod. What's your use case for this?

There are various properties like startPositions, endPositions, restPositions, any many other, but some of them has no effect and others are not what would be expected.

"startPositions" and "endPositions" are particle positions at the start and end of the whole simulation step, respectively.
"prevPositions" and "positions" are particle positions at the start and end of the current substep, respectively.
"restPositions" are the positions at rest (used to determine whether particles self-intersect at rest, and deactivate self collisions for those that do). "renderablePositions" are the values actually rendered to the screen (which may be interpolated or extrapolated from start/end positions depending on your solver interpolation settings).

Usually, you'd work with positions and renderable positions. These are conceptually equivalent to rigidbody.positon and transform.position in Unity.

I would expect particle delta to show something, but I think they are reset during simulation.

position/orientation deltas store the change in position/orientation applied during each constraint projection. These are reset to zero after each constraint type has been projected (so that the next constraint type has a clean slate), they aren't useful unless you're writing custom constraints.

Is there way to "pin" my burst job to main job queue? I wish I could just add my job handle to be executed with all other stuff without need to complete job instantly.

This is not something that's exposed, but you could modify ObiSolver's source code to give yourself access to the simulation job handle. Note that the lifetime of the job is rather complex and varies depending on the synchronization method used (see "Bolting Obi onto Unity's timestepping": https://obi.virtualmethodstudio.com/manu...eloop.html). It's very easy to break things if you're not intimately familiar with how and when physics should be updated.

kind regards,

[Qriva0]

Thank you very much! This is a lot of useful informations!

Yes, by taking it out of the solver, modifying its blueprint and then bringing it back in. But this of course means re-loading the rod. What's your use case for this?

I would like to kind of bend the rod, or change shape and make it permament (for runtime duration). That would be similar to deformation related stuff and because it's possible via forces, I thought it could be possible to change during runtime. Also another case is that I might want to turn rod into bendable tool, for example imagine tool used to explore pipes with camera on the tip, it might be possible to steer that cable, bend left/right to change direction, so in reality it changes it's default shape, however it still bends properly from that rest shape.

Is there way to access rod default shape?

This is not something that's exposed, but you could modify ObiSolver's source code to give yourself access to the simulation job handle. Note that the lifetime of the job is rather complex and varies depending on the synchronization method used (see "Bolting Obi onto Unity's timestepping": https://obi.virtualmethodstudio.com/manu...eloop.html). It's very easy to break things if you're not intimately familiar with how and when physics should be updated.

Yeah, that is the reason why it would be nice if there was simple functionality to "attach" some job/jobs to chain, some method or event where job can be added and then that job would be for sure completed in obi OnSimulationEnd event. That would ensure that whatever runs it's executed in the same pipeline with Obi.
I am aware I can most likely hack the whole thing, but it's better to not touch the source code, so it would be nice thing to have.

In any case thank you for help, I will check and try things you mentioned

[josemendez]

Thank you very much! This is a lot of useful informations!

I would like to kind of bend the rod, or change shape and make it permament (for runtime duration). That would be similar to deformation related stuff and because it's possible via forces, I thought it could be possible to change during runtime. Also another case is that I might want to turn rod into bendable tool, for example imagine tool used to explore pipes with camera on the tip, it might be possible to steer that cable, bend left/right to change direction, so in reality it changes it's default shape, however it still bends properly from that rest shape.

That cannot be done by altering the blueprint, since changing the blueprint requires re-creating the rope from scratch and reloading the entire thing anew - current velocities, positions, etc will be lost. Visually, it would look like the rod loses all momentum and "pops" into its new shape.

What you want to do is called plastic deformation. Elastic materials return to their previous pose after external forces cease to be applied, elastoplastic materials however retain part of the deformation induced by stress (could require little or a lot of stress, depending on the material's plastic yield), even after external forces are gone. Some constraints in Obi support plastic deformation (eg. bend/twist,  shape matching constraints). If you just want the rod to be able to bend and keep its shape, adjusting plastic yield and creep in the rod's bend/twist constraints will do this.

Note that stretch/shear constraints do not support plasticity. To fully support this, the implementation of stretch/shear constraints must be heavily extended. Will add this to our roadmap as a feature request.

Yeah, that is the reason why it would be nice if there was simple functionality to "attach" some job/jobs to chain, some method or event where job can be added and then that job would be for sure completed in obi OnSimulationEnd event. That would ensure that whatever runs it's executed in the same pipeline with Obi.
I am aware I can most likely hack the whole thing, but it's better to not touch the source code, so it would be nice thing to have.

There's not a single point in the code where the main simulation job ("simulationHandle", in ObiSolver.cs) is completed(). Will see if it's possible to expose an event that gets called before completing the job, passing the handle to the user so that you can insert custom jobs at the end of the simulation pipeline without the need for an intermediate Complete().

kind regards,

[Qriva0]

What you want to do is called plastic deformation. Elastic materials return to their previous pose after external forces cease to be applied, elastoplastic materials however retain part of the deformation induced by stress (could require little or a lot of stress, depending on the material's plastic yield), even after external forces are gone. Some constraints in Obi support plastic deformation (eg. bend/twist,  shape matching constraints). If you just want the rod to be able to bend and keep its shape, adjusting plastic yield and creep in the rod's bend/twist constraints will do this.

Well, I want it to bend, but from some defined shape and control the exact shape of the rod.
I think good example is medical endoscope - doctor can bend the tip with control handle, so default "rest" shape becomes different, however the whole thing is still a bit elastic. As you said, ignoring the simulation problem when blueprint is changed, that is what I want, to change initial shape, but not just swap, but "define" new or rather modify every frame.
Actually how rod keeps it's shape? Does it try to keep relative orientations always the same?

[josemendez]

Well, I want it to bend, but from some defined shape and control the exact shape of the rod.
I think good example is medical endoscope - doctor can bend the tip with control handle, so default "rest" shape becomes different, however the whole thing is still a bit elastic.

Ok, so that's not plasticity after all - there's no external forces involved. You can manipulate the rest orientation of each bend/twist constraint yourself (by writing to their restDarbouxVectors array), or you can use a ObiBone which does exactly what you're looking for: it creates and drives a rod-like structure using a skeleton. This way you can just rotate the skeleton joints to the shape you want the rod to have, and it will try to follow it while performing the simulation.

Actually how rod keeps it's shape? Does it try to keep relative orientations always the same?

For each constraint, it measures the current darboux vector and tries to keep it the same as the rest darboux vector. If you want the nitty-gritty details, it implements this article.

kind regards,

[Qriva0]

Ok, so that's not plasticity after all - there's no external forces involved. You can manipulate the rest orientation of each bend/twist constraint yourself (by writing to their restDarbouxVectors array), or you can use a ObiBone which does exactly what you're looking for: it creates and drives a rod-like structure using a skeleton. This way you can just rotate the skeleton joints to the shape you want the rod to have, and it will try to follow it while performing the simulation.


For each constraint, it measures the current darboux vector and tries to keep it the same as the rest darboux vector. If you want the nitty-gritty details, it implements this article.

kind regards,

Thanks for the hint! I am going to try it.