Customize the contacts-cache SaveState function.

[AndreaCatania]

Hello, I need to send the physics state, recorded using SaveState, from server to clients so that they can simulate the physics in a deterministic way.
I need to optimize the data sent, so I'm writing a custom SaveState that collects only the body state that the client is simulating. Notice, the server is simulating a large world and the clients are simulating only few bodies around the character.

So far this task seems doable, as the SaveState is already public, however the Contact cache is private and I'm wondering if I can PR a change that makes it public so I can write a custom SaveState that fetches only the contacts relative to the bodies simulated by the client.

Can I open a PR that allows that?

[jrouwe]

Hello,

The contact cache is something I'd rather not expose as it contains a lot of internal interface that users are not supposed to call.

I'm also not 100% sure if the simulation will be deterministic if you don't have the exact same bodies on both sides (it may be but is totally untested). In any case you need to be very careful in synching BodyIDs between simulation.

What about adding 2 virtual functions to StateRecorder that you can override?

Something like:

bool ShouldSaveBody(const BodyID &)

and

bool ShouldSaveConstraint(const Constraint &)

So that the contact cache (and body + constraint manager) can ask the recorder if the state of a particular body / constraint needs to be saved.

[jrouwe]

Do you think that would be ok making EStateRecorderState a paramter of RestoreState instead?

I had that at first but it results in crashes if you pass the wrong value (which is very easy because the parameter has a default value) so I made it into what it is now. For context, saving a single dynamic body uses 142 bytes in single precision mode and more in double precision mode, so I don't think that byte is too much data.

[AndreaCatania]

Yeah, make sense, my idea is to reduce body size as much as I can, using delta update (and keeping ""static"" parameters, like the friction, mass out), by implementing a custom SaveState. I will use the default SaveState + the filter, just to store the needed contacts.
At some point I want to do something about BodyID too, as I think we can shrink it even more. Eventually, I'll figure out how to keep that byte out as well. In any case, I'll share with you some ideas, maybe we can figure out a way to make SaveState really customizable and granular while keeping the API simple.

[jrouwe]

Yeah, make sense, my idea is to reduce body size as much as I can, using delta update (and keeping ""static"" parameters, like the friction, mass out), by implementing a custom SaveState. I will use the default SaveState + the filter, just to store the needed contacts.

Actually it was always the intention that only the state that got modified by the simulation was saved (constraints already do) but it appears that too much body state was saved. I've reduced the amount of state that gets saved to that what is simulated in #654.

[AndreaCatania]

Oh wow, this is going to be useful, thanks a lot!

I was wondering about this:

inStream.Write(mAllowSleeping);

Is this going to change at runtime because of the Physics Simulation? If not, we should consider to keep it out, and leave to the user the responsibility to network it.

[jrouwe]

It's unfortunately needed because the vehicles switch allow sleeping mode at run time