My ships become very unstable with the PGS physics solver

Is it even possible to set collision groups on client? It doesn’t seem to work online for me. The wiki says they can only be created and configured on server. (see limits)

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You can’t create the groups on client, however you can change a parts CollisionGroupId on the client.

Hi CaptainMarcin - that would be great if you could provide a repro (model, level, instructions) for this issue.


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Here’s a repro pgsrepro.rbxl (796.4 KB)

It only seems to happen with 2 or more players, and online, the unwanted effect is much more intense than in studio. To reproduce it, at least two players must be walking on the ship, high walkspeeds are recommended.

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Any update on this now that you have the repro? I have similar issues.

SerClockwerk - we are still looking into it and will notify this thread when we have some results.

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Hey CaptainMarcin, SerClockwerk - we have a fix that appears to address the issue you found with 2 or more characters on a ship using the PGS solver. At this stage we’d like to limit the new code to specific games.

Please let us know which of your places are experiencing this problem, and we will activate the fix for these places and let you know when you can try it out.

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The problem is present in this game of mine:

Currently, it doesn’t have PGS enabled to avoid the issue.


CaptainMarcin - the fix has been activated for this place (for PGS enabled). Please try it out and let us know if you see improvements.

Edit: Unfortunately it’s still reported by players (I confirmed it too):

The issue is still happening, so I had to switch back to the old physics. The issue still exists and has not changed at all since the fix.

Also, could you please activate the new code for this place as well? It’s my test server:

Thank you.

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CaptainMarcin - When you confirmed it, was it the exact same problem you reported previously? Or was there some improvement?

We have enabled this code for your test place. Please keep us posted on its behavior.


When I tested it, the problem was exactly the same - the ships were easily blocked, moved and kicked by characters, causing chaos.
I’ll check soon if there are any improvements on my test server.

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EDIT: Ignore this, I learned how to read.

Can you provide a test level that simply spawns one of your ships? I need to be able to test this in Studio, but your entire place is too heavy to be able to play around with safely.

You can either uploaded it to a different place on ROBLOX or send it to me VIA PM if you don’t want to share your tech with people.

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Could you please verify this again? I was able to test the fix we applied to the two places you linked above:

One caveat I may have found was that for the first 30 seconds of the game the fix may not fully activate itself because its tied to testing whether your machine can handle extra performance of simulating MORE humanoids on your machine.

If you are available, I could test this with you if you’d like.

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I’m testing it here right now, I will post about the results soon:


Man this place takes a LONG time to load for me.

Should we consider using the simple-repro level you posted to verify behavior?

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It seems that the behaviour of the ships when players walk on them has improved a lot, ships don’t get kicked away anymore, but return to their former position. There is still a lot of shaking on some of my ships though, when tested with multiple players. The video with it is here:

The ship which was given in the repro (blue cargo ship) doesn’t have this issue at all now, however, it’s still noticeable in some cases, even though the effect is not as “crazy” anymore. It’s definitely going in a better direction. If anyone would like, I can upload another repro, this time including the ship from this video (cruise ship).
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How well does your machine handle the performance on that level? Does your game stay at 60 FPS or does it drop sometimes?

Can you do the repro you posted again, but this time with Micro-Profiler enabled (See: Steps to Report a Bug).

I want to verify whether it looks like your perf drops down to potentially cause the fix to disable itself.

Also another repro with the above ship would be nice!

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Here’s another video, this time with Micro-Profiler:

There were 3 players aboard, not counting me.

I̶ ̶p̶e̶r̶s̶o̶n̶a̶l̶l̶y̶ ̶d̶i̶d̶n̶’̶t̶ ̶e̶x̶p̶e̶r̶i̶e̶n̶c̶e̶ ̶a̶n̶y̶ ̶f̶r̶a̶m̶e̶r̶a̶t̶e̶ ̶d̶r̶o̶p̶s̶,̶ ̶i̶t̶ ̶w̶a̶s̶ ̶k̶e̶e̶p̶i̶n̶g̶ ̶w̶e̶l̶l̶ ̶a̶t̶ ̶6̶0̶ ̶F̶P̶S̶.̶
Actually, I do have very slight drops sometimes, but I would not say they’re caused by the movement of the ship.

I will send you the cruise ship on PM.

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Can you please test this on the default Roblox pirate game in studio?

ShipRepo.rbxl (1.1 MB)

Also you can fix humanoids colliding with the ship by using the new collision groups. Humanoids don’t collide with the ship on the captain’s client, but on their client they do. But I would prefer if there was some more default behavior instead.

Also I don’t like how limiting smooth terrain water is; so my mate Siren10 is working on triangle water.