[Full Release] Important Changes to Inertia and Volume Calculations

No it shouldn’t result in any performance difference, it’s only a correction to the way certain part masses/volume & inertia are calculated.

The point of doing this in the first place is to allow you to adjust your game to the change. If it remained optional, it leaves an enormous amount of surface area in the engine for bugs to happen and it creates a lot of extra code in the engine to maintain. It would mean adding the same new feature twice in two pieces of code.

In this case, it would also mean testing new physics content with the feature on/off and painstakingly tuning it until it works for both, and, well, that’s just not feasible. That would then mean that any new features would either not support the old way, or Roblox would need to make the feature change with the setting, and that too is problematic and adds more code and surface area.

I think it’s pretty safe to say that if there were better options Roblox would take them, but, at least keep in mind they’re inputting effort to allow people to adjust first.

I recommend reading this quote from tnavarts:


Will this have any negative performance implications?

No - Even though we switched Wedges and CornerWedges to calculate angular inertia in the same way as any other arbitrary mesh, there’s very little noticeable perf impact.

Our angular inertia calculation for meshes is pretty fast.

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They could just make it deprecated, where it would still exist somewhere in the engine and when it inevitably breaks it would get removed, no need to maintain it. Roblox’s major issue (and the reason I am moving to a different engine) is that they aren’t making backward-compatible changes. For example, the recent removal of public audios broke a lot of games that were abandoned by their developer (one such example are the old After The Flash games). In Roblox, I don’t have any guarantee that the things I coded today won’t break tomorrow when new unannounced updated releases.

Thank you for the update’ I remember people going nuts cause Roblox’s physics were not that accurate in the past. Funny how people switch up fast after they get what they want. Writing systems is no easy work and to be frank, I can’t imagine why you want a more inaccurate old physics calculation method that is said to be no faster than the new one. I’ll be able to create real time cutscenes maybe more attractive.


Just checked the physics of my carts (a combination of massless imported meshes, blocks, and cylinders) and there was no discernable change in movement. There does seem to be a more reasonable balance to the cart when at rest.

Enabled! Thanks!

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Just to be clear I don’t necessarily need a workaround for my situation if you have better things to be doing, I can likely sort it out myself with a body gyro, but I will still share a file with the affected character for if it is useful for other purposes.

The easiest way to see the difference is to flip yourself and move in a straight line. With it enabled you can go a longer distance upside down before the character will manage to right itself. With it disabled it will usually right itself shortly after starting to move.

Private message:

Hi! This is great, but it’s going to break my moment of inertia calculations. Can I get a way to compute or query moment of inertia? This would make this sort of change a lot nicer!

Also, this is going to break old boats in my game, which is problematic for gameplay. I think this is going to be ok, but long term back compat would have been nice.

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I agree with this and often am upset by various decisions as well, however I still also agree with Roblox’s stance in these particular cases. I think this way about many of the settings that are being added on Workspace like this one. It’s a difficult dilemma.

In the case of audio, I absolutely think Roblox could have been significantly less destructive in the way that they went about the whole audio purge thing, I posted a lot about it under many of the announcements. I think it was a rather impulse-driven decision and I think the outcome was rather poorly thought out, especially because I feel like all of the destructiveness could’ve been avoided even with all of the audios being gone.

Ultimately the choice is for Roblox to deal with the complexity & clutter and such in the engine, or it’s to accept that it will cause problems, and to try and evaluate those problems and hope that they are not significant enough to matter. Either way, it is a loss for us, it’s just one way is a less direct looking loss.

I do not think that that means Roblox’s decision is always correct (cough on the audio thing especially) but I am a little more apt to be supportive of Roblox I think with that in mind, I just hope that they end up trying to go for slower, more careful solutions than some particular examples, because sometimes I feel like they decide before they even actually ensure what the heck the outcome’s gonna be, sometimes it seems like there is an assumption and a lot of information is missed or the assumption is wrong.


Will this affect spring suspensions? Is mass distributed evenly across parts or also models?

What is the reason for you to know the moments of inertia?

This change affects only Block, Wedge and CornerWedge, so if you have models build of this parts, they will be affected… nothing changing for regular meshes, they continue to be solid

Updates like this one always make me wonder why things have always been calculated incorrectly and why no one at Roblox looked into it sooner.

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I can’t speak to his case, but when working with a dynamically controlled Torque, it’s useful to know the principal axes and inertias to get the desired response & damping.


There’s another big issue with CornerWedges and WedgeParts… you can no longer climb up them like usual, for some reason it’s collision box acts like a standard part, ruining a lot of old games / new ones using the two.

You could previously use them as stairs, ramps, so on and so forth, but now their collision boxes are straight up broken.

Try walking up the slope part of one and you’ll see the ghost collisions for yourself.

This gets kinda annoying, and I would appreciate if there could be an explanation from your end as to why this happens, as it may not be intended behaviour.

If it is intended, I’d like to ask; Why?

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Could you provide a repro case? There were no any changes to collisions of wedges for very long time.

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Thanks so much for all your feedback! Later this week on Thursday, June 16th we will begin the Opt-out Phase. The Opt-out phase will last for three to four weeks, before moving to the full release. During this time we invite any and all feedback! Thanks again.


@vespa_rbx Is this now enabled by default?

Starting now we are in Opt-out Phase, if you see some strange physics behavior in your experience, try to explicitly disable the functionality to verify if it is related to inertia changes.
Thanks! Any feedback is welcome!


@vespa_rbx The “timeline at the bottom” Button does not work https://gyazo.com/97c0084511e1b4b9f367465fd949d782

thanks! the link has been fixed