Modifying mesh prebuilts changes physics model in unexpected ways. Please explain!

[Thecla]

So occasionally I will buy a finished skybox or scene or backdrop and either modify it or break it up to use it in other builds for my sim. On several occasions removing a *seemingly* unrelated piece will change the physics model. For example, a backdrop that has a doorway...I unlink and remove some walls or other piece next to the doorway...or even some distance from it, and I'm no longer able to pass through the doorway. And the removed pieces are not invisible non-phantom mesh parts, such as for walls and stair cases. What is happening here? It's more that I want to understand the principles and practical aspect than finding a solution, although perhaps a better conception of what is going on will allow me to find workarounds.

Here is a specific case, an elevator backdrop from MINIMAL. I removed the two side panels on the iteration on the left, and the result no longer lets me pass through the door (fyi the upper floor exterior wall is simply set to transparent, nothing else was removed).

 

[Zalificent Corvinus]

7 hours ago, Thecla said:

So occasionally I will buy a finished skybox or scene or backdrop and either modify it or break it up to use it in other builds for my sim. On several occasions removing a *seemingly* unrelated piece will change the physics model. For example, a backdrop that has a doorway...I unlink and remove some walls or other piece next to the doorway...or even some distance from it, and I'm no longer able to pass through the doorway. And the removed pieces are not invisible non-phantom mesh parts, such as for walls and stair cases. What is happening here? It's more that I want to understand the principles and practical aspect than finding a solution, although perhaps a better conception of what is going on will allow me to find workarounds.

There are THREE physics types you can set for a link in a linkset, , 'prim', 'convex hull', and 'none'. Root prims cannot be set to 'none'. So, if one chops out certain links, including, say the root prim, some other link becomes the new root, if it's physics is set to 'none' it changes to one of the other two, for example.

Sometimes, links set to 'none' or 'convex hull' might change to 'prim' when the linkset is edited to remove links. Not only can this make it impossible to walk into the build, it can also cause the Land Impact to change radically, I've seen a 100 LI house leap to over 300 LI when 1 link was chopped out because of physics type swaps on some other links, I've never found a logical reason for some swaps, they just happen, and all you can do is cycle through the linkset and check the physics type on each link, and if it's "new root prim" related, either add a new root prim or unlink another part, and then relink it as the new root.

 

[Aquila Kytori]

 

 

On 8/27/2023 at 4:29 PM, Thecla said:

What is happening here? It's more that I want to understand the principles and practical aspect than finding a solution, although perhaps a better conception of what is going on will allow me to find workarounds

Perhaps the best single tool to trouble shoot physics problems like that is the View Physics option in the Firestorm > Build floater > Features panel > Eye icon to the right of the Physics Shape Type drop-down.

With this enabled you should be better abled to find what is blocking any doorways etc.

For an object consisting of a single mesh object then it is not difficult.

 

Enable the view physics option (eye icon) and the physics/collision surfaces of the selected object will be displayed in blue :

If this mesh object was set to use the Physics Shape Type option Convex Hull then there would be no openings in the physics shape and an avatar would be unable to enter the building.  (For Convex Hull imagine the object wrapped in cling film).

 

For your more complicated building consisting of a link set of  many mesh parts then its a little more complicated. Some parts may have been uploaded with there own dedicated physics mesh like in the example above but others will have been uploaded without having any specific physics model assigned to them when uploading to SL. These parts may have been been assigned a Physics Shape Type: None after uploading and rely on a completely separate mesh object (or even SL prims) for its collision surfaces.

This physics mesh object will have been assigned a physics mesh to use for collision surfaces at the time of uploading into SL then set to use the Physics Shape Type: Prim.  (Here using this Prim option is telling the viewer to use the physics mesh that was assigned in the mesh uploader as collision surfaces.)This physics mesh object will be set to 100% transparent.

(Note; This single physics mesh object could have several other mesh objects (set to None) in the link-set relying on it for their collision surfaces.)

Example below is the same building but uploaded as 3 different mesh objects, Outside_Walls, Inside_Walls and Physics.

 

If for any reason one or more of the objects has its Physics Shape Type set to Convex Hull then all openings will be blocked :

 

 

So to trouble shoot,  enable the View physics tool to see how the opening is blocked, then with Edit linked checked, run through the Link set until you have that part selected  and then check its Physics Shape Type status.

 

Edited August 29, 2023 by Aquila Kytori

[Chic Aeon]

I was thinking along the lines of Aquila yesterday when I read this but knew she would explain better and with pretty pictures.

As to the "WHY" of your problem, it "could" have happened if the uploader "made" the physics model (almost always a bad idea) rather than having built one in their 3D program.   The more complex a building is -- especially when it is not divided into sections with their own physics models and then put together in SL, the more issues can arise.

To be fair to the creator, IF the building worked well as designed, they did their job.  There is never any guarantee that an item can be modded as the purchaser wants  even it it IS modifiable.  

[Thecla]

OMG thank you SOOO much for all this information. This is invaluable. It never occurred to me to check the physics of the prims after unlinking one of them. It was not the root prim, and I thought nothing else could have changed. I am truly grateful for the time you took to respond.

[Thecla]

An update. Aquila and Zalificent you have changed my life, lol. Went back easily identified the offending prim and modified it to suit my needs. And I had no idea that objects set to convex hull have that physics property. Super useful to know!

And the backdrop worked beautifully out of the box, it was my meddling that caused problems. Also Rafael (I think that is his name, I'm not positive) of MINIMAL has been super responsive and helpful in the past when I have asked him questions. While his scenes/backdrops are optimized for what they were intended for and not disassembly or "parting out", he has no issues with that and will gladly share information to help you with that. And now I'm better equipped to make the best of everything I mod. Thank you so much