Triangle physics behaving like Convex Hull and filling in the shape

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Edit: I've just had chance to get back to my good PC and try more things - analysing the physics on the floor I talk about below, and uploading it seperately, makes it behave as I expected it to, but it makes the rest of the house completely broken physics-wise when I upload it all (about 50 parts) as one linkset.

I'm working on a house and it's going well, including most of the triangle based physics. I have two problems though that seem odd and I'm after advice please...

The ceiling and next floor mesh, and just one doorway get their triangle physics filled in exactly the same as if it is set to convex hull when uploaded to SL. Concentrating on the ground-floor ceiling/first-floor floor, I have a stairway cutout that gets filled in so I can't get up the stairs, and the L-shape gets filled in too.

The physics is the same size and shape as the mesh, scaled to 1, and has two layers of triangles: the lower layer (for the ceiling) has normals point down and the upper (for the floor above it) with the normals pointing up.

I've tried a few adjustments to the triangles but get exactly the same result when uploaded: The physics shape when set to prim behaves exactly like the convex hull physics shape, even though the Physics Shape view shows it how I have made it. I can stand on the staircase cutout (and that filled-in L-corner) when it's set to prim just the same as if it's Convex Hull, and can't walk up the stairs.

Pictures will show what I mean:

Physics mesh:

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Physics View as Convex Hull:

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Physics View as Prim - the 'gaps' are still filled in even though they do not show as such,  and the physics behaves exactly as the above convex hull:

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Hi

Unfortunately what you describe is normal behaviour for non Analyzed Physics shapes of meshes that are less than 0.5 meters "thick" .

https://community.secondlife.com/t5/Mesh/Making-physics-HELP/m-p/1751753#M18002

Have a look at message N° 15 and 16 ....

Quoting Drongle :

"It's quite complicated, so I will try to explain the relevant background. First, there is a mechanism whereby the physics shape type used by the server switches from Prim to Convex hull without telling the viewer. This happens when one dimension of an object reaches a certain minimum. This minimum used to be 0.01m, but was made much bigger some time ago. This broke a lot of content, including thin walls like yours. It doesn't show up on the Physics Shapes display because the change is hidden from the viewer. So you can only detect it by trying to walk through (bug #1)"

The work around is to add some extra geometry, a single triangle will do, so that the meshes bounding box will be at least 0.5m thick. In the case of your ceiling mesh it could be added at one edge of the visible mesh where it would be "flush" with one of the walls of the building.Have the normals of this extra triangle facing into the wall so that it will be invisible when rezzed. Don't forget that this added triangle will have to be included in the Physics mesh.

And as the Uploader doesn't like small triangles in the physics mesh you may need to experiment a little. If you find the physics weight has increased because of the added small triangle in the Physics mesh then try making it bigger (higher in your case) so that you can use a larger triangle in the Z axis  in the physics mesh.

Best is to avoid this situation completely by including the walls or part of the walls in the ceiling mesh and so increase the boundng box dimension in the Z axis.

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Thanks, I had missed Drrongles post and I think it solves a mystery: once in a while in one of my houses, the stairs will be temporarily blocked. Never saw anything like it. Now I think I know what is going on and can fix it.

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Interesting thread, thanks. I've read it and saved it to my 'useful bits' folder. That answers what I'm experiencing.

I'm doing test uploads now to optimise LI (and to an extent convenience) based on what I join and what I don't, so the problem has cropped up at the relevent time, at least. I'll most likely do what you say and join to the above and/or below walls to avoid the issue,

Funny how simple something can look at first, only to find something that completely blows that comfortable feeling out of the water. Oh well, I guess life would be boring if things just worked.

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Pamela, I just wanted to say that yours are among the builds that I enjoy looking around taking inspiration from. In terms of quality I mean, not plagiarism.