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To Analyse or not to Analyse, that is the question....


Anna Nova
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OK I have a very simple, but large 30M diameter, octohedron with a 25M octohedral hole in the middle, made in Blender.  Each of the 8 segments is identical, and consists of 4 quads (8 tris after conversion): two are the inside and the outside edges 0.1m wide, two are the identical faces with the 22.5 deg ends.  This is to be the awning round an octablonal structure.

I export it from Blender 2.78, using the preset for SL + Static.  The .dae file contains what I would expect.

In Beta, I import this using the same file for both visual model (High) and Physics.  The file is so simple, anything I could make would be just as complex.

Looking at the preview of the physics I see that the yellow physics model fits, it is co-incident on all faces and edges, and does NOT cover the hole in the centre.  On import without analyse, rez and set to prim, this mesh does NOT, repeat NOT, show a collision surface over the hole using the Render Metadata->Physics Shapes tool in Developer, yet you can walk your avatar over the hole in mid (simulated) air!!!!!  As a prim, this comes in at 1.0 download, 1.0 physics, 0.5 server, and 349 display.  But is useless.

If I do the same thing and Analyse, the physics model is similarly co-incident and now the usual pinky-glow. On import and rez, switch to prim, the mesh behaves as expected and you fall through the hole.  The picture seen in Render Metadata->Physics Shapes is IDENTICAL to the one not analysed.  But as a prim it is 0.9 download, 2.9 physics,  0.5 server, and 339 Display.

Is there a bug in the Render Metadata->Physics Shapes tool?  Or better still, is there a tool that actually shows the physics in world?

 

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Without "Analyze", you get a triangle-based pysics shape*. Inworld, anything with a triangle-based shape which has any dimension less than 0.5m uses the default (unu-analyzed) single convex hull for physics instead of the specified shape. That is the default convex hull you get when you set the shape type to "Convex Hull" inworld, but when the mesh is that thin, it is still used when you set it to "Prim". The triangle-based shape will still be there in the asset, but it isn't used. If you stretch it to 0.5m thick, then it will switch to the expected shape. The other way of making it have that shape ius to add some geometry that makes it at least 0.5m thick while leaving most of it thin.

By the way, if you are using a triangle-based (un-analyzed) shape for something thin like this, you should delete the narrow edges from the mesh in the physics shape file, because narrow trianglkes are heavily penalized in the physics weight and have little effect on the collision behaviour.

The viewer physics shape display doesn't reflect this "secret" switch to convex hull. It's done by the server, and the viewer doesn't seem to know about it. To see the actual physics shape on the server, you can use the Build-Pathfinding menu: -Linksets to set the object to be a Static Obstacle (remember to Apply Changes); then -Rebake Region, to rebake the navmesh;  then -View/Test and check static objects, to see the shape. Physics shapes of static objects will be red. You might have to move the object out of the way, or to mak it transparent to see it. (Note that the collision shape of a static object will stay where it is if you move the object, until the region is rebaked again.)

 

ETA: Tested the alternatives for your octahedral ring - If we make it 0.5m thick, so that the triangle based shape can actually be used, the (un-analyzed) triangle-based physics weight with the mesh with removed edges is 0.5. If the edges are left there, then the triangle-based weight is 7.2 and the analyzed weight is 2.9. So, if the thicker version is acceptable, there's a big saving using the un-analyzed shape. If it has to be thin, then analyzed is the only option.

I also thought I would be clever - tilt both rings 1 degree so that the bouding box was 0.58 thicj although the ring was still only 0.1 thick, then titl it back inworld so that it was flat. Then we would have a shape with a hole in a thin ring. That was true, BUT it runs into an old problem/bug with the server physics weigh calculation - the physics weighjt was more than 38000 !!! It got returned.

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Thanks Drongle, I am almost able to understand all that....  (at least I think I do!)  What a ride this stuff is!

I'll try some more things myself now. I can't see any reason why this can't be 0.5 thick, as long as most of the thickness is transparent, and Over it rather than under it.

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