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Mesh Physics Issues


cscott0108
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I'm having an issue with Mesh Physics, I'm having more that than but I'll get to it after I figure this problem out. I've gone and made a house in Blender 2.77 and I've created a separate Pysics file to go with it. When I import the file the preview of the physics looks good but when I continue with the upload the physics are not what were previewed. And I cannot get into the house what so ever. 

Then, when I click 'Analise' It seems to a little better, I can get closer to the structure but not much and the preview has a weird shape and none of the openings are still accessable. I'm not sure what I'm doing wrong. I can upload photos if needed.

I'm not creating block based physics just plane physics as I figured I'd get what I need from it. Any Ideas will help here.

 

I am really new at this and new at SL in a sense I've been here a year but havent really played much in that year.

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This is a known problem with the physics engine of SL. It has problems dealing with very narrow triangles and all objects with dimensions less than 0.5 m along any axis are automatically treated as convex hull regardless of what hysics shape you choose.

The solution is to use analyzed physics but you have to do it right. Create a separate mesh for the phsyics shape constins of three cubes separated by narrow gaps, one at each side of the doorway and wone above it. (You can sometimes skip the one above and make it with just two cubes but there's not much to gain from it.)

One important tip if you want to make good mesh btw: Never ever let the uploader generate any of the models, make them all manually. This is espeically important for the physics model but also for the LOD models you need.

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hi :)

There is one important final step you need to take that you haven't mentioned......

Once you have rezzed your house inworld you need to Edit it and then open the Features tab of the edit panel.

There you will find an option called Physics Shape Type. The default is Convex Hull, to enter your house you need to change this to Prim.

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Aquila Kytori wrote:

There you will find an option called
Physics Shape Type.
The default is
Convex Hull
, to enter your house you need to change this to
Prim
.

Of course, I forgot to mention that! You did change physics type after you uploaded, didn't you? ;)

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cscott0108 wrote:

So I need to do boxes around the doors but use planes for the rest of the house?

You made the whole house from a single mesh? That's not usually recommended but it would eliminate the too-narrow-mesh problem I mentioned so unanalyzed physics should work.

Generally speaking, use analyzed physics models made from boxes for walls. And use as few boxes as possible of course - you don't need window openings in the physics model for example. You've probably come across houses with faulty wall physics, walls you seem to climb a bit up when you walk into them, walls you can walk halfway through, walls you can walk completely through. The first of these problems is inevitable with unanalyzed physics, the other two are caused by faulty unanalyzed physics. You can easily avoid them all with analyzed.

Use unanalyzed physics or good old fashioned prims for floors. It is very difficult to get correct elevation height walkign on an analyzed physics model and unless you do it right, you may end up with those pesky floors you can't rez anything on. Unanalyzed physics solves both these problems.

I usually use unanalyzed physics for roofs too since I regard them as walkable surfaces but most house builders don't bother with such subtleties, they're happy as long as you can't just drop straight though the roof.

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Okay, Thank you. I'll add boxes around the doors, I didn't include window holes because of the vids I watched but many of the vids didn't explain the opening issues I was trying to keep the prim counts low which is why I tried to add just planes for walls/door openings. But I did notice all of what you said. I added my own roof because I noticed SL seemed to auto stretch the physics to match the shape.

 

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I'm not sure what you mean by single mesh or single wall mesh but yes I have only 1 object for the house and 1 object for the physics layer. But the house walls are double thick. I just wanted to use a single wall physics to save on prims but I may have to add photos of the house in blender then the house in world. Because the house still didn't let me in but I noticed that analizing the object seemed to look better. But there were areas I think I need to add boxes around the eves and porches because the planes didn't work there either.

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Just a note:

If you use the planes method and follow the simple rules about bounding box size, avoiding small or thin tris/planes in the physics mesh, making sure that each visual mesh object has its own physics mesh object  and not touching the Analyze or any other buttons in the physics tab of the mesh Uploader, then using the plane method throughout your build works fine.

The advantages of the planes method is that you get EXACTLY the collision surfaces you created in your Physics mesh model and also that the physics cost will generally be lower than had you used the box method and Analyzed it.

If you start trying to mix the two in the same mesh object then .......... well don't.

To do that successfully you would need to divide up the visual mesh into smaller separate objects and upload each separately along with its own physics model , analyzed or not Analyzed.

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cscott0108 wrote:

Okay, Thank you. I'll add boxes around the doors, I didn't include window holes because of the vids I watched but many of the vids didn't explain the opening issues

I've never seen any videos explaining how to make SL houses. Any instructions not made specifically for Second Life will be useless there since SL uses the HAVOK physics engine which works very different from what most other 3D environments have,


cscott0108 wrote:

I was trying to keep the prim counts low which is why I tried to add just planes for walls/door openings.

The physics weight doesn't necessarily depend on the number of triangles in the physics model. It's quite a lot more complicated than that. You do want to keep the overall shape of the physics model as simple as possible but complete enclosed "boxes" can often give lower weight than individual single planes.


cscott0108 wrote:

I added my own roof because I noticed SL seemed to auto stretch the physics to match the shape.

Yes. The uplaoder will resize all LOD models and the physics model to match the overall size of the main mesh.

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cscott0108 wrote:

I'm not sure what you mean by single mesh or single wall mesh but yes I have only 1 object for the house and 1 object for the physics layer.

You also need separate files for the mid and low models and for some of the parts the lowest LOD model too. In the uploader window there are menus with the options "Generate", "Load from file" and "Use LoD Above" for Medium, Low and Lowest Level of Detail.

  • Choose "Load from file" for each of them and select the model you made specially for that level.
  • Sometimes you can use "Generate" and set the triangle limit to 0 to "zero out" the lowest (and sometimes even the low) level but be careful when you do that. The "Generate" option has no practical use apart from that.
  • The "Use LoD Above" option is sometimes necessary for small objects but very rarely for something as big as a house. That is, if it's a very simple mesh (below 10-50 triangles depending on the complexity of the shape) you might as well use LoD above all the way since it won't affect the I for such meshes anyway.

There are four reasons why it's generally better to make a house from several parts than a single piece:

  1. LOD and land impact: You often want better LOD for outside surfaces than inside ones.
  2. Land Impact (again): Splitting will usually give lower land impact. You should especially try to avoid mixing big and small triangles in a single mesh unless you know exactly what you're doing.
  3. Practicality: Smaller modules are often easier to handle than big bulky ones.
  4. Faces: A single SL mesh can only have eight faces and you usually want more than that for a house. Do not under any circumstance try to upload a mesh with more than eight faces to Second Life! If you do, the uploader will try to be helpful and split it up into several meshes for you and it always gets it wrong. The uploader always gets it wrong when it tries to be helpful but when it starts splitting your meshes, that's the worst of all.
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