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How do people make low prim count mesh stuff?

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   Well, i have never tried a building physic but i fixed the physic of one part of a building with the mesh in Pic.1. It is set as convex hull. i can move around/inside it without any obstacle. It just has alittle hovering (Pic.2) but i noticed lots of buildings have it.

   The second i want to say...Isn't it hard to make a mesh with proper looking, proper LOD for 1,000 m. and low LI, especially if it includes rounded geometries. Even it is set to 2,000 m. as default at Mid Graphic Settings.

   As the last, to exclude lowest LoD, is one dimention of 10 m. enough or do all 3 dimentions have to be 10 m. at least ?




Edited by Memo Opaque
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The mesh in your picture will work when set to convex hull if each of the boxes is a separate object (a linkset). So each box will just have it's own single convex hull, hence it works correctly. If it is a single mesh, a single convex hull will encompass the entire mesh, when set to convex hull. That's the default physics shape the importer assigns to any mesh uploaded, even if you don't specify a custom physics mesh at all. When set to Prim, it's using the custom physics mesh, either as a triangle based mesh (not analized), or the specified mesh decomposed into a set of convex hulls (analized).

For the LI calc, and the dimensions, it's the radius of the bounding box of the mesh. The formula to calculate the radius is this:

r = Sqrt(X*X + Y*Y + Z*Z) / 2

And the radius that determines which LOD is dropped out of the land impact calculation is this:

Lowest dropped at r > 5.43 m
Low dropped at r > 10.86 m
Medium dropped at r > 43,44 m

So if the BB radius is more than 10.86 m, only the Medium LOD will have an effect in reducing the download weight of the mesh. You can load "Use Lod above" (the medium LOD in this case) in the Low and Lowest slot as well, without increasing the land impact.


Edited by arton Rotaru
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i used 6 prims for it inworld and linked them. Then i saved the linkset to my desktop and reuploaded it as single mesh. And thank you for the detailed explanation about radius. So from the formula you shared, i can see it calculates radius as half of 3 dimentional diagonal of a rectangular prism.

Edited by Memo Opaque
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19 hours ago, Memo Opaque said:

It just has alittle hovering (Pic.2) but i noticed lots of buildings have it.

Yes, you can't avoid that if you use hull physics (that is if you get the uplaoder to analyze the physics). All serious mesh hosue makers are aware of that though and the solution is usually simple: don't use analyzed phsyics for walkable surfaces.


19 hours ago, Memo Opaque said:

The second i want to say...Isn't it hard to make a mesh with proper looking, proper LOD for 1,000 m. and low LI, especially if it includes rounded geometries. Even it is set to 2,000 m. as default at Mid Graphic Settings.

Tell all the LoD butchers out there! There are so many meshes in SL that requires you to lag down you viewer by increasing the LoD factor to 2 and even to 4 - and there are even meshes that won't work with the LoD factor maxed out. If you know how to make mesh, there is so little to gain and so much to loose by skimping on the LoD models.

We've been discussing this many times before and I know my view isn't a particularly popular one but stick to it: What you make for your own enjoyment and use is nobody's business but yours. But if you are planning to sell your builds, make sure they work with RenderColumeLODFactor set to 1. Because if you can't do that, you're not good enough a mesh maker to charge money for your works.

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  • 2 years later...

Hello, all. I am new building and some of you have been my best references in my learning, especially Chic Aeon. I have read all the comments and I would love write mine 3 years later to say:

  1. Yes, Sketchup is not the best software to do mesh stuff, but is not so bad. Its tools and ways to do things are more simple than Blender and is nice for people like me that have the custom to draw with pencil from scratch and compose the 3D from the 2D basement, step by step.
  2. Following the advices of some of you. I started to learn Blender, so tough! But the latest versions since 2.8x are very friendly and now Blender develope an add-on to import/export from/to sketchup making easier to clean up the geometry, the principal reason related with this troubles shown in the discussion. I'd tried and is very useful! And now I love Blender too!

In my experience using Skectchup to do mesh and upload to SL I have learned:

  1. The real trouble is how each software calculate the geometry. Since Sketchup planes have two faces by default, Blender planes have just one and to make a wall is necesarry to use two planes or a cube or use nodes to indicate the faces, but, in this case, exporting to SL info is missing instead what happen with Sketchup.
  2. Sketchup make the UVmap automatically, but of course one can edit it in some limited ways, against what happen in Blender where it takes more care and time to do. BTW the new versions of Blender have a new "tools" to make UVmaps and also to asignate individual map faces apart of the assignation of materials what is great!
  3. About tiny tris... This is a very often modeling mistake we do, no matter which software we are using and SL show it better if we use to analize when uploading and not always can be solved simplifying with % of resctriction. Most of times the cause is the existence of lost vertices and/or edges mixed inside the geometry and is very difficult to see with any software, Blender included. But, I suggest to clean up the model using the Blender tools for that. The old tool was "delete duplicates" now, in new versions we have more options, or at least with different names and better organized. So, the most usual are now: "Delete loose", "Limited disolved", "Merge by distance" (similar to delete duplicates). I suggest use these ones in first place and repeat all the necessary times until Blender can't clean up more. The other tools can distort the model if we don't have care. This way is easy to diminish the geometry and simplify it before upload the model.
  4. The width of walls of a house have to be around 15 to 30 cm (same with the floor) otherwise, the physics model let the avatar walk though them by little pieces, the knee, for example. So, I suggest not to erase the planes of the walls, but let them hollow when simplify them to do more eficient the collision model. I suggest this because some erase the exterior face for example if the logic says no avatar will walk near, through or against it. Do things against fools or mistaken or dump avatars. Yes, it arise the geometry and upload price, but not necessarily the LI.
  5. If the model draw curves. be sure make them as simple as you can, example, cilinder with no more than eight sides to avoid tiny tringles and multiplication of them inside the geometry. But, again, it all depends of the model, is not the same a mesh house or a sculpted mesh. Don't be slave of the LI concerns, but think efficiently to reduce lag. 
  6. Many think that Location, rotation are very important to affect the geometry and... sorry, no. Are important yes to define the espacial relation of the models against their origin, but it doesn't affect directly on the weight of the geometry and LI or LODs. Affect, yes, the UVmapping and appearence of materials and this way, indirectly, on lag.
  7. When uploading, very simple models don't need a physics model layer and, no matter if the model is made with Skectchup or Blender or Maya or 3D or whatever you can use the inbuild options for physic in the upload window, but, yes, a physic model layer can reduce substancialy the LI of all the model, but not the price to upload. I discovered that combining the model, with the physic model and using analize and simplify options help to reduce LI too and here is something very important that just one builder in SL could enlight me about: the colors code for analize the physic and geometry of a model and their meanings.
  • Blue -Purpule= All physic is ok, but... Is important to define if the shape of the physic is the correct one and the colors don't give any clue about with the exception of how are organized and distributed along the surfaces or showing the colision hull.
  • Green = often is related to lands physics useful for landscapes.
  • Orange-Yellow = The geometry is complex but not a trouble.
  • Red (often as lines and dots linked) = there is a mre or less serious trouble with geometry and there are vertices, edges or faces loose degenerating the final hidden geometry of the model, it means the necessary review and clean up of the model.

And finally, but not finally, LODs are very important and affect as the LI as the upload price. They can be define in the original model in Blender but it affect the renderization inside the Blender no necessarily when uploading to SL. Is important to define the LODs when uplading to control the tirangles and the lag, depending of the kind of piece of model we are uploading and modeling, is not the same LOD for a planter than for a house.

These topics are little hard to understand for some latin people like me. If around are some latin people I have being filming a serie of tutorials in spanish to show and explain my learnings about all this. You can see them or share with your friends following this link to my channel: 


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Sketchup, MD, mudbox, Zbrush, the big appeal of all these tools is that they abstract away the geometry itself to let you focus on design and design only.

The problem with this is that, if you are modeling for SL, at some point you are gonna need to "pop the hood open" to speak of and work at the individual polygon level, either because you need to build a good collider, or because you need to fine-tune your LOD models, or simply bring your polygon count to something acceptable for real-time rendering.

So all those "efforts" to avoid traditional modeling are ultimately in vain because if you want to do a decent job, you will have to get down to it eventually.

Edited by Kyrah Abattoir
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