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Old 7th May 2008, 03:39 AM   #1
Dania Daviau
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Default Not understanding AC3D - Secondlife-Sculpties, please help

Hi all

I searched now a longer time for this, but either I use the wrong search words or there is no such feature.

Just at the beginning: yes I understood several tutorials and I got the plugins that were made for secondlife. I am able to make basic sculpts. However as it seems the possibilities are very limited by the following reasones. the conversions are/cannot be accurate.

A sculpt map is in its base 32x32 dots, each dot defining a ... dot, lol, means its place in the defined sculptie space.

To work for sculpties one would put ones dots to get accurate results. Means the vertex method would be what I wanted to go for. Now my crux begins.

Now my problems begin. I don't understand the definitions. if I look at the sphere I want to begin with I see 12x12 dots. Dividing gives me 24x24. dividing again gives me 48x48. How can I make an accurate sculpt with that ?

Another thing. if I click on surface -> calculate surface area it calculates 14x12 = 168 surfaces. What is the definition based on it.

Maybe it is the wrong program which would be a disappointment since I looked at it, liked it and bought it for that purpose since there is also publicity made to announce this program exactly for secondlife.

But i would really love to work directly on the sculpt definition = with the 32x32 dots. Since everything else is a hopeless approach from the beginning with inaccurate rounding on the conversion.

Do you know any tricks, features I overlooked, expriences ? Please help me.

kind regards
Dania
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Old 7th May 2008, 10:17 AM   #2
lisa
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Default Re: Not understanding AC3D - Secondlife-Sculpties, please help

The current sculpt map exporter for AC3D works in UV space, so it's not sensitive to the number of faces in the model. It is however sensitive to the placement of the vertices in the Texture Coordinate editor.

If your "dots" are evenly gridded in the TCE, you should get fairly accurate results.
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Old 8th May 2008, 06:36 AM   #3
Dania Daviau
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Default Re: Not understanding AC3D - Secondlife-Sculpties, please help

Thank you

That is what I assumed. Well it is quite ok, but leave the quest open for a real sculptie tool. I can handle it fairly well since I know how sculpties work, I paint the really precise ones in Photoshop. I think the basic problems are that using the faces is the wrong approach, it should use the dots if it is a sculpt. As you mentioned, I will get an approximate result which is most likely satisfactory for organic builds. I looked for a tool like that since organic forms are difficult to achieve working dot by dot. However when I saw that it is possible to also extrude the dots understandably I got the wish to make precise sculpts as well But knowing the limits of a tool is also knowing how to use it, iAC3D will definitifely have its use for me.
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Old 8th May 2008, 11:58 AM   #4
lisa
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Lightbulb Re: Not understanding AC3D - Secondlife-Sculpties, please help

I'm working on something that won't require a map so it can "use the dots" as you say, but the trick is determining the connectivity of the surfaces. Most of the sculpty editors either rely on starting with an exact mesh where the connectivity is known (i.e. you must start with a specific shape that can be modified, but vertices cannot be added nor removed); else they rasterize the faces in a way similar as the AC3D plugin. The only way to be able to use the vertices instead of an approximation is if the connectivity of the vertices is known, so the puzzle then becomes determining the connectivity of an arbitrary mesh.

Unfortunately, for reasons inexplicable to me, the SL sculpted prim format requires a mesh arrangement of vertices instead of just a vertex list. Had they
used the exact same data format and treated it as a vertex list forming a triangle strip, *any* polygon editor could produce *exact* meshes of completely *arbitrary* shape. (Within the limit of 8-bit precision of course, but that's a seperate issue.) Sigh, but alas, that's not how it was written. I assume the reason they did it that was was LOD, but there are better ways to solve that problem. (Edge collapse and\or allow people to supply a LOD texture.) But it is what it is, so ya' work with what ya' got, right?

The trick is for an arbitrary mesh, algorithmically re-arranging it into a grid. Again, the problem isn't the vertices--if I could have any arrangement of vertices, this would be done already--it's determining the connectivity, i.e. which edge is "up" when you unwrap it. "Up" for the very first face isn't bad, but when you start solving it's neighbors, up isn't up anymore, up might be left or sideways. I've got some ideas on how you could compute this for a mesh made of regular quads, but it's a lot more difficult as soon as you start allowing other face types including degenerate quads, aka triangles.

I'm open to ideas on this one. It has applications well beyond SL--for example, triangle stripping for games is essentially the same problem mathematically. In theory, I could do as some tri strippers do and use degenerate triangles to try and enforce connectivity where none exists, but that gets a bit sticky, because again SL is expecting a gridded mesh. Even the best strippers in existence usually limit their strip length, but SL needs what is effectively a perfectly stripped model in *two directions* instead of just one. Non-trivial.

Here's some research on the subject:
http://www.opengl.org/resources/code...es/node18.html
http://www.cs.queensu.ca/~jstewart/strips/algorithm/
http://www.cs.umd.edu/projects/gvil/papers/av_ts.pdf
http://portal.acm.org/citation.cfm?id=1101426
http://users.pandora.be/tfautre/soft...ripper/how.htm

The alternative, of course, would be to add a mode that allowed for a mesh with a known connectivity... this would give you 100% precision, at the expense of not being able to add or remove vertices. (You could still scoot them about, but I've never been a fan of that.) I could still use the UVs to track the vertex indices, basically data stuffing, but I'm concerned this would confuse people: it would be dis-allowable to move coordinates in the TCE in this case. This isn't terrible, as truthfully the layout in the TCE would be as it would be in the final sculpty anyway, but people seem to have a lot of confusion when it comes to texturing sculpted prims. SL does *not* let you change the uv coordinates of a sculpted prim, they are bound to the layout of the model but that doesn't stop people from trying things that would not ever possibly work.

Short of storing things in the UVs, I'm not sure how else to keep track. AC3D re-orders vertices to whatever is most optimal for display when it saves, so unfortunately I can't rely on vertex order. I know other plugins, like the Poser Morph Target editor have the same issue and work-around it in various ways, incl. external save files. Multiple UVs (very high on my wish-list for AC3D features :wink: ) would allow plug-ins to use an alternate set of UVs for tracking vertex indices but that feature doesn't exist presently. Even if it did though, like I said, this would be relying on a preset mesh, which isn't ideal. I'd really like to come up with an algo that can do it for an arbitrary mesh, but as previously mentioned, the connectivity problem is non-trivial... people have been trying to solve that problem for triangle strippers for years, and AFAIK no really good solution exists.

I'm still poking around for ideas, got a few things I'm going to try... if anybody's got any theories, post 'em here.

Last edited by lisa; 8th May 2008 at 12:08 PM.
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Old 8th May 2008, 12:30 PM   #5
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Default Re: Not understanding AC3D - Secondlife-Sculpties, please help

Oh, BTW: 100% precision of the vertices does *not* lead to 100% precision sculpt maps. Sculpty earth (http://slurl.com/secondlife/Areumdeuli/129/131/125) was generated completely algorithmically so the maps are in theory 100% precise, but there's still a nominal amount of pixel-drift regardless.

This is in part due to SL's 8-bit precision limitation. Some numbers are simply impossible to represent in 8 bits.

[EDIT: clarification on "algorithmically"... it's from the actual satellite elevation data, but I didn't use a 3D modeller to make the model. I wrote a computer program that would translate the satellite data directly into sculpted prim format.]

Last edited by lisa; 8th May 2008 at 12:32 PM.
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