Newbie Loft issue

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 From:  how (BLACKSURFACE)
2219.1 
I am having an issue with the loft function and having the wrong desired effect.
I attached a .png of the resultant of lofting curves in Moi and the desired effect I want to achieve (as done in Max).
I also attached a .3dm file of the original curves and resulting solid.
Any insight would be appreciated.

-how



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 From:  Michael Gibson
2219.2 In reply to 2219.1 
Hi how, welcome to the forum!

When there are a different number of segments between 2 curves that you are lofting, Loft just doesn't really know how to do anything other than a kind of global averaging process which you're seeing there in the result you don't want.

When that happens, the solution is to use Edit/Separate to break your curves down into individual segments and do the Loft between individual pairs of segments so you can control how things are getting matched up.

So for example in this case, use Edit/Separate and then do the Loft with just these 2 curves:



That will produce a result like this:




Repeat that on the other 3 similar pairs (so using Loft 4 individual times) to get this result:




That has now got one portion of your desired result built. Now to do the corner pieces, there are a couple of different ways but I would do it by drawing a line here:



Now select these 3 curves (to select the edge of the surface, select the surface first and then with a second click you can "drill in" to select the edge):



And then run Construct/Planar to build a planar surface there:



You can then repeat that with the other triangle pieces to build all the corners, and also the bottom square cap can be filled in that way as well.

Once you have all those surfaces created, then it is a good idea to select them all, and run Edit/Join to join them into one connected solid object. You'll want to have surfaces joined when you export to a mesh - joined edges get handled specially to be sure to get a common vertex structure on shared edges to avoid any cracks. When you have separate unjoined surfaces, it is possible for them to get meshed independently and have slightly different mesh structures which can make little tiny cracks in there, so make sure to join surfaces up to avoid that.

I hope this helps! Let me know if you have any more questions,

- Michael

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 From:  Michael Gibson
2219.3 In reply to 2219.1 
Also one other note - to get a nice clean result it would probably be best to drop some of those vertical lines all the way down from the top profile, like this:



Then you can get a kind of cleaner result without any extraneous edges in it:



I've attached the .3dm model file of the result here.

Also one other note - since your piece is symmetrical once you have build one piece of the corner you can actually generate all the other pieces by Transform/Mirror which is faster. You can use the midpoints and corner points of the top profile to snap the mirror line on to, let me know if you need more info on doing that part. That would actually work for the Lofted pieces as well actually, you can just do 1 Loft and the use mirror to generate the other 3 loft pieces.

- Michael

EDITED: 6 Dec 2008 by MICHAEL GIBSON


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 From:  how (BLACKSURFACE)
2219.4 In reply to 2219.2 
Wow.

Incredible. I am not just talking about your easy to understand answer and resultant but also the incredible response time to this post.

I have had this software since early April and am FINALLY getting some time to get into it (just got laid off). The program is amazing and has great potential for an area I look forward to diversifying myself into.

I look forward to being part of the community.

A fellow Washington resident.
-how
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 From:  how (BLACKSURFACE)
2219.5 
Thanks I will look into this method.
Anything that results in a cleaner model and less steps to accomplish it in is a plus in my book.

Thanks again...and get some sleep.

-how
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 From:  DannyT (DANTAS)
2219.6 In reply to 2219.1 
Hi how :)

With nurbs I sort of try and break down the elements of a model to basic geometry where possible.
I try to use solid modelling as much as I can and if there is no other choice then I get to the surfacing side of modelling.

Another way this can be done is you use Construct>extrude for the top cross, then apply Construct>fillet>chamfer to the edges, highlighted in the picture.





Cheers
~Danny~

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 From:  how (BLACKSURFACE)
2219.7 In reply to 2219.6 
Hey Danny.

Thanks for your feedback and alternative technique. There's more than one way to skin a cat (or thread a screw...next step) and I always appreciate different solutions and thought methodologies.

I've been working with polys for the past 12 years in the game industry and am trying to get back into the swing of things with NURBS. I haven't used them since AliasPower Animator and I look forward to using Moi3D and the amazing forum resource.

Merci
-how
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 From:  DannyT (DANTAS)
2219.8 In reply to 2219.7 
No Probs, how!
BTW, Michael doesn't sleep, I've got my suspicions that he's a cyborg :)

---------
~Danny~
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 From:  Michael Gibson
2219.9 In reply to 2219.8 
I don't need to sleep, but my battery packs have a long recharge cycle.... Commencing... ;)
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 From:  Michael Gibson
2219.10 In reply to 2219.4 
Hi how - that sucks about getting laid off, but at the same time it is cool that you're focusing on learning new tools!

Like Danny mentions it can be a good idea speed-wise to use the "solid modeling" tools when possible, which basically means starting with a larger primitive solid shape made by something like extrusion, then using booleans and fillets/chamfers to modify the solid.

One common thing to use for stuff like that is to draw a curve and then do a boolean of the solid with the curve as the cutting object, to slice the solid up where the curve is projected on to it.

There is nothing wrong with doing a "surface modeling" approach where you do stuff like build individual surfaces and then join them, but it can take longer to finish, the solids tools are kind of like a "batch" operation. Like for instance doing a boolean with a circle curve as a cutting object does a kind of batch of extrusion + trimming + joining pieces together all in one sort of "higher level" operation.

One thing I've noticed is that people with a heavy poly modeling background do tend to focus on the "low level" surface construction tools more than thinking of using the solid modeling tools. It's certainly understandable since booleans and cutting operations tends to be generally avoided in poly modeling. In NURBS modeling they tend to be much more of a "first class citizen" and more of something to try first, that is one of the biggest differences in optimal workflow.

Another semi-related area is if you're trying to build a more complex model with some smoothly curved surfaces in them. In NURBS modeling you generally want to try to identify some larger kind of "panel sheets" that make up the overall form of your shape (as a kind of extended and simplified panel) and trim that to make many of the final edges in the model, rather than trying to do something like Loft or Sweep directly along irregular boundaries, which is more trying to build things directly to the final edges rather than producing those final edges through intersections. A sort of avoidance of intersections is another thing that I see often with people from a poly background.

Here are some examples of generating something through building extended panels and then intersecting them:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2164.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=2164.4


One other area that tends to be a bit strange for poly modelers is the way that "underlying surfaces" work in NURBS. That means that you some edges of your surface may be a "Trim curve" that lives on the surface, and the surface can be larger and not necessarily have any of its surface control points actually aligned with that edge. That's why you can't typically just grab any edge vertex point of a NURBS model and just move it around like you would on a poly model, because that edge can be kind of floating somewhere interior to the surface rather than being one of the surface's own natural edges. But the payoff of this system is that this is fundamentally what makes Boolean operations work better in NURBS than on polygons, because when you do cuts of a surface it only calculates new trim curves and the underlying surface itself remains exactly the same. So the surface itself does not "degrade" with repeated trims or booleans like it does with polygons where you get fragmentation with each cut. Some more details and illustrations on the underlying surface structure here.


Anyway those are just some common issues for people coming from a poly modeling background, hope some of this is useful!


- Michael
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 From:  WillBellJr
2219.11 
Michael, at times I'll try trimming a solid with a curve and end up with a hole in my solid?? The trim op will seperate the surfaces as I want but deleting the unwanted piece left the surface unclosed.


What I typically do then is extrude the curve into a surface or solid and then subtract...

Actually now that I'm typing this, perhaps I should have tried a boolean operation with the curves instead of a trim?

I usually think trim with curves and boolean with solids - I have to give this a try...

-Will

PS - DUHoid! That was it - boolean instead of trim! Boy knowing this would have saved me some time the other nite! :-P

EDITED: 6 Dec 2008 by WILLBELLJR

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 From:  BurrMan
2219.12 In reply to 2219.11 
THe other thing I think of Will is with bool dif it will punch through the solid. Using trim I can only select one side to remove. So on a sphere, a circle can trim a hole or punch a tunnel
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 From:  Michael Gibson
2219.13 In reply to 2219.11 
Hi Will - yeah the way that it works is that Trim is more of a surface operation where it will cut the "skin" of a solid and the result will be surface fragments and not a solid anymore until you use Join later on to join those fragments back into a solid.

Booleans try to operate on solids as a volume and will try to give you solids as the result.


But either of them can work with curves as the cutting objects.


When using a curve as a cutting object in Trim, it is the same effect as if you projected that curve on to the surfaces.

When using a curve as a cutting object in Booleans it is similar, except instead of just projecting the curve on to the surface it is the same as the curve being extruded into a solid (if the curve is closed, or a surface if the curve is open) and then that solid being used as the cutting object.

So for instance if you use a boolean with a circle as the cutting object, the hole that is produced will include "side walls" which are the result of the circle being extruded into a solid cylinder.


As a kind of demo, take a sphere, and then draw a line that crosses the sphere so that it appears to be divided in half in one of the Top, Front, or Right views.

If you select the sphere and use Trim on it, and pick the line as the cutting object, your result will be 2 half sphere surfaces that are not solid pieces, just the surface of the sphere "skin" cut into 2 pieces.

If you instead select the sphere and use boolean difference and use the line as the cutting object, you will get 2 solid fragments that have the plane of the extruded line incorporated into them.


One of these operations is not automatically "better" than the other, but basically Trim is a more "low-level" operation which works at a surface level.

The booleans are basically a wrapper around doing several things like an extrude, trimming, throwing away of different parts automatically depending on where they are located in the volume, then followed by a join. You can do all those operations manually using only Extrude, Trim, and Join, but that is kind of bundled up for you in one batch operation with the Booleans.

So if the booleans match what you want to do, you can save time using them instead of Trim since they bundle up several operations together.

- Michael
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 From:  Michael Gibson
2219.14 In reply to 2219.12 
Hi Burr, yeah basically another difference of Trim is that it dices up the surface and then lets you manually pick which pieces you want to remove or keep.

You can pick just one single cut piece to remove, which lets you do something like remove only one area where a projected circle cuts your solid to make a hole there.

Also with Trim you can cut everything up and leave all the pieces behind by pushing "Done" or doing a right-click instead of picking any pieces to remove.


Booleans are different - they are focused on automatically deciding which pieces to throw away by where they are located in the different volumes. Like a Union throws away the pieces internal to both pieces, Difference throws away the part that is only inside the subtraction piece, etc...


If you wanted to do a hole that did not go all the way through an object, you could still do that with a boolean but you would need to extrude the curve into a solid yourself, extruding it to the depth you want to make the hole and then using that extruded piece as the cutting object in the boolean rather than the curve.

When you do a boolean with a curve, the extrusion that it automatically produces always goes through the full length of the object.

- Michael
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 From:  BurrMan
2219.15 In reply to 2219.14 
Many uses with the 2 tools. I had in mind from one of your tutes where you just wanted to "remove" 1 surface so you could do a blend.

So with the trim you could hit your sphere:



Then select "Only" the surface closest to your blend to be removed, to leave the opposite side untouched:



Select the edges for the blend:



And Poof! A good reason to leave open holes on the surface:



Burr

EDITED: 19 Jun 2012 by BURRMAN

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