Alias General Discussion

Alias General Discussion

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cekuhnen
Posts: 857
Registered: ‎07-26-2012
Message 1 of 13 (478 Views)

Curve Snap tolerance issue - does not really snap onto curve but next to it !?

478 Views, 12 Replies
09-28-2012 10:32 AM

Hi there,

 

I encounter an odd behavior in Alias.

 

When I snap onto a curve the CV is palced not onto the curve but more next to the curve and this of course afterwards produces issues with the surfacing tools telling me the profile curves do not intersect.

 

Here is a screenshot and as you can see left all snaps fine but zoomed in right shows you it is NOT on the curve.

 

Any idea or explanation?

*Expert Elite*
cekuhnen
Posts: 857
Registered: ‎07-26-2012
Message 2 of 13 (470 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

09-28-2012 03:27 PM in reply to: cekuhnen

Nobody has an idea? This is like a roadblock to me. I cannot imagine that Alias has such a terrible snap onto curve tool.

Valued Contributor
JohnDixon8251
Posts: 60
Registered: ‎01-26-2010
Message 3 of 13 (461 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

10-01-2012 10:06 PM in reply to: cekuhnen

There was no screenshot attached, It might be helpful. Alias curve snap works fine so there must be some issue. Are you sliding it back and forth on the line to make sure it is on the line? If you are then it should be right on the line. If you are snapping to a curve that does not have a lot of points on it then it may look like it is not on the curve when it actually is but you said it was causing issues when building surfaces so that should not be it. Maybe you could attach a data file and I can look at it.

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cekuhnen
Posts: 857
Registered: ‎07-26-2012
Message 4 of 13 (456 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

10-02-2012 07:11 AM in reply to: cekuhnen

There is actually a screenshot in my first post, you do not see it?

 

As it looks like Alias does not render cuvres smoothly and thus visually represents points not being on the curve.

 

However I have a wire atatched and I cannot get the Bi Rail with 3 gen curves to work because Alias tells me they do not touch each other.

Valued Contributor
JohnDixon8251
Posts: 60
Registered: ‎01-26-2010
Message 5 of 13 (451 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

10-02-2012 09:52 AM in reply to: cekuhnen

Interesting, I am not seeing the screen shot.

On your wire file, everything seems fine. I was able to build the birail with no problems. I even turned the tolerences way up (very tight) and it still built with no errors. The only way I could get an error was to turn Rebuild Interior Gen Curves off, but this fails because of non evenly spaced knots.

I have attached the built surface. You can see if it has issues when you read it in.

 

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cekuhnen
Posts: 857
Registered: ‎07-26-2012
Message 6 of 13 (449 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

10-02-2012 10:13 AM in reply to: JohnDixon8251

Thank you for getting back to me.

Attached is a screenshot but ziped into an achive.

 

This way then you can download and see the image. As it looks however it truly snaps just visually is not representing it right.

 

I am sorry for asking, could you explain the reason why I need to turn rebuild on? You mentioned because of uneven knot spacing. I do not get this problem in Rhino for example with the same curves.

 

Are the cuvres I drew bad or is this just the way Alias works?

 

Claas

Valued Contributor
JohnDixon8251
Posts: 60
Registered: ‎01-26-2010
Message 7 of 13 (443 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

10-02-2012 10:35 AM in reply to: cekuhnen

I was a little bit off on my last comment as it would fail when rebuilrd rails was turned off (not Rebuild Interior Gen Curves) and this is because your 2 rails have different structures. When a surface is generated, isoparms are created based on the structure of the rail curves. If you are building a birail with 2 gen curves then Alias can adjust the placement of Isoparms (adding extra isoparms wherever there is an edit point on each rail) but when you have 3 or more gen curves you are forcing the location of the isparms where they should not be (Not where Alias wants them).

If you allow one or both rails to be rebuilt then it is not a problem, Alias can adjust one rail to match the construction of the other rail. I always recommend that people not worry too much about having rebuild turned on unless there is a particular reason. As far as using the same curves in Rhino, there may be some rebuilding going on, I am not familiar with the settings in Rhino, but without some rebuilding, there would be the possibility of crossing isoparms or other issues.

*Expert Elite*
cekuhnen
Posts: 857
Registered: ‎07-26-2012
Message 8 of 13 (440 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

10-02-2012 10:55 AM in reply to: JohnDixon8251

Hi there,

 

so as far as I understand you correctly the build issue Alias presents is because one the of rails has two and one has 1 span. Rebuilding one span so it has two spans should then in theory make the bi rail with 3 gen curves work.

 

I tried this but Alias still tells me that I need to turn rebuild on at least for one of the rails, even while they in this file attached here have the same degree and spans now.

 

 

Out of curisity, is rebuilding a surface something bad or good? As I read it my curves are not good so rebuilding a surf should be a good thing. But how much will a rebuild surface follow the originial input curves? I assume because I rebuild it the surface will fit?

 

Any advice or tips?

 

Rhino has a rebuild option to manually turn on but in my years of working with it I never encountered a bi rial command with 2+ gen curves to fail and forcing me to use a surface rebuild.

 

Thank you very much for your time.

Valued Contributor
JohnDixon8251
Posts: 60
Registered: ‎01-26-2010
Message 9 of 13 (438 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

10-02-2012 11:51 AM in reply to: cekuhnen

On rebuilding the curve to make them the same, I encountered the same issue. It still has to do with the knot spacing, so for instance, if you move the gen curves to the ends of the rails it will work fine. I don't know anymore about why it is like this. As far as rebuilding, I always use it. If, for instance, you have curves with not enough points on them, (too much curvature for the number of points) then when you create a surface with rebuild off, the surface may build but you will get warnings of curve fit failures, meaning the surface has left the curve. With rebuild on, it will add as many isoparms as it needs to stay on your curves, thus making a better surface. There are some people who are very concerned with having any isoparms in their surfaces and insist on only single span surfaces (Bezier surfaces), but they would tend to build things differently and would probably be more likely to have rebuild turned off.

*Expert Elite*
cekuhnen
Posts: 857
Registered: ‎07-26-2012
Message 10 of 13 (427 Views)

Re: Curve Snap tolerance issue - does not really snap onto curve but next to it

10-02-2012 03:28 PM in reply to: JohnDixon8251

Single span surfacing is also something that is new to me, while it makes sense considering how edit points on a curve lead to curvature flow problems.

 

Reading through your posts makes me feel like everything is actually working well and I only need to get adjusted to the way how this works in Alias compared to Rhino.

 

 

Thank you very much for your help and clarifications!


Claas

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