Constant path length in bending animation?

[toph]

Hi,

first to introduce myself: I visit this forum more or less regularly, but didn't post yet, because I would still call myself a beginner in ASP, so I don't see myself qualified to give useful help to others at the moment. But I found many answers to my questions over here as a quiet visitor in the tips & technics section and by using the search function. I have a long years background in animating in Flash (since Flash 4), so I may have a little better understanding then a total beginner.

But today I have something I want to ask and hope someone can give me some tips:

I need to do an animation showing how different pole vault poles bend.

Now I'm wondering if there is a workaround, to keep the length of the poles constant while bending them in ASP8? Or do I have to get that done by trial and error?

My way to bend right now is to do an point animation, but this gives me the problem, that I don't have see a chance to control the path length.

I would appreciate any help and hope to be more helpful myself in future.

Thanks in advance,
Christoph

[Víctor Paredes]

I don't understand what you want to do (probably my fault because of my basic English). Can you explain it again or post some images to see if I can help?

[toph]

Hi Selgin,

thanks for replying. Maybe it's a fault of my basic English skills and using an online dictionary.

Here is a fast sketch, of what I have to do:


I need to show the behaviour of different long and hard poles used by the same jumping person. And so I need to animate the different bendings of the different poles, but the length of the one pole at a time (or simplified the animated path) has to stay constant while it's bent, because a pole doesn't stretch or get smaller while it's bent.

Or in other words: I need to change the form of a path of a certain length and the length of the path has to stay the same while it's transformed from a straight path to the wanted form.

I hope it gets more clear and understandable know. If not: Native speakers please don't hesitate to help out, if you understand, what I want to say.

[Víctor Paredes]

If I understood good, here you have a tutorial with what you need:
http://dl.dropbox.com/u/54411/videos/Tu ... torial.mp4

[toph]

Hey Selgin,

wow, thank you very very much! - I didn't expect to get a video demonstration of what I was looking for. And by the way: Yes, that's exactly what I was searching for!

I was thinking of using bones, but had the wrong approach. I would have tried to do it with way more little bones, what would have made the whole thing very hard to handle and to look good.

Thank you again! It's soo cool to have people around here, that help out in such a professional way.

Have a nice day,
Christoph

[hayasidist]

Hi,

sorry to be a pain - but when the line here is bent, it looks as though it is actually longer than when it was straight?? The total length of the two bones is the same as the length of the line when it's straight, but when the line bends it (nearly) forms an arc of a circle with the bones as chords inside the circle. (as in the pic here grabbed from about 1:35 in the tutorial and also as at 2:14). To preserve length the line of the pole needs to be much more closely aligned to the line of the bones (and not almsot totally outside). Could do that by animating bone length or realigning the bones to be on the outside of the curve.



the physics of flexible bodies is way out of scope for this forum but the textbook approach approximates a flexible body as a set of coupled rigid bodies implemented by a chain of alternating rigid elements and joints actuated by springs and dampers. The spring stiffness and damping coefficients are functions of the material properties and the geometry of the flexible elements. So the instinct to go with multiple small bones is closer to the way that materials scientists would tackle the problem.

as an aside, and why this subject of preserving length is of intererst to me is the way that AS stretches and contracts lines (rather than preserving line length) .. this is part of the problem with elbow etc joints. If AS could "unfurl" wrinkles on the outside of a joint and vv on the inside ... (that's on my list...)

But whatever .... Visually, it's unlikely that anyone will notice (and the length difference is small where the angle betwen the bones is small). and if Christoph's aim is just for illustration rather than serious physics is it really a problem if the pole stretches a bit?!

[toph]

and if Christoph's aim is just for illustration rather than serious physics is it really a problem if the pole stretches a bit?!

Hey hayasidist, thanks for your reply and it looks like you're right, that the described method doesn't preserve length exactly. But like you wrote above: It's not that obvious and will work for my current task.

I had to animate a side view of the move of a bullwhip some time ago and tried that by applying many small bones to my whip illustration in ASP. That was a real pain in the a**. Took me days and some sleepless nights and didn't look very good at the end.

[hayasidist]

Selgin has published some good stuff on rope and hair and other "floppy" stuff that you might have seen.. which might make the next bullwhip-like project a bit easier?!

viewtopic.php?t=15647

and there's a point animation of (a strand of) hair in wind -- the file was pelo.anme ... I'm guessing that was Selgin too but maybe I'm wrong because I can't find that thread right now (only another 102000 articles to look through ..).

[slowtiger]

For very fast movements which will not occur often it can be a timesaver to not use bones at all, but animate frame-by-frame with a switch layer or with point animation, instead of rigging with numerous bones.

Also I'll repeat the advice to use sketches (from anywhere, on paper, scanned in, or video) as reference. Much faster work. Use sketches, grids, arcs on a reference layer. (to be erased later)

[toph]

Thanks again for all the helpful answers.

The Bullwhip animation had the "small" extra task that there should be a loop running through the whip and it had to be slowmotion, because it was used as an explanation for the resulting crack of a whip (what is caused by the end of the whip being accelerated to supersonic speed).

So I had to add many bones but the final result what I came up with finally still looked bumpy and the curves got an edgy look, I wasn't able to get rid of.

Maybe I missed something and maybe one of the problems was, that I had to bend not only a path, but a form back then, because a bullwhip is reducing its diameter from start to end (what is essential for being able to accelerate it to supersonic speed).

I just searched the file from then and loaded it up, so you can have a look what I did, if you like to:
http://www.soehngensbuero.de/public/bul ... xample.zip

Regarding to SlowTigers tips: I was lucky enough to have screenshots from a video as guiding reference.