clipka <ano### [at] anonymousorg> wrote:

>
> Also, if 2-axis rotation of asteroits were more realistic, Kubrick would
> have made his special FX team work overtime to make the impossible possible.

Hmm; I think the physical set-up for filimg two-axis rotation would have
presented a problem.

Having worked in the physical special-effects field -- :-)  -- my initial
thought would be to build a simple 2-axis motorized rotation gimble, to hold the
asteroid model-- with a single rod attached to the 'inner' gimble for skewering
the model and holding it. (A chicken on a spit). For simplicity, think of the
inner gimble as rotation around x (in POV-Ray terms), and the outer gimble as
around z.

Depending on the camera position, the attachment rod's point of contact with the
model would be invisible most of the time (hidden by the asteroid itself.) But
eventually, that attachment point would rotate into a position that's visible to
the camera (with the rod obscuring a small part of the model, 'in front of it'
so to speak.) Having realized this problem from the get-go, I would have said
that, no, it can only rotate around one axis. Sorry, Stanley.

There's a much easier method to do this, of course: aim the camera at the
ceiling of the filming stage, and simply drop the asteroid from a good height,
past the camera. (Filming it in slow motion.) But Kubrick being Kubrick, he
would probably have given an emphatic thumbs-down to such a crude and
uncontrollable method.

Just sayin'  :-D

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"Kenneth" <kdw### [at] gmailcom> wrote:
>
> ...I'm working up a 'better' example-- with a
> few new realistic constraints added. I'll post that asap.
>

Two of THESE animation examples are what my own mind's eye wants to see in
POV-Ray, for free-fall.

I actually like all three, but I like B) and C) better.  The two-axis rotation
of C) *might* not be physically correct, but it looks cool anyway ;-)

The red dot represents the 'center of mass' of the object (just a guess, of
course), and the rotations are around that point. Those 'off-center' rotations
automatically make the result look more realistic, IMO.

For the two-axis rotation, it looks like the 'degree' of one or the other needs
to be constrained, possibly so that the two do not exceed a certain ratio(?).
1:1? 1:0.5?  Otherwise, maybe that's when the odd 'reversed rotation' appearance
starts showing up?

For finding the *actual* center-of-mass (for a simulated POV-Ray animation
anyway), I think there might be a relatively simple scheme based on the 'reduced
mass' concept I previously mentioned. (That concept at its simplest takes two
separated masses-- like two Suns orbiting each other-- and comes up with a
single mass, the rotation point for the pair.) Even though my animated object is
a *single* object, it's made of simple separate parts. The *individual* parts
could be thought of as separate Suns (in a step-by-step fashion for the
computations?) Anyway, although a POV-Ray object has no real 'mass', that can be
simulated too (for an object made of simple geometric parts like spheres,
cylinders, etc.) Assuming that the parts are all made of the same stuff--the
same density--, the VOLUMES of the parts can be a stand-in for their separate
masses. And the volumes can be computed from the surface area(s).

So far, it's just an interesting set of ideas, that I haven't tried working out
yet.
---------------

The codes for the animation examples (a 400-frame animation):
A)
object{OBJ
rotate <-4000,0,0>*clock
rotate 360*rand(S) // arbitrary rotation
  }

B)
object{OBJ
rotate 360*rand(S) // arbitrary rotation
rotate <-4000,0,0>*clock
rotate 360*rand(S) // arbitrary rotation
  }

C)
object{OBJ
rotate <-4000,0,0>*clock
rotate 360*rand(S) // arbitrary rotation
rotate 800*clock*y
rotate 360*rand(S) // arbitrary rotation
  }

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Attachments:
Download 'rotations_in_1_vs_2_axes_redone.mp4.mpg' (3646 KB)

"Kenneth" <kdw### [at] gmailcom> wrote:
> my initial
> thought would be to build a simple 2-axis motorized rotation gimble...

Gimble-- That was the name of a big department store in New York City. Duh.

GIMBAL

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On 10/10/2018 17:06, Kenneth wrote:
> "Kenneth" <kdw### [at] gmailcom> wrote:
>> my initial
>> thought would be to build a simple 2-axis motorized rotation gimble...
> 
> Gimble-- That was the name of a big department store in New York City. Duh.
> 
> GIMBAL
> 

Hands up all those that noticed.

I certainly didn't.


-- 

Regards
     Stephen

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Hi(gh)!

On 03.10.18 17:58, Stephen wrote:

> Well here is my tuppence worth.
> I think that it is the ratio of the rotations that makes 3 axes look 
> odd. in this animation the X axis rotates 3 times for one rotation of 
> the Z axis and the Y axis rotates twice.
> (I am wedded to cyclic animations)

May I do Atari ST and Commodore 64 versions of this and re-publish them 
on my YouTube channel, of course crediting you? Also your other 
animation works?

See you in Khyberspace!

Yadgar

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Hi(gh)!

On 08.10.18 05:14, dick balaska wrote:

> Mine looked a lot like yours, especially 3A, so I don't need to post
> them, but I did end up with this, which *technically* has 3 rotations :)

You must have set the brilliance value for the day side quite high... 
too bad that I never have been in space to tell the difference from 
reality... but one should never say "never"! Who of us POVers would not 
like to take at least a low orbit vacation in those inflatable Bigelow 
Airspace hotels?

Besides that: may I AtariSTficate and Commodore64ize your animation and 
re-post it on my YouTube channel, of course crediting you and liking to 
the original? May I do this also with you other POV-Ray animations?

See you in Khyberspace!

Yadgar

Now playing: Give Me Love, Give Me Life (Roger Hodgson)

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