Derive the rotation value between two nulls?

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  01 January 2018
Derive the rotation value between two nulls?

I've got a shot that I will be adding a CG cloth simulation for a flag being planted in the ground. We shot with a practical pole and tracking markers on the pole. I'll be tracking the points as 2D Screen space tracks and I'm hoping to somehow attach a null to the vector between these two tracking points so I can attach a cloth sim to the cylinder.

What would I need to rig up in Xpresso to get this rotation value between the two nulls?

Attached is an image to demonstrate what I'm trying to achieve. 
 
  01 January 2018
I might be misunderstanding your situation but here's a couple of options:

https://vimeo.com/64786739

http://sockgardener.tumblr.com/post/48915744796
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  01 January 2018
Mathmatically the angle between two vectors is the ACos of the dot product of the normals of the vectors.
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  01 January 2018
Srek that sounds legit, I just have no idea what that means.


The constraint tag for the midpoint is handy, but more specifically what I'm trying to get is a Cylinder that will rotate according being tied to a Null on top and a Null on bottom, so if the Nulls move left and right the Cylinder will rotate accordingly. Does that make sense?

Basically in this case, it's a cheat to be able to use 2 2D point tracks and derive a poor man's "object track" of the cylinder of that flag pole.  I know I can also rotomate the cylinder, which I might be forced to do if I can't come up with a solution, but I'm hoping to use track data.
 
  01 January 2018
Very simple solution.
Use the two nulls as PSR Constraints on the cylinder, and then add an Aim constrain "looking" at one of the Nulls that will force the cylinder to rotate.
Scene file attached:
https://www.dropbox.com/s/p48tjbjg4...%2001A.c4d?dl=0
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  01 January 2018
YUP!!!

Thanks noseman, EXACTlY what I was looking for. And thank you so much for sending along a project file. You are my hero.
 
  01 January 2018
Now that that's figured out, one other question I had was -- 

Can the C4D Manual Point Tracker spit out Nulls for the trackers? I know this be done in Syntheyes, do a point track and export it and bring it in as a Null. I was curious if it was possible to do this right in C4D?
 
  01 January 2018
Originally Posted by bowserlm: Can the C4D Manual Point Tracker spit out Nulls for the trackers? 
Yes.
One thing to remember is that you need at least 7 good tracks for an object track.

If you have Cineversity, please check out my Tracking Series:
https://www.cineversity.com/vidplay...side_cinema_4d/
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  01 January 2018
No not for an object track, literally just to turn one Manual Point Tracker into a Null with Screen Space tracking data.

In Syntheyes, I could track those points as 2D point tracks, export those, and bring them in as Nulls. Obviously it doesn't represent the true 3D motion, but since the camera is locked off it allows you to do Screen Space object tracking. Basically it's a 2D cheat.

I was just curious if I can get manual tracking data out of C4D without any kind of official "solve".
 
  01 January 2018
Originally Posted by bowserlm: I was just curious if I can get manual tracking data out of C4D without any kind of official "solve".

Although it sounds logical to do so, it actually doesn't make sense for a pure 3D tracker.
Another option would be After Effects. You can use the old 2D tracker to track 2 point and export the nulls directly to C4D.
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  01 January 2018
Sure, not in the context of doing a 3D solve, but since C4D now actually does have a decent point tracker in it it would be great it if would give us access to that data. Because for any locked off shot, or tripod shot you could do a lot of things with a simple 2D screen space track. 
 
  01 January 2018
Originally Posted by bowserlm: Srek that sounds legit, I just have no idea what that means.
For your problem Nosemans solution should be the better option. For future reference, here is how you get the angle between two vectors in Xpresso using the dot product and ACos

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  01 January 2018
Since I think R19 there is a "MotionTracker User Tracks" node in the presets. You can find it under X-Pool / System Presets / General Presets. It allows you to extract the 2d (screen space) position of your manual tracks. I guess that is what you are looking for. You can also specifiy a camera distance and it will return the global and camera space 3d position for that distance, which saves you a bit of Xpresso math. 


 
  01 January 2018
Originally Posted by Havealot: Since I think R19 there is a "MotionTracker User Tracks" node in the presets. You can find it under X-Pool / System Presets / General Presets. It allows you to extract the 2d (screen space) position of your manual tracks. I guess that is what you are looking for. You can also specifiy a camera distance and it will return the global and camera space 3d position for that distance, which saves you a bit of Xpresso math. 


Now that's a subject for a quicktip ;-)
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