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Placing or attaching a 3d object to a moving subject.

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MaxX Kentaur

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Placing or attaching a 3d object to a moving subject.

PostThu Mar 26, 2020 7:00 pm

I am trying to add a 3D Knife (FBX) into the hand of a person in a sequence. Im not sure on the correct way to do this. The only way I know how is by placing the sequence in the timeline then opening up Fusion, add fbx, merge, 3d cam, ect and then attempt to place the knife in front of the clip and key frame , one frame at a time. Is there a better way to do this?,

Can you add the 3D object with Tracking?

Any help would be awesome.

Thankyou
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Knife in hand.jpg
Here is a pic of the model in the hand
Knife in hand.jpg (274.64 KiB) Viewed 129 times
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Thomas Milde

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Re: Placing or attaching a 3d object to a moving subject.

PostSat Mar 28, 2020 12:13 pm

Hi,

is there anything you could track? Is the person in the original clip holding anything like a fake knive or some other object? If yes, you cold track that and at least have a position for your 3D-knive.
My experience stops when it comes to planar tracker and 3D-objects, so maybe someone else could explain further.
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Bryan Ray

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Re: Placing or attaching a 3d object to a moving subject.

PostSat Mar 28, 2020 4:21 pm

Knives can be difficult because it's hard to see how it should be rotated. Usually I try to get a good 2d track on the hand (point or planar—if the hand is small in the frame, then Fusion's planar tracker probably won't do a good job. If you have Mocha, that's often better), then position the knife in 3d space so it will fit well in the reference frame.

Let the track take care of the overall motion and micro-jitter, then do some animation in 3d to handle rotations. You'll probably want to place the pivot of the 3d knife is a little behind the hilt, near the actor's middle finger. People tend to let a knife rest on their middle and use their last two fingers to control its pitch, so it usually rotates around that spot. Swords often rotate from the actor's wrist because they'll have a firmer grasp. That's a simplification of the mechanics, of course, but it's usually good enough for this purpose.

From there, it's 'rotomation.' Frame-by-frame match animating the CG knife to what the prop handle is doing.
Bryan Ray
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MaxX Kentaur

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Re: Placing or attaching a 3d object to a moving subject.

PostTue Mar 31, 2020 6:56 am

Thank you for your replies!

The only part I am confused about is after tracking the hand, how I take the information and apply it to the FBX model.
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Thomas Milde

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Re: Placing or attaching a 3d object to a moving subject.

PostTue Mar 31, 2020 2:16 pm

Unless there are newer ways now, you have to publish the result of the tracker by rightclicking on for example "path center" (the label, not the numbers) in the tracker inspector window, and rightclick on the X, Y and Z labels of the object in the object inspector window and select "connect to -> Tracker1 -> position x".
The exact menu entries might vary depending on the tool you actually used, but the secret is to publish stuff, and connect other stuff to it.

Any other possibility would be appreciated
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Bryan Ray

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Re: Placing or attaching a 3d object to a moving subject.

PostTue Mar 31, 2020 5:36 pm

I wouldn't try to push the Tracker info into the 3D world. You'd have some trigonometry to do to get the exact z position and scale right. Just apply the tracking in 2d:

Capture.JPG
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Bryan Ray
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