Tracing the path of a moving object filmed with a moving camera

[Markalark]

Hello there!

I'm trying to trace a path that follows a moving object. I've managed to get a nice effect using a particle system linked to a null that has tracked the motion. Nothing was hand animated, it's 100% position tracking. You can see the result here:

https://www.youtube.com/watch?v=915G6XPNnmo

How would I go about doing the same thing with a moving camera (let's say, a simple arc around the person)?
I know I can recreate the camera movement with 3d camera tracking, but would the particle system leave it's trail in 3d space (with depth variation?) I get the feeling that it would track the motion, but in a 2d plane... Is it possible? Should I track the camera first? The moving object first? I'm kinda stumped...

Thanks!

[Mylenium]

It would work the same, but of course the tracked position would be a result of both motions combined and not necessarily physically plausible. You may need some expressions to subtract the camera motion. Really depends on the specifics of the shot and what you can get away with.

Mylenium 

[Rick Gerard]

The most efficient workflow depends entirely on the shot. Camera Tracking requires a fair amount of fixed geometry (objects that don't move) to be in the shot. If you were to walk around the same actor on the same set that you have in the example, there would be very little to track because there is nothing on the wall, and the actor is moving around a bit. 

You would still need to motion-track the actor's hand. Mocha AE would be my tool of choice for that. You would also need to do some rotoscoping work to separate the actor from the background so that the particles could go behind the actor as the camera moves around him. That would require you to at least estimate the camera movement. If there is enough detail on the set you use, and the camera move is carefully executed, you should be able to pull it off. 

The composite would have you place 3D layer as a guide layer in the same plane as the actor to verify that you have the camera move properly tracked. Once that is done, the coordinates for that layer would be used as the center of the particle system's 3D world on a 2D layer. A rotoscoped copy of the original footage would be used as a track matte to hide some of the particles as the camera moves around the actor. You'll probably need more than one copy of the particle layer, one for the particles between the actor and the camera, and one for the particles behind the actor. 

If you have a sample shot that shows the camera moves I can be more specific on the workflow. 

[Markalark]

Ok, here's a new file that shows a bit of what I'm trying to do...

https://youtu.be/nE5SKFk_H3I

WHAT I CAN DO

-I can track an object (in this case an orange pencil sharpener attached to a moving fan - Nobody was home to help me!)
-I can wiplink a particle system's emitter position to that tracking null's position so that it traces that object's position and leave a trail in 2d space

-I can track the camera's motion (see the Orange solid with a T on the table in front of the fan)

WHAT I CANNOT DO

-Have the particle trail remain at the position of the tracked object in 3D space. I don't know how to go about in doing so.

Thanks for the help!

[Rick Gerard]

You are going to need to trim the shot to just the frames you are going to need in the final edit, pre-compose, trimming the Pre-comp to the length of the layer and moving all attributes, Camera track the shot, establish an origin and ground plane on the table, add a Solid and Camera using the same target you used for the Origin and ground plane, then add another 3D solid, Shift + Parent the new solid to the Ground Plane to match the position and orientation, then stand up the new layer by rotating 90º in X (name it Fan) and position it so that it tracks the fan. Then you need to carefully adjust the position using just the X, Y and Z coordinates until you get the new Fan layer positioned precisely aet the center of rotation for the fan. When you get that done, animate the Y rotation of the Fan layer until it matches the rotation of the fan. Now add another new 3D layer that is only about 200 X 200 and shift + parent it to the Fan solid to snap it to the same position then use only the X, Y and Z values to move the new (tracker, name it tracker) solid so that it matches the position of the object you are trying to track. 

This is going to take some time. If you use an actor instead of a fan, it is going to take a lot longer to get he "tracker" solid to follow the movement of the actor in 3D space. Motion Tracking with Mocha AE won't cut it, you'll have to set the position manually.

When the tracker solid looks like it is perfectly tracking the movement you want to track, add a Point Light to the scene and Shift + parent the Point Light to the Tracker solid. Name the light Emitter and then build your particle system using Particular on a new solid and use the Emitter Point light as the emitter. 

When you have all of that done, you may have to start creating track mattes or obscuration layers to hide some of the particles. 

If you have Mocha Pro, there is a 3D tracker that you can use to do a lot of the work for you and it will make your life easier. 

Once again. Carefully planning the shot and executing the camera movement will make this kind of project a lot easier. 

[Markalark]

Thanks for that info. But basically, you're telling me that I cannot track an object in 3d space without keyframing it manually?

[Rick Gerard]

Mocha Pro can do the kind of tracking you want to do, but it is incredibly complex to mix camera tracking and motion tracking. 

Your sample shot would be a lot easier to use for a demo if you had not added the particles. 

Another approach would be to motion-stabilize the fan so it does not move, add your particles to the motion-stabilized fan, then put the motion back in the shot. 

When you shoot something like this, understanding the mechanics of tracking and blocking your shot, so you don't back yourself into corners is the most critical part of the workflow. You can end up with a shot that takes several days to complete or one that takes a couple of hours. 

If you want to share a sample shot without added particles, I may have time to throw together a simplified tutorial. 

[Markalark]

Yeah, it would be easier, but the entire point of this shot is to trace in 3d space a moving object. If you have another way than through a particle system, I'm all ears.

Cheers!