How do I anchor particles to a different reference frame than the emitter?

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Level 1

10 points

How do I anchor particles to a different reference frame than the emitter?

I've been playing with motion for a few hours and I can't figure out the combination of behaviors to get this working: I have an object in my shot (a ball), that is rolling along. I can create a smoke emitter that tracks the ball easily enough - no problem. However, the framing of the ball in the shot changes, so the particles (smoke) that have been emitted, should track the movement of the frame (instead of the emitter, which tracks the ball). I can easily create an object that tracks the frame, but I can't figure how to get the emitter to track the ball while the emitted particles track the frame.

Any suggestions would be a big help (an actual example, even better!).

Motion 5, OS X Mavericks (10.9.1)

Posted on Jan 27, 2014 9:36 AM

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Answer 1

fox_m

Level 6

12,220 points

Jan 27, 2014 12:54 PM in response to James D M

I'm having a little trouble figuring out what you mean by "frame". A frame of what? There are individual frames in video which are essentially a still image. There's camera framing - or the viewpoint at which you look at a scene. There are frames that "bound" objects... I'm not clear on what you want the smoke particles to track.

I will tell you this: once a particle has left an emitter, it's just like a bullet leaving a gun. It has it's own trajectory and its own destination -- there is no way to apply trackers to individual particles. You can "solidify" the "system" (capture all currently visible particles into a kind of "image sequence") by cloning the emitter.

For the most part, there is no necessity to use a tracker for an emitter. You place an object (like your ball shape) and the emitter in the same group (align the emitter to the point on the ball you want particles emitted from) and animate the group containing both.

Below shows a "ball" and smoke emitter inside a group (you can see the group's bounds - grey box) along with the animated (keyframed) motion path (red curve -- for the same group). You can also see how the emitted smoke particles are following the path of motion only because that's where they are being "deposited" at this specific instance in time. It's a very natural looking effect.

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Answer 2

James D M Author

Level 1

10 points

Jan 27, 2014 1:39 PM in response to fox_m

By "frame", I mean in the conventional photography/cinematographic sense (field of view of a camera) or the traditional physics sense (as in a reference frame).

In your example above, there is an emitter following an object moving along a path. It makes it look like smoke is coming out of the object, and there's a plume. That's quite simple.

Assume that the ball is stationary and you place place a particle emitter on it to general smoke. If you move the ball left, the emitter also move left, and the particles at the emitter's original location move straightup and new particles are emitted in the up direction along the path. That makes sense if the ball was moving, but if the ball appears to move left because the camera is panning right, the emitted particles shouldn't be spread between the emitter's start and stop locations, the particles should move with the emitter.

The really tricky part is that the camera is panning (in the sense of the shot for the scene) AND the object in the shot that the emitter is tracking, are BOTH moving. So, the velocity of the particles should be their velocity as they come off the emitter PLUS the velocity of the movement of the scene.

If you had a shaky shot you could easily stabilize the scene to make sure it doesn't move, then setup your emitter path in that stationary scene. However, if you want to keep the movement of the scene from the original shot, you need the emitter to move relative to the object and correct the particle motion for the movement of the scene. If you don't, the particles follow a regular trajectory that's out of sync with the movement of the scene.

Mathematically, it's very simple, the particles have a velocity relative to the emitter. Within the scene, their realized velocities should be the velocity relative to the emitter plus the velocity of the change in the scene. This is what Motion Tracking gives you for objects and emitters, but I can't figure out how to make particles move to compensate for the motion of the scene. They don't "track" anything.

The obvious way would be to create a Motion Tracker that moves with the scene. Then create an emitter that follows and object in the scene, and then make the emitted particles trajectories match the movement of the scene. I can't figure out how to do that.

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Answer 3

fox_m

Level 6

12,220 points

Jan 27, 2014 10:52 PM in response to James D M

Assume that the ball is stationary and you place place a particle emitter on it to general smoke. If you move the ball left, the emitter also move left, and the particles at the emitter's original location move straightup and new particles are emitted in the up direction along the path. That makes sense if the ball was moving, but if the ball appears to move left because the camera is panning right, the emitted particles shouldn't be spread between the emitter's start and stop locations, the particles should move with the emitter.

You can have particles follow the position of the emitter by sliding the Attach to Emitter (near the bottom of the Cell Controls pane) parameter to 100%.

You can control the directions particles are emitted with the Emission parameters. With the 3D checkbox checked, Emission Latitude is the elevation; Emission Longitude is the angle around the Y-axis; and Emission Range is the "cone" angle of the emitted particles.

Particles emitted with Attach to Emitter set to 100% will keep their original relationship with the origin of the emitter wherever it goes and follow the emission path set with the Emission parameters.

HTH

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