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Looking for insight on a motion track shot

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Jon Orchin
Looking for insight on a motion track shot
on Aug 13, 2017 at 2:01:34 am

Hi guys,

I have just a little experience with motion tracking. I have done some planar tracks with mocha ae (mostly screen replacements) and some 3D camera tracking to track text and signs in shots.

I am not sure how to proceed with this shot. Its a long shot of a woman getting on a lift and then it moves down to the ground. The camera pans with her the whole time. I need to track a sign onto the front of the lift (see attached screenshot for a rough mockup). See the shot here:





I did a brief test with mocha but didn't get a good track at all. The lift itself is very jerky. Before I really dive into trying to get a better track in mocha I wanted to ask what you all thought would be the best way to try and execute this shot. To me it seems very difficult but I don't have enough experience with this type of shot to be sure.

Any insight or direction would be greatly appreciated.

Thanks!



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Steve Bentley
Re: Looking for insight on a motion track shot
on Aug 14, 2017 at 4:56:41 am

Well you have a couple of issues there. First you don't have a lot to work with as far as perspective goes. You can only see the top rail of the bucket really. Its a shame that instruction card on the front with the yellow and blue arrows isn't bigger because that would be perfect (and might still be). But the wider you can get the tracking points apart the more accurate the angles usually are.
But that being said, often perspective doesn't matter and its often better to get a good track off one point and then do the perspective by hand (if even needed) after the fact with a corner pin or skew.

Second, as you pointed out, the massive jerk when the lift starts. There is going to be so much motion blur during that section that it will be very different from the rest of the track and hard for the track area to lock on. You might have to do those few frames by hand and then resume the track.
Make sure the motion blur is turned on for the layer that is following the track or it won't look right at all.
One other thing that you need to be aware of with video cameras is that if the motion is fast enough you can get a distortion since the sensor isn't fast enough for the motion (telephone poles going by can have a curve to them). For these situations we'll often take the last good frame and the next good frame, line them up and move them by hand through the same kind of motion while dissolving. That way the move is clean, but so blurred you would never know that it was just a set of stills. Just take the blur off and track the shot again after this fix is added. Then use the real footage or substitute the blurred doctored clip.


There is a massive lighting change as she drops below the floor level. you should stop the track there a few times and restart so the system can accomodate the new look of the points you have chosen to track. Thatr yellow color is really going to affect the green dots.

I see you have some green dots to track but if you are adding other spots, keep in mind that reflective things don't make for good tracking points (the lift tubing for instance) because the reflections keep changing so the color of the area you have chosen to track will keep changing and these reflections can change faster depending on whether the reflections are facing camera or are around the curve on a tube.

One of the tricks I use for trouble tracking shots is to isolate an edge with a slit mask. So a vertical black mask precomped onto the footage that only lets through a few pixels , and then track in either the X or Y only. You have to do this a few times in both X and Y but it will let you isolate something that has good contrast but coudn't be tracked because it is just one long line. There is the straight part of the plate in front of the talent. Good contrast between the white of her and the gray of the plate. But it's just a straight edge. Put a thin vertical mask on it and the system will have no problem tracking it's vertical movement. Then pick something else in the area and put it through a slit mask to get the horizontal movement (like the vertical of the same plate where it crosses her dark midsection).



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Jon Orchin
Re: Looking for insight on a motion track shot
on Aug 14, 2017 at 1:43:46 pm

Wow, thanks for that lengthy reply!

I really appreciate that info, helps me understand how to go about processing a shot like this. This seems like too much for me to handle for the time being, so I will most likely have to pass the shot on to someone else.

A couple follow up questions if you don't mind:

1) What kind of tracker are you speaking of using? 3D or the regular tracker in AE (maybe both? I ask because you only spoke about the x and y when tracking with the slit mask)

2) Briefly how would you handle the lift itself starting off screen and moving on?

3) I was not involved at all in the shooting of this scene and did not advise them to use those green marks. Are those even useful? Could this scene have been marked up differently to make it much easier?

Thanks again, I am just trying to wrap my head around all the possible ways to track different footage.


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Steve Bentley
Re: Looking for insight on a motion track shot
on Aug 14, 2017 at 2:48:31 pm

The AE tracker could do this with the tricks I mentioned. Mocha would probably do a better job and does a better job tracking backwards and extrapolating off screen.
Again you can cheat here - if you make a sub comp with the footage and a still of the first frame of the footage (where you have a good image of the lift and the point you want to track), overlay that still over the footage and hand animate it past the start of the shot. this way you can see if the motion looks real enough. So you are moving a still beyond the boundaries of the frame and the the zero time marker. Once that looks real enough you can track that new sub comp past the start of the original footage. Then turn off the still.



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Jon Orchin
Re: Looking for insight on a motion track shot
on Aug 14, 2017 at 10:09:41 pm

Thanks! Looks like I have a lot to learn on the VFX side of things (I started in AE just doing basic motion graphics).

I have one more shot that I thought I could handle but it ended up giving me a hard time.







I tried to track the surface in Mocha and I tracked her and put that layer on top but it still had an issues sticking to the plane of the screen. What is the best way to get a clean track with the camera moving left to right and the woman stepping in front of the screen?

Thanks again for all the help.


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Steve Bentley
Re: Looking for insight on a motion track shot
on Aug 14, 2017 at 11:24:01 pm

Depends on what goes in that screen ( I assume it's the screen right?). You could track just one point and hand off to another point as that point gets covered by her. Remember to track something that is at the depth the screen is at not the talent or it will never track. You can always pass off the tracking chore to something that is still on screen after your main point goes offscreen - you can move the detecting box in AE any time you like (with some modification keys) and the track will continue as though you were tracking the original point but now locked to the new detail.

I know the first idea probably is to do a 4 point corner track but you might not need it. If on a dolly you don't need rotation (if handheld use two points to get the rotation) but I doubt you need a perspective track here. Get the motion to match and then use the green to key out the image. If nothing graphicy touches the edge of the screen you probably won't see incorrect perpspective that creeps in over time, but you might have to initially corner pin the comp that will shine through the matte, you just won't do that for every frame with 4 tracking points - its very very hard to track 4 points and keep them all going together when the shot is out of focus. You could also hand keyframe your corner pin if it's really needed, Just keep in mind the start and stop ease of the camera move.



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Jon Orchin
Re: Looking for insight on a motion track shot
on Aug 16, 2017 at 1:17:07 am

Awesome sounds like I can handle that!

Last question: How do I move the detection box in the AE tracker? I paused my track and tried to move it but then AE moved the whole track. I did some googling about that technique but couldn't find anything. What am I missing?

Thanks!


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Steve Bentley
Re: Looking for insight on a motion track shot
on Aug 16, 2017 at 2:50:13 am

If you alt drag inside the detection box that will shift the focus to something new to track in the frame. While the box will move and the path's one point will move, you will notice the attach point does not move, and that's the point that is sending data to the receiving layer.
Once you hit play again to keep tracking or retract a section the path will shift to this new object but not the attach point. (thats the little white cross)

So if you had two planes flying along at roughly the same speed and you were tracking one of them. you could stop the track, alt move the detect box to the other plane and continue the track. The path would jump to the second plane for that part of the track but when you send the data to the receiving layer it will continue on along the original track vector assuming the planes are moving the same way. What happens is an offset is included in tracking data when the tracker box is moved with the alt key, so the sudden shift of the tracker box isn't noticed.



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Jon Orchin
Re: Looking for insight on a motion track shot
on Aug 16, 2017 at 12:56:27 pm

Boom. Thank you so much. I've got it now!


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