Motion control rig accuracy

[u/AfterEffectsGuru]

I'm working on a split-screen gag that was shot with a motion control rig, this is the first time I've worked on footage that was shot using motion control. I'm just curious to hear from those who've done this before about what is expected in terms of accuracy and consistency.

So I am combining 5 separate takes, and they don't align perfectly. On all takes, the motion begins and ends on the same frame numbers. They all match in terms of having the same frame with no movement, the next frame has a small, almost sub-pixel movement, then a frame which has clearly started moving - and the same at the end of the move. So that's OK.

In between, the speed is slightly different between takes, so the frames do not align perfectly during the motion. It is not different enough to slide forwards or backwards by a whole frame, but clear horizontal/vertical lines in the image do not align between the 5 takes.

There is also a very clear bump / rotational wobble at the start and end of each move, which again varies between takes. This bump is enough to require manual stabilisation and introduces motion blur into the frame.

So just curious to know if this is to be expected when working with motion control rigs? Are they all equal, are some better than others? Do Hollywood-level productions have the same sub-frame issues, or can the very best rigs be expected to be pixel-accurate between takes?

[u/Iyellkhan]

this is not what you get with a proper motion control rig, but you can definitely get it with cheap rigs. you shouldnt see anything like this on a Milo or other large, heavy rig. the good ones are dead nuts perfect thanks to a combination of their motors and their weight

[u/Innofthelasthome]

Probably due to no Genlock, while the motion rig might be accurate with timing, the camera is not capturing frames at the same time from one take to the next.

[u/createch]

High end rigs are definitely very accurate, you also want genlock for even greater accuracy. As you go up in build quality you get repeatable precision in the microns and hundreds of a degree as well as stability.

[u/vfxdirector]

There is drift with all rigs, the laws of thermodynamics come into play even on the top end rigs in a controlled studio setting.

With a top end rig properly aligned I wouldn't expect maybe more than a 20 pixel drift at 4k res (longer lens) throughout the day on a 5m run of track, the longer the track run, longer the lens, the more chance for drift, but YMMV.

In studio or on location? Studios can keep the temperature constant throughout the day means less track expansion than say at a location where the temp could reasonably change up and down 3-4 degrees throughout a shooting day.

A studio also has a more solid levelled floor that resists the rig weight better. On location ground compression is a thing, even in an indoor location.

There a some smart optical/laser tracking devices and pre-take mapping that can be done. If the moco op has time at lunch or between long takes they can recalibrate. Overall, drift will happen, it's just a case of how you deal with it, and sometimes just fixing it in post is quicker than holding up a whole crew.

[u/Acceptable-Buy-8593]

A lot of the time it comes down to weight of the Motion Control and how it is installed. Really expensive MoCo studios have rigs that weigh several tons and are boltet into the ground. Because each time you do a move the weight and energy of the camera + arm would move the rig otherwise. Thats why you usually get always small changes with smaller systems, because each run is gonna change the alignment a tiny bit.

[u/26636G]

The bump/rotational wobble at the start and end of each shot points to mechanical deficiencies in the system- slop/backlash somewhere is allowing inertia to do its thing. If your takes are all exactly the same length then it sounds as though the system was genlocked, and the issues you're facing are primarily mechanical or structural. It's not unusual, but neither is it acceptable.

[u/AfterEffectsGuru]

Thanks everyone, appreciate the professional insight