Home of Professional Cinematography since 1996

Shooting VFX Twins With Moco and The Alexa

Published : 14th July 2012

Dear All,

>Someone has come up with a job which is a movie with the same person playing both leads (identical twins) leads.

>I'm worried about how to make a video assist for this ... The way it should be done is the first pass should be recorded and then the playback mixed with a live signal(the second pass) for the director so the director can direct the artist.

>I'm going to put this together this week, but I'd love a second opinion from the brains trust before I stiff myself roundly ...

>In case any of you are thinking "Why is this on this list"? ... Well it has everything Digital video, HD-SDI, SD-SDI, PAL, VFX .. In fact how more General can you get ?

>The big problem with moco playback is the shutter sync problem ... If you start to add time base correction into the mix then one frame (played back or live) can be one frame delayed, and the output on the directors monitor goes "screwy" every-time the camera moves. As you can imagine this does not give people confidence in the motion control rig. (Note this is not a problem that will occur when the move is comped in a computer).

In the "Old days" if you were shooting with a regular PAL video camera you could get a Black and Burst generator and feed the output from that into the genlock input of the camera and into the video input on Kuper
(or Flair) then for playback you could feed the B+B into the camera and the moco rig and the Video machine then you could mix the output from the video machine with the output from the camera. You achieve the
sync by triggering the motion control rig from timecode from the video tape (there is a fixed number of frames delay but once you find them out it's a constant) There may be a TBC on the output from the video machine
but it will still sync with the timecode. However the camera and the moco rig are both syncing to the same signal.

With film cameras you did a very similar thing you disabled the sync between the camera and the video assist (a simple interlaced type). of course you had to disconnect the camera - video sync and at 24fps you
had 99 problems but the image's were on top of each other (you had flicker like you would not believe, unless you had a pelicule reflex system).

So now I'm thinking about doing the same thing with the Alexa ... Assuming that the "Way to go" is to down res the whole operation to 625 line PAL ...

I've gone through all the options and I can't see a way to make the thing sync .. if it was at 25fps then I could build a box to generate a B+B signal from the Alexa frame pulse ...

What does the brains trust think ? Am I missing something blindingly obvious here ?

Justin Pentecost
Motion Control
London
http://www.portablemotioncontrol.com


Since the Alexa records to SxS cards why not just drop the Prores footage into After Effects or even FCPx for playback out via DVI-SDI?

A Qtake System would be a great piece of equipment to allow u to do both?

Best,

Dane Brehm
ICGDIT : HiSpeed
Currently in Portland, OR


Dane Brehm wrote:

>> Since the Alexa records to SxS cards why not just drop the Prores footage into After Effects or even FCPx >> for playback out via DVI-SDI?

But this has to be in real time .. the director is directing the talent from the output by the time it's been put into AE the shot is over ...

Hence the Analog video ...

Justin Pentecost
Motion Control
London
http://www.portablemotioncontrol.com
+44 7973 317241


Justin Pentecost wrote:

>> But this has to be in real time .. the director is directing the talent from the output by the time it's been >> put into AE the shot is over ...

How about a frame syncing switcher like one from Panasonic or Blackmagic? I believe both offer adjustments to the frame sync to allow for accurate timing of any source to each other as well as the option to perform either up or downstream effects.

Aaron Owen
San Francisco based shoot/edit/Future Sailor in the US Navy


Justin Pentecost wrote:

>> Someone has come up with a job which is a movie with the same person playing both leads (identical

>> twins) leads.

You should do some research on the production of "Dead Ringers," directed by David Cronenberg with a terrific dual performance by Jeremy Irons. I believe Cinefex and/or American Cinematographer had stories on it at the time. Live encoding motion control was done for the first pass, with the dolly grip and camera operator essentially shooting as they normally would. Then the system went into motion control playback for the second pass. Pretty advanced for 1988.

Mitch Gross
Applications Specialist
Abel Cine Tech


Justin Pentecost wrote:

>> I'm worried about how to make a video assist for this ...

The latest firmware for Alexa (SUP 4.0, now in beta release) allows for input to the camera for 4444 video, meaning a genlock sync image input for compositing reference. Perhaps this could be useful in locking your images.

http://blog.abelcine.com/2011/06/23/arri-alexa-firmware-4-0-beta-release/

Mitch Gross
Applications Specialist
Abel Cine Tech


Aaron Owen wrote:

>> How about a frame syncing switcher like one from Panasonic or Blackmagic? I believe both offer

>> adjustments to the frame sync to allow for accurate timing of any source to each other as well as the

>> option to perform either up or downstream effects.

I think Aaron is on the right track as this is for live reference only... Using a small switcher with frame syncs on each input should get this done without requiring any synchronization of the cameras themselves ... which sounds like one headache too many in this kind of situation! This is assuming that you can afford to be off by a frame or so, because frame syncs will do that... so it's a matter of how precise your preview monitoring needs to be. If the director is using it as a reference to direct talent you should be fine, but if you need frame-accurate synchronization, the frame syncs may introduce some error.

Something like this : http://tinyurl.com/3nyzjqn may do the trick

I've seen some small switchers like this that have on-board recording capability, aimed at live events production so you can record production elements ahead of time - those might be useful, although I suspect you're probably better off recording the first pass independently.

----------
George Hupka
Director/DP
Saskatoon, Canada
Listmum, Cinematography Mailing List


All blindingly easy twenty years ago with a PVR system and standard def video... doing it now you will end up with a one frame delay no matter what you do.... but at the speed most actors work, one frame won't kill you - let me snoop around and get back on this in a day or two


Mark Weingartner
DP, VFX Supervisor, and Stereographer
Los Angeles based but rarely home these days.
Represented by Schneider Entertainment Agency
http://www.schneiderentertainment.com/view_talent.asp?dept=visual%20effects%20supervisors

Someday I'll update my reel


>> The latest firmware for Alexa (SUP 4.0, now in beta release) allows for input to the camera for 4444 video, >> meaning a genlock sync image input for compositing reference.

Did they fix the issue where the time code displayed on the side of the camera doesn't match what is being recorded on the SxS cards, nor what is being embedded on the HD-SDI output?

Because while ARRI seem to think this is a "cosmetic" problem, it really REALLY isn't any such thing.

Bob Kertesz
BlueScreen LLC
Hollywood, California


George Hupka wrote:

>> This is assuming that you can afford to be off by a frame or so, because frame syncs will do that... so it's a >> matter of how precise your preview monitoring needs to be.

To slip by a single frame is not a possibility, I'm accepting that there will be a delay .. According to Arri's documentation there is a "Less than or equal to 1 frame delay" on the monitor output .. which suggests
that this delay is a bit elastic ..

I think you and Aaron have the correct motorcycle. Downconverting to SD is not an option so I have to make it work with an SDI mixer somehow ... With any luck if I can get the images close enough then the mixer will not delay one image more than the other ...

Testing this week .. I'll let you all know ..

Justin Pentecost
Motion Control
London