Stereoscopic with two 7D's

[Adam Stanislav]

If they are out of sync, they are out of sync. It does not matter how long or short a shot is. They do not get the more out of sync the longer you shoot.

It is not that the two cameras do not shoot at the same speed, they do. The reason they are out of sync is that they do not start at the same moment. Let us say, for example, that you are shooting at 25 fps. One camera starts at one moment, the other starts 1/50 s later. That means they are 20 ms, or half a frame, out of sync. They are that way right from the start and they continue to be that way whether you are shooting for a minute or for an hour. If there is any kind of movement, especially lateral movement (that is left-to-right or right-to-left movement), objects will appear at a wrong depth. You can have a body 2 meters in front of the camera but its arm can appear 1 meter or 3 meters or whatever other distance from the camera. Or the body can be walking at the side of a pool of water but appear to be walking on the water, even while its feet are clearly touching the firm ground. This causes disorientation in the viewer, as his brain is receiving conflicting information.

For proper stereoscopy both cameras have to run perfectly synchronized, so this does not happen.

[Tony Reidsma]

Yeah well.... I'm shooting and it's coming out pretty nicely. I'll post samples when I'm totally satisfied with the subject matter.

[Steve Shovlar]

Originally Posted by Adam Stanislav

If they are out of sync, they are out of sync. It does not matter how long or short a shot is. They do not get the more out of sync the longer you shoot.

It is not that the two cameras do not shoot at the same speed, they do. The reason they are out of sync is that they do not start at the same moment. Let us say, for example, that you are shooting at 25 fps. One camera starts at one moment, the other starts 1/50 s later. That means they are 20 ms, or half a frame, out of sync. They are that way right from the start and they continue to be that way whether you are shooting for a minute or for an hour. If there is any kind of movement, especially lateral movement (that is left-to-right or right-to-left movement), objects will appear at a wrong depth. You can have a body 2 meters in front of the camera but its arm can appear 1 meter or 3 meters or whatever other distance from the camera. Or the body can be walking at the side of a pool of water but appear to be walking on the water, even while its feet are clearly touching the firm ground. This causes disorientation in the viewer, as his brain is receiving conflicting information.

For proper stereoscopy both cameras have to run perfectly synchronized, so this does not happen.

That is true Adam. I think what Tony was trying to say was that two cameras will never run at exactly the same speed therefore after a period they will stray. Genlock is esential.

NOw I only own one EX3 but I do own two HC5's ( small little consumer HDV cams) They can't be genlocked obviously, but you can by the Colorcode lanc controller.
LANC and this should start the two cameras off at exactly the same time. Obviously no Genlock so after a while the frames will not be accurate so short clips are important to avoid straying.

Am I right?

[Pavel Houda]

I don't believe that using LANC, in general case, one can achieve good enough frame synch in each instance. At least I couldn't do it. I have a clip here: YouTube - LANC Sync Experiment - 3D Stereoscopic HD . Each device implements the LANC protocol at a different level. There is just no way to guarantee that the camcorder controller S/W will respond to serial protocol command and control piece of H/W with the exactly same delay. That would depend greatly on what other real time tasks they have to serve (RTC, display control, switch scans,...) and how the code is written. Furthermore, due to the battery voltages being slightly different and crystal clock and other oscillator tank component tolerances, the controlling clocks will not be exactly the same, so there will be a longer time drift. IMHO, the only way to assure acceptable synch is by H/W re-sync at each frame - genlock. I know that there are the LANC Masters, etc. on the market, but I don't see how they can GUATANTEE the camcorder response in each case, since the S/W and LANC command implementation in each camcorder can and does vary. Often there are slight differences in the same model. These devices can only command the cameras at S/W level, over the b-directional LANC I/F, not at the shutter H/W level, that actually determines what the sensor can see. I would sure be interested about how can that be achieved with the LANC in general case. It can be "good enough" for slowly moving subjects, while not moving the cameras, but in general case, there can be a lot of motion within a frame or field, which will result in different shot in L/R views, in general case.

[Giroud Francois]

that is where shooting 60p become interesting. the frame rate is fast enough to make the eventual difference negligible.

[Adam Stanislav]

Originally Posted by Steve Shovlar

I think what Tony was trying to say was that two cameras will never run at exactly the same speed therefore after a period they will stray. Genlock is esential.

I was replying to Bicky in that message, something about being safe with short shots. Maybe I did not word it right. My point was that even if the two cameras did run at exactly the same speed, they still need to be synchronized (yes, by genlock) or else you can get some very strange results. Even a few milliseconds off sync can cause serious issues.

[Tony Reidsma]

Originally Posted by Giroud Francois

that is where shooting 60p become interesting. the frame rate is fast enough to make the eventual difference negligible.

Thanks for this bit.

Thanks for all the replies....most helpful.

[Steve Shovlar]

What are the smallest and cheapest cameras out there with Genlock?

[Pavel Houda]

Originally Posted by Giroud Francois

that is where shooting 60p become interesting. the frame rate is fast enough to make the eventual difference negligible.

That would be worth trying. I know that Sanyo makes couple of consumer grade 60p camcorders. Can you tell me what other models are there? Does Canon make any? What editor and stereo compositor S/W can handle 1080/60p footage? I would buy the Panasonic 3D rig, but it costs almost as much as my wife's car. If I bought it, I would end up with Tiger Woods in some sort of 3D rehab and divorced. The lawyer would then get himself the Panasonic on me. ;-)

[Giroud Francois]

the sanyo FH1 i use for this purpose (3D) are 1080p60, but 720p60 is ok too and is found in many cameras and DSLR and is compatible with bluray (1080p60 is compatible with almost no support).
720p60 3D give 120p while interlaced. many screen and projectors already support this frame rate.
And you can still drop half of the frames and get a 3D at 60p (2x30) that should be viewable.
I think the good thing with 60p is since the pricture is progressive (no upper/lower frame), there is a big chance that you can sync to the closest frame with pretty good accuracy.
at worse if a left frame start in the middle of the right one, you only get 1/120 difference.
if it start just before or after, by aligning to the previous or next frame, you get much better than 1/120.
in my case , i have seen no problem with my FH1.

http://www.giroud2.com/download/test_left.avi
http://www.giroud2.com/download/test_right.avi

[Kaspar Kallas]

Hi

The problem is not sync drifting but inter-frame sync, meaning the sync difference less than 1/24th of a second for DCI cinema. This is enough to introduce serious problems.

-Kaspar

[Heinz Bihlmeir]

Originally Posted by Pavel Houda

I don't believe that using LANC, in general case, one can achieve good enough frame synch in each instance..

The trick with LANC controllers is to switch both cameras on at the same time. Assuming the cameras are identical and are equipped with the same firmware, the sync generators in both devices should reset and start aligned. By comparing the LANC bursts of both cameras (which are synced to field transitions) the controller can measure and display the field offset of both cameras. By delaying the power-on sequence of one camera, fine tuning is possible.The problem is that the startup mechanism is analog and depends on battery power, temperature and circuit tolerances. The sync drift after power-on is relatively small and depends only on the accuracy and variations of the crystal oscillators in both cameras.

There are no practical reasons, why cameras with genlock option are relative expensive. With modern chipsets this functionality could easily be implemented in compact prosumer cameras.

[Pavel Houda]

I will have to look at the consistency of the sync shift after the power-up, based on your input. How do you precisely delay the boot sequence of one of the cameras? How do they keep long term sync?

[Adam Stanislav]

Originally Posted by Kaspar Kallas

The problem is not sync drifting but inter-frame sync, meaning the sync difference less than 1/24th of a second for DCI cinema. This is enough to introduce serious problems.

Exactly! The two cameras must be 100% synchronized at all times. The only exception to that would be if nothing in the scene is moving, but that kind of defeats the purpose of using video cameras.

[Heinz Bihlmeir]

Originally Posted by Pavel Houda

I will have to look at the consistency of the sync shift after the power-up, based on your input. How do you precisely delay the boot sequence of one of the cameras? How do they keep long term sync?

If you can program a micro controller to control both cameras via LANC, the following sequence can be used:
(0) Power-on both cameras
(1) Send a power off LANC command to both cameras
(2) Power on both cameras by grounding both LANC Lines for 100ms
(3) After release both cameras will turn on
(4) Measure the offset of the LANC packets of both cameras to calculate the requred offset

Repeat the same sequence and apply the calculated delay

The diagram may clarify the sequence.