9 Replies Latest reply on Apr 28, 2014 10:12 AM by Szalam

# How to apply camera transformation to another clip?

Hi,

I have an actor on the left half of the screen filmed with a dolly shot.

The same actor on the right half of the screen filmed with a dolly shot that the camera operator tried to do at the same speed and positions as the first dolly shot.

I wish to composite both halves together using one of the dolly camera shot movements so it looks like there are twins in one shot.

I've done a 3D Camera tracker on both clips so I have two cameras. In my mind I need to do an inverse camera transformation of one of the cameras, say on the right camera. Then attach the left camera as parent to the right clip. This in theory would make the right clip move in sync with the left camera dolly?

How would I do that, how do I get an inverse transform of a camera so that I can apply the transform of another camera? Or is there a better way for me to achieve this.

Hope that makes sense?

Thanks for any help,

Jules

• ###### 1. Re: How to apply camera transformation to another clip?

3D camera tracker is probably the wrong tool. Sounds like a simple key and timing issue. Two layers, one above the other, both with the background removed adjusted for timing. If there is any tracking required it would be on something that does not move on the set and you would just track one shot and apply the tracker to a null to be parented to the other shot or applied directly to the other shot. You are probably making this much too hard.

• ###### 2. Re: How to apply camera transformation to another clip?

I think I tried that but it won't work because both shots are dollying to the left. So choosing a target that doesn't move does not compensate for that fact that both cameras are effectively moving.

That's why I thought of it mathematically, if I could reverse transform the camera of one shot and then apply the camera transform of the other than that shot should adjust itself per frame to move in time with the other shot.

Or am I missing something?

Jules

• ###### 3. Re: How to apply camera transformation to another clip?

I'd have to see the shots. If you have one shot and the key element is moving, track the key element then apply the tracking to a null. The null will now move with the key element. Take the second element and add it to the scene, adjust it's position, then parent the second element to the null. If the camera in both scenes is moving at the same speed and in th same way you're done if the timing of the moves is exactly the same.

If the timing is not exactly the same you have other problems that have to be solved. One option would be to stabilize (remove all movement from both shots) then adjust the position and timing of the composite and re-introduce the movement from one of the shots. Again, I'd have to see the shots to tell which technique would be best.

Here's a short tutorial, very short tutorial on how to stabilze a shot, make a composite, then re-introduce the motion to the composite. I hope it helps. If not, show us the shots.

• ###### 4. Re: How to apply camera transformation to another clip?

Thanks Rick for the detailed response.

I fall into the latter where the camera timing is not the same as they are clips from different takes.

Hopefully this Photoshopped composited image will help. This is from a test on a smaller scale. The real shoot I'd want the actor walking a fair distance rather than just a couple of steps.

So in take 1 the actor walks away and the camera is dollying from lower right to upper left.

Take 2 the same actor is walking towards and the camera is again dollying from lower right to upper left (at the same camera starting/ending and speed as is humanly possible to the previous take).

The arrow in the picture just shows the direction of travel of the camera, the camera is further away of course and pointing towards the actor, locked off at the same angle for both takes.

Ideally in this scenario there would be a computerised motion control rig that can repeat the dolly shot movement exactly across different takes, but I don't have access to that sort of equipment.

In the scene the actor meets himself in the middle and they have an interaction.It's not just dolly, I'd like to do other moving shots such as handheld shots etc.

I think it maybe too much for the stablization method to handle given that the dolly shots would travel a long distance, two or three screen lengths possibly. Plus the camera is moving towards and away from the subjects as well, so there is scaling involved.

That's why I came up with the idea of using the 3D tracker in After Effects to grab the 3D camera transformations/coordinates and then try and remove the camera movements by applying an inverse camera matrix transform to say take 2, and then parenting this take to the camera transformation from the 3D camera tracking from take 1. In After Effects, applying the inverse of a camera transform to a clip doe not seem possible?

So am I wondering is there a way in After Effects that would make two different shots with similar camera movements to look like they have been shot in one take?

Thanks for help,

Jules

• ###### 5. Re: How to apply camera transformation to another clip?

Hopefully this Photoshopped composited image will help. This is from a test on a smaller scale. The real shoot I'd want the actor walking a fair distance rather than just a couple of steps.

So in take 1 the actor walks away and the camera is dollying from lower right to upper left.

Take 2 the same actor is walking towards and the camera is again dollying from lower right to upper left (at the same camera starting/ending and speed as is humanly possible to the previous take).

First of all, congratulations on running tests before you do it for real!   That's excellent thinking!   Too many people take it as an article of faith that they have a fail-safe plan, and get burned in the end.

From your description, you will need a FAR BIGGER green screen background -- you're going to run out of key wall in a big hurry!

Lighting will also be crucial to avoid excessive spill on the subject(s).

And you're right: motion control is the no-fail way to pull this off, but you STILL might be able to do it if you mechanize the camera movement -- i.e. eliminate a human operator physically moving the camera -- and provide good, repeatable mechanical timing cues to the talent.

That's a complex shot.

• ###### 6. Re: How to apply camera transformation to another clip?

Hopefully this Photoshopped composited image will help. This is from a test on a smaller scale. The real shoot I'd want the actor walking a fair distance rather than just a couple of steps.

So in take 1 the actor walks away and the camera is dollying from lower right to upper left.

Take 2 the same actor is walking towards and the camera is again dollying from lower right to upper left (at the same camera starting/ending and speed as is humanly possible to the previous take).

First of all, congratulations on running tests before you do it for real!   That's excellent thinking!   Too many people take it as an article of faith that they have a fail-safe plan, and get burned in the end.

From your description, you will need a FAR BIGGER green screen background -- you're going to run out of key wall in a big hurry!

Lighting will also be crucial to avoid excessive spill on the subject(s).

And you're right: motion control is the no-fail way to pull this off, but you STILL might be able to do it if you mechanize the camera movement -- i.e. eliminate a human operator physically moving the camera -- and provide good, repeatable mechanical timing cues to the talent.

That's a complex shot.

• ###### 7. Re: How to apply camera transformation to another clip?

Thanks, I will certainly have to get a bigger green screen for the actual shoot.

I think a way to do what I need is to manipulate the camera transformations in Adobe ExtendScript? But it will necessitate me to learn the language and object structure just to pull this off.

But unless I'm completely off track, no pun intended, it's doable in the scripting language.

Still, I would love an easier way.

Jules

• ###### 8. Re: How to apply camera transformation to another clip?

I really don't think that 3-D camera tracking is your solution here. If it were me I would use a stand-in and audio playback on the set. You do the shot with two people in the scene them you remove one of them and replace him with the main actor in post. Audio playback on set will give the main actor cue points for his dialogue.

In post you lineup the shots as best as you can, use time remapping to adjust the conversation and roto or key out any overlaps. I did a similar thing with a steady cam shot years ago, way before 3-D camera tracking. The camera followed the main actor as he met his double then the steady cam orbited around the pair as they were discussing safety issues in a factory. There was no green-screen, just actors that could hit their marks, roto, background replacement, motion tracking, and time remapping. Careful blocking and a couple of rehearsals and about four takes was all it took to get the shot ready for post. In the end the character meets himself in the middle of the factory floor and the camera moves around the hair as they discuss safety issues all on location. The only tools used and post were hand masking for roto, Photoshop to fill in the holes for the background, and mocha for motion tracking.

• ###### 9. Re: How to apply camera transformation to another clip?

Something that comes to mind is using a body double and just replacing the head/face where needed.