Tracking and depth of field

M

mörkö

Member
Hi all,

We are planning a shoot with a lot of public locations such as metro stations etc.,I would like to replace or cover a lot of things already there, like advertisements, signs, and possibly add some colored elements to walls etc. As a production/set designer, I would usually I would do this with practical modifications, but for various reasons I'm exploring the possibility to do this digitally in post.

We are shooting with Arri Alexa on steadicam, and the DP is going to go with a narrow depth of field. That's what I'm most worried about. Does the background being out of focus make it impossible to track? Are there any tricks to make out of focus objects more easy to track? Any other considerations?

Also, as we use steadicam instead of fixed camera, I assume 2D tracking is not enough.. so 3D tracking is the only way to go?

All replacements would be rectangular, in the backround, out of focus and sitting in a wall, floor or other simple straight surface.

It's somewhat low-budget, and can't afford to hire a pro SFX artist, and I would be the person to do it whether it is done practically or digitally. I've only played around with tracking and 3D, done some small tests, but very willing to learn - I don't want to take risks with this project, and if you guys/gals think there could be huge problems I wouldn't want to put myself in a situation I can't handle.
 
Answer is.. it depends.

For DOF
If it is so shallow, and sign you want to change is so far away, then yes, a blur is hard to track and replace smoothly.

For tacking 2d vs 3d, it depends on your camera moves, you can get a lot done with mocha in AE
 
The good news is that the viewer won't be concentrating so much on out-of-focus objects, so tracking isn't as critical as it is with in-focus objects.

But if the background objects will always be out-of-focus to the extent of being unrecognisable, I'm not clear why you need to replace them in the first place?
 
If properly tracking and planting objects there is hard, could it work if I made a not-so-perfect, or even animate a moving mask by hand, and just apply some color filter magic to change the colors of the existing, blurred signage. I guess it could work at least when the wall/background of the print is dark, or different enough?

Main reason I want to replace unrecognizable things is that most of the colors/signage already there don't match the overall look of the film - I'd like to keep the palette limited and use just a few strong colors. In these places, we can not do much with light, and I'm afraid these scenes will look too different compared to the rest of the film. Some locations are too plain and I would like to add bigger colored elements of my choosing. We might shoot some prints in focus (a fictional ad campaign), and it would be nice if in the rest of the shots, we could see similarly colored signage blurred in the background.

I'm going to make some test shoots, and would like to know options/considerations so I can get proper test footage and then play around with AE or something to see how it goes.
 
Yeah, applying colour filtering to a moving mask may work, but could be a whole lot of work. As with any sort of manual tracking, it very much depends on how smooth the camera movement is - steadicam is at least better than plain old hand-held :)
 
Motion tracking is calculated at the sub-pixel level - trying to manually track things (pixel level) can be nightmarishly hard. Out of focus could be your friend in that case, unless foreground (sharp) objects cut across those planes.

You really want to test the hell outta this; with the Alexa, I'd definitely look into shooting the largest possible frame, backing off, shoot locked down or with minimal, smooth & linear movements, and then add any handheld look in post. (I've slowly amassed a lot of "black x on white card" handheld footage; track it and use the data on static or smooth shots. )
 
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