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For an upcoming shoot I need to create a classroom set, but I don't have access to the school tables I need to create the desired effect; instead, I was thinking about filming with simply IKEA tables painted in blue/with tracking markers and replacing with a 3D model. I've never tried anything like this before, nor do I have PP/AE (I mostly use Hitfilm basic and Blender, + Fusion 360 for 3D modelling (my only paid software)), but I need the actors to interact with the table, placing objects on it, etc. so I can't just superimpose the model over a blank plate.

I'm mostly trying to find the most fail-safe way over necessarily the easiest, although I don't exactly want to spend much money, hence why the chroma key solution looks appealing. The end goal is to map the model onto the real table (which is of the same shape), not to use chroma key necessarily.

I was wondering if anyone has any experience with this sort of thing, and if so, I'd like to hear any potential pitfalls or advice about full-scale object replacement of this form (maybe chroma key is not the simplest method?). Advice on lighting the complex shape sufficiently, or placement of tracking markers, is also appreciated.

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    Do not paint the tables, wrap them in green fabric. It is cheaper and you will not mess the tables. – Rafael Nov 12 at 14:21
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+100

What you are thinking about is a very great way to save production costs and is used in many cases on many sets. Here is what your pipeline would look like:

  • Shoot the footage with green tables
  • Track the movement of the camera (using pfTrack preferably, but after effects can do the trick too)
  • Importing the tracked movement into any 3D-software such as blender
  • modeling the table so it fits the dimensions
  • moving the 3D-camera using the tracked data, so the 3d-camera matches the real camera.
  • rendering the sequence (preferably as an image sequence, such as .exr or .tiff)
  • importing the 3D-asset into the compositing-program (such as After Effects)
  • Keying out the green table and "replacing" it with the cgi table.

These steps are requiered only if you have a moving camera. If you don't, you can scrap points 2, 3 and 5. So if you want to save effort in the post-production, you can try to lock down your camera and put some artificial movement on your footage once it's composited.

Now, the tricky part is getting the shadows and the perspective right. Make sure to pack something like a grey-ball, or something matte and something shiny, so you can hold them in the frame to have a reference for later. You might need to fake the contact-shadows of the arms and hands when they are placed on the table. All of this is manageable from inside After-Effects though, no need to fire up heavy artillery like Nuke.

  • Thanks for your answer! It seems very detailed. Just one question: could you please expand a little on how the shadows could be mapped? I understand the part about getting the correct colour/angle using reference objects, but I don't really know how to know where the arm is in 3D space to tell AE/Blender how to render the shadows. – Geza Kerecsenyi Nov 12 at 17:16
  • I wouldn't take the approach of trying to render the shadows in 3D. In most cases, it is convincing enough to draw a translucent shape inside a compositing program such as After Effects which mixes with the original footage. If you want to be absolutely correct, you would most likely have to model the arms and hands in blender from scratch and put up a shadow-catcher which you will render in a separate pass, so that you have an image sequence of only your shadows. I would recommend the shapes inside After Effects though. – Florian Claaßen Nov 13 at 8:08
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It might be false economy to try to do this in post. If you cost the time you will spend getting it to look halfway decent, it might work out a lot more expensive than going to your local furniture dealer or antiques shop and asking if you can borrow / rent a desk that looks like what you actually want.

A lot of smaller shop owners are happy for the publicity and will loan stock items for short periods in return for credits, or let you rent items, particularly things like antiques and second-hand goods. This is a pretty standard thing to do for indie film-makers, and I think will probably work out a lot more convincing than trying to do it with 3D.

  • Thanks for your answer. Out of interest, when you say that small shop owners usually lend or rent items, where is this? I live in the UK, so I was wondering if this was custom here, too. – Geza Kerecsenyi Nov 12 at 7:22
  • This is from experience working on indie films in Australia. I never had any thing to do with the actual negotiations, the art department people did it all. The deal was that we had insurance for anything we borrowed, and the fee varied according to how receptive the shop owner was. – stib Nov 12 at 17:08

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