How to place actors in a 3D model?

So, I am making a short film, and I got Hitfilm Pro for it. I have six actors who will be standing in front of a green screen. They're going to be composited into a 3D model of an alien hangar bay. Once I have the model and the footage, how do I line up the shots? Like if the camera moves while the actor is speaking how do I get the 3D camera in Hitfilm to move with it? Like panning around the room? And how do I actually put the actors in the middle of the model?

Comments

  • This video should answer some questions.

    https://www.youtube.com/watch?v=1H7JwDKHGqw

  • edited December 2018
  • Revisiting this because I still need one more question answered.

    If my camera is moving when I do the green screen shot, like panning around or moving around the talent, how do I match the Hitfilm 3D camera to match?

  • @HeySiri ; I think I might know this one.  Those little tape markers you see all over greens creek material in bts footages are there to use with a tracking program like Matchmove (now obsolete unless you can find it) to create a camera solve and apply that data to the camera in a 3d package to replicate the movement.

  • @Palacono ; D'oh, Face palm!  

  • So I'd make a point on my green screen (something easily trackable), then when the camera moves I'll be able to match the background to it with a camera solve...

     

    @tddavis what if my camera is panning around my talent? Basically, if the tracker won't be visible the entire time.

  • edited January 15

    @HeySiri ; It's not just one point it's a bunch of plus sign made with a contrasting tape or something.  I'll try and find a reference image.

    Edit:  Actually found this incredibly detailed video.  0:44 sec shows what I was talking about, but tons of useful info here ion the whole video.

    https://www.youtube.com/watch?v=0Qhe7IO2hwY

  • @tddavis And is Mocha able to camera solve like this?

  • @HeySiri ; According to the video Palacono linked yes.  I, myself, have never had a lot of luck with tracking and I barely understand Mocha which is partly why I forgot about it in my early post. :) Only partly the rest is old age...  I did play with MatchMove once upon a time but there again I wasn't satisfied with the results.  And I've tried a course in Blender where I failed at tracking there too!  I'm sensing a theme here.  I just suck at tracking!  So I try and avoid it in my little ditties I put out.  Just goes o show, you can know most of the basics but the application is a whole 'nother ball of wax.

  • Triem23Triem23 Moderator

    @tddavis tracking is a mystical art, Terry. I've had clips I COULD NOT TRACK over hours of time and many tries. Then changing one parameter value (point track) or slightly moving a mocha spline or deleting a spline keyframe and the same clip becomes solidly tracked.

    @HeySiri there are two basic kinds of trackers, point and planar. Hitfilm's 2D tracking is points. Ae's camera trackeis also point based as is Blender's, PFHoe, and the majority of trackers. 3D point trackers create lots of points - and by this I mean points of contrast. These points become a "cloud" of point data. Based on how these points move relative to each other the tracker tries to generate a 3D space. Note the point tracker does not know what was tracked, how big it is, how far away it is.... It's just looking for contrast points and tracking them all in 2D, then comparing these points to each other to generate depth. There are infinite mathematical solutions to each shot. The tracker picks one. Any three points can then be used to define a triangular plane.

    Advantage of point tracking is creating a lot more reference points in a scene to align objects. Disadvantage is motion blur ruins your track. 

    Planar tracking is different. Planar tracking has the user define an area then tracks the movement of the pixel texture within the area. Planar trackers assume everything tracked is a flat rectangle. When setting the surface in a planar tracker the angled lines (not 90 degrees) are used to attempt to figure out a 3D orientation for the tracked surface (which is why you give mocha a focal length. Focal length affects apparent motion/angle). Again  the planar tracker doesn't know what's being tracked, its size or its distance, it's just looking at how two (or more) rectangles move relative to each other to fake a 3D space. 

    Advantages of planar tracking. Motion blur doesn't kill your track. Disadvantage is a less accurate 3D world space and fewer reference points. 

    Motion tracking a green screen in 3D space is tricky, no matter what. However  in mocha you have to have two defined surfaces in different positions and/or rotations from each other to calculate a 3D space. A single greenscreen might not give you enough spatial information to generate a 3D solve if the camera can't see the floor. There's no 100% yes/no answer to "can mocha track this." The answer depends on if you can give mocha enough data to do its job, and that's done with testing and planning. 

    Tracking markers.... Often the cross of green tape is used under the belief it can be easily keyed out without having to roto. Often this actually hurts the track.... Green on green might not provide contrast for a tracker. I've seen green, red and blue tape used on screens.

    Do a Google image search for tracking marker. You'll see crosses and circles and trangles and other odd shapes. Anything that is a point of contrast can be tracked.

    https://openclipart.org/image/2400px/svg_to_png/182832/motion-tracking-marker.png

    This tracking marker is a larger pattern. Something like this provides enough area for a planar track and contrast for a point track. The pattern also is different if you rotate it. This type of tracker mark would be used for a multi axis roll on the camera since the VFX supervisor/tracking artists would know how it's supposed to look "right way up" vs "upside down." this spacial information is useful for anyone doing virtual sets since this shape tells you if your camera is rolled or tilted sideways. 

    Of course this type of tracking marker requires roto to remove, but it's giving a lot more useful information than a green on green cross. 

    So  to sum up, it doesn't matter what you use as tracking markers, but for any 3D solve you need multiple markers on different surfaces at different depths and/or angles to give the tracker data to create a 3D world.

    Bottom line, you need to test this before you shoot your film. Tracking is almost never as automatic or simple as it usually looks. Remember  in a behind-the-scenes thing you'll always be shown things that worked well (and not the rejected and bad stuff) and, unless a tutorial is specifically called something like "Tracking a problematic/complex/difficult/impossible shot" then the tutorial is using footage the tutorial maker already knows tracks easily and quickly. 

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