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how to place the best tracking points
Hi there. I am emabrking on a shoot which amy involve alot of tracking. I have had some problems with this in the past and i was hoping someone could point me to a guide on how to place the most effective tracking marks on set. What shapes/colors are best, are grids good, or more random placement. I will be traking in aftereffects, Maya Live, or Boujou, whichever will do the job.
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2 Replies
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I always try to get dark green tape. It should be dark enough to have a visible luminance difference between the greenscreen, yet still be keyable (or taken out with something like Key Correct’s Alpha Cleaner). The green tape really helps when there is motion blur… If you have white tape you should avoid intersections with the actors, since white tape will have to be painted from the edges of the foreground motion blur. After getting burned a few times, I now bring my own tape. Usually the production companies do not know what to do… I have requested dark green and got fllourescent green which looked identical to the greenscreen. I made them re-tape the whole thing (60’x60′ 2 screens took all night). Recently the grips only had “electrical” tape which was highly reflective and turned white when lights hit them, so do not use reflective tape.
If the shot is pretty planer, then you should use crosses for tracking marks. If you have rotation or severe craning, then triangle markers might work better.
Make sure that you have enough trackers in frame at all times. This will depend on what you are tracking. If you only are tracking position, then you only need one marker in frame. If you need to track rotation or scale you need two, and a corner-pin will need four. If one goes out of frame, make sure there is one to continue the offscreen tracking. In reality, the more the merrier if you have good green tape. I sometimes find myself rushed to get set up, and will get trackers in the specific area for the first shot, then continuously add trackers throughout the day to the rest of the screen… it doesn;t need to be pretty… sometimes you just have to rip some tape and slap it up there.
If you are planning on doing a 2D track, then you should look at the video assist and make the trackers the appropriate size depending on how far they are from the camera. I suggest something like 50px x 50px lines 5px thick on a D1 frame, 100×100 on HD. This should be ideal for a 2D tracker. I like 1/4 inch tape for most shots… usually the gaffer’s tape is 1/2 or 1″ which is too wide.
3D trackers like a lot of points to grab onto. In general I like smaller tracking marks than 2D tracks, and a lot more of them. I have had large tracking marks and found that the Boujou’s trackers end up clinging to the corners of the marks (ie. 50 trackers stuck to one cross on the edges). 3D trackers do a statistical analysis of a ton of points, then throw away bad tracks automatically. This makes them a lot more lenient than 2D tracks which can suffer from jitter. Therefore, I have found that shifting channels and crushing the image prior to 3D tracking will help sometimes even though it is deteriorating the image (ie. do not do this with 2D tracks).
3D trackers need to be placed on planer floors and walls. Do not put any markers on the transition between the floor and the wall.
Try not to do zooms if possible.
If you are shooting on film, get a simultaneous flat pass out of the telecine. This should be a “raw” pass that the telecine’s color corrector never touches. This should be the shot that you track, then apply the data to the beauty pass. Any processing done by the telecine will deteriorate your image quality… even though the raw pass will look low contrast it still contains more information. We have also had success doing red passes in telecine to pull out hard to see bright green trackers that would have been impossible to track without crushing it. It’s much better to do this in telecine than in AE…
Anyway, those are some tips that I learned the hard way.
-Matt Silverman
http://www.phoenixedit.com -
Wow. This is great info. Thanks Matt!
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