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Anyone have good pointers for setting up a chroma key situation for small-scale dolly & crane shots as well as hand held in front of green screen?

Do I need different shapes or will crosses do?

Do i need to place markers on a stationary object in the foreground of the acting area as well for reference points closer to camera to help DaVinci 16 or PrePro determine what's going on? 

 

 

 

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Impossible to say without knowing what kind of vfx elements you are going to add afterwards and the special paths the camera is moving and what kind of tracking software you use and how wide the shot is etc. 

It may vary from a single tracking marker to hundreds and the optimal marker shape can vary as well. Sometimes it may be even more practical to shoot a certain shot without the greenscreen with using normal background to get enough tracking data and then just roto the subject instead of keying. 

A basic distinction would be whether you will do 2d or 2.5d tracking or 3d matchmoving and whether you use point trackers or planar trackers or both and how many you need to see at minimum on each frame to be able to track reliably.

Have you hired a vfx supervisor for the shoot, he/she can check the shots you want to do and tell how it's easiest to archive them? 

We can try to give some suggestions here as well if you tell all the necessary details about the scene and individual shots 

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Woh...ok i opened up a large world here...jeez. So I'm not with a large company, I'm just a guy shooting video, so there's no experts on board to consult.

I had no idea about the vastness of this...looks like I've got something new to learn about. 

My scene is a parody of a heads-up no-limit Hold Em scene, the simulated location: a casino. I wanted to move the camera on a 2 shot against a greenscreen. Or handheld. Not a big deal to keep it immobile.

At this point it's more important to get the basic shot locked down. The cam mvt isn't so crucial. But i would like to ask:

 

What's this parallax effect? How is it different from simply photographing movement where that which is closer appears to move faster than that which is farther away?

When I look at these examples

of course i sense something markedly different, but i can't put my finger on what precisely is going on. Is something exaggerated?

 

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I think that when you are doing moving camera on chroma key, it's important to consult with the person who will be creating the digital FX and composites.  Ask them what kind of markers they will need to track the new backgrounds.

And, if you really can not consult with your fx artist...  Keep in mind that it's best to be able to see at least 2 tracking marks on the green screen at all times.  So let this be your guide as to how to space them.  Also, the tracking marks are "tracked" by finding contrast in them against the green background, so, if they are also green tracking marks or crosses, they should be of a different brightness to the green screen background.  I would think that if you use something like black tape for the tracking marks, they will be easiest to track, but the chroma key will fail around these marks and need small amount of rotoscoping to fix the mask.  This still might be preferable if the back ground green goes out of focus and lowers the contrast of the background.

If there will be multiple layers of background elements added, then it's time to think about tracking markers placed in front of the green screen on stands (hopefully wrapped in green cloth).  Theses should be placed at a distance that will simulate the intermediate background element.

If none of this makes sense to you, it is imperative that you consult with your VFX artist before shooting.

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Sometimes the green screen can not be placed at a distance that simulates the background plate.  And in this case it's up to the artistry and skills of the VFX artist to create the best tracking of the background.

I have an example of this at the 3 minute mark in the clip linked below.  This shot was made in a studio set of a boardroom in an office tower.  The city seen out the window is computer generated.  Even though we used tracking marks on the green, it was probably not very helpful for the FX artist as the distance to the green does not match the distance to the city out the window.  So this became a not easy and expensive DFX shot.

The chroma key shot begins at 3:00.

 

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Yep it is mandatory to consult the person actually doing the vfx. But you still need to know how do the tracking for the different style of shots if the vfx person can't be on the set to supervise the vfx shoots to ensure that they are done correctly.

Yes it is usually (though not always) possible to get somewhat good or at least usable results even if the vfx plates are badly shot but it will require A LOT more work. Basically if a well shot vfx shot could be finished in 1 hour in post the badly shot one can take anything from 5 to 20 hours to do. Even 100 hours or more if one has really screwed up on set. The challenging thing is you may not know whether you have screwed up or not until the post persons tell you so much better to plan these things beforehand to save everyones work :) 

in the case of vfx the fix it in post may mean saving 10min on the set but doing 100 hours more work in post to fix the shot...

if talking about two dimensional tracking one needs at least one tracking point for being able to track pan and tilt and one additional point to be able to track roll (rotation). Then it is possible to track a flat background plate or other element to the same distance than the marker was on set. Longer distance than the marker can usually be somewhat faked manually. 

if you have a flat background plate which changes its angle during the camera move (2.5d tracking) (like a text which is projected on a wall the camera passrs horizontally) then you normally need at least three to four markers depending on how you do it in post. This can also be done with planar tracking which tracks the surface texture and shapes of the area pointed to it instead of using normal point tracking. Planar tracking generally needs a large enough distinctive plane it can follow which has surface texture on most of the area so for example a normal wall with wallpaper might do but a featureless piece of greenscreen may not. Planar tracking is often used to fix shots where the point trackers were incorrectly placed and thus unusable.

3d matchmoving is needed if you need to attach cg elements to the camera move so that there is very noticeable perspective/parallax change and the cg elementd need to replicate that change too. You have to basically track the 3 dimensional path of the camera to replicate the camera move afterwards in the cg program to be able to get the same camera move to the cg elements. This is very distinctive of the normal approach of tracking because you are not following a photographed single markers path but are using dozens og tracking points which the algorithm uses to calculate how the camera originally moved in three dimensional space. This is a very challenging type of tracking to do without experience because it can fail completely if there is not enough points or the points are not constant in certain parts of the move (covered temporarily by the actor etc) and it is challenging to get enough tracking markers available if shooting greenscreen. Modern trackers can use both point and planar trackers for matchmoving so you can normally use most of the fixed objects in the shot for tracking. This is the type of tracking where you need to have lots of parallax tracking points on different distances from the camera to be able to track the shot and you need a lot of markers as well. 

 

You can use green painted pieces of kapa cardboard to host markers if the are needed on different distances. Easier to control shadows than with cloth. 

Point tracking generally works best by locking to a 90 degree or sharper angle corner of high contrast difference. That is why the traditional cross marker has evolved, it has lots of 90 degree high contrast corners to use. Another good marker type is a triangle which is more seen on matchmoving markers but can be used for normal tracking as well. It has the advantage of being a bit easier track if its blurred. 

With greenscreen and basic tracking you can often place the markers outside the actors movement area so that they can be just masked out easily without rotoscoping. If doing matchmoving I advise shooting about 20% wider so that there is unused extra area on the sides which is cropped in post and you can place as many markers than you can fit there to be sure the shot does not fail

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I think it's certainly worth lookin into how much a good vfx guy costs, even for this shoot. Would be worth it to learn from her/him AND get a nice product. 

Thanks guys!

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