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camera movement and green screen


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#1 ben jones

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Posted 23 January 2006 - 06:11 AM

Hello every one. I havn't been on the forums for a while as I have only just come off a 5 month job as a camera trainee. It was my first job since graduating from the surrey institute (where I studied cinematography). I had a great time, but found the whole thing exhausting - I guess this is normal when your tring to start out!

Anyway, Im going to be dp'ing a short film in march (probably on mini dv - either a dvx100 or xl2 with pl mount and primes) which involves two men in a boat (one being death) heading towards a tunnel at night. I havnt discussed the exact lighting requirements with the director as yet, but I do know that he wants to shoot the whole thing using green screen. I have spent all day reading all the related posts on the forums, and I must say that I have learnt so much from you all! this really is a fantastic place. What does concern me though is working knowing that the commpositer is very inexperienced with this being an almost no budget production.

keeping the camera locked off when shooting, I guess, would make things alot easier for the digital artists and compossiters as they dont have to work so hard with creating background movement to match that of the cameras. However, if the story so requires a fluid style of camera, then how do I make that compromise? Is there anyway of making the compositers job easier if I am required to use movement? I know im pretty inexperienced my self, but I do understand the importance of communication between all areas of a production. Ive noticed people mentioning shooting empty "plates" for the compositers etc. Im afraid I dont understand this, so some explanation would be wonderfull.

Aso, there is the possibility of shooting on HDV. does anyone know how easy it is working with HDV from a compositers possition? I read that people experience problems because its mpeg2. is this true?
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#2 Stas Tagios

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Posted 23 January 2006 - 08:18 AM

Hi,

I don't have a lot of green screen experience but I'm now posting a short I directed that involved a number of green screen effects shots and composites (though we tried to achieve most of our effects practically), so I can give you some answers based on my admittedly limited knowledge of the subject.

With regards to choice of camera, the less compression of your signal, the easier it will be to key out the green screen and composite in another background. It can be done with DV (my short was shot with the DVX100), but it won't look as good or work as well as a less compressed format like HD. Consider using Panasonic's new HD camera, the HVX200, which is shipping now in limited quantities, as it can shoot true HD footage at 1080 24p and 720 24p with a 4:2:2 compression. (Read plenty more about the HVX at DVXuser.com and Panasonic's pro site).

"Background plate" usually refers to a piece of footage that is composited into a live action scene to replace any areas that are green screen -- for example, you shoot two actors standing in front of a green screen, then shoot a shot of a city street and composite it to the green screen shot to make it look as though the actors were standing on the city street (the city street shot would be the background plate, which would fill in all the green screen area).

In my short, there were scenes involving a puppet that's come to life, so for some effects shots, we would shoot the puppet on set, being manipulated by the puppeteer, then shoot the exact same shot with just the set, to allow for rotoscoping the puppeteer out (in post, you mask out the puppeteer, and the resulting hole left in your footage is filled in by the shot of the set sans puppeteer (an "empty plate").

We shot minimal green screen, relying mostly on practical effects and locked-off shots from which I could rotoscope the puppeteer, since in my case, I found I got better results that way than with green screen, given the limited budget, though obviously thre requirements of your film are different.

When dealing with camera motion and green screen, tracking marks (often white or black X's) are placed at regular intervals on the green screen so that compositors have a reference point for any camera movement, and they can then track that movement and apply the same motion to the effects or background plate.

Check out the excellent magazine Cinefex (cinefex.com) for extensive articles on FX-heavy movies, and also, I think the extended "Sin City" dvd has a bunch of info on shooting with green screens.

I haven't shot any HDV footage, but I've seen some and was not impressed with how the compression scheme handled motion.

B)-->
QUOTE(major B @ Jan 23 2006, 03:11 AM) <{POST_SNAPBACK}>

Hello every one. I havn't been on the forums for a while as I have only just come off a 5 month job as a camera trainee. It was my first job since graduating from the surrey institute (where I studied cinematography). I had a great time, but found the whole thing exhausting - I guess this is normal when your tring to start out!

Anyway, Im going to be dp'ing a short film in march (probably on mini dv - either a dvx100 or xl2 with pl mount and primes) which involves two men in a boat (one being death) heading towards a tunnel at night. I havnt discussed the exact lighting requirements with the director as yet, but I do know that he wants to shoot the whole thing using green screen. I have spent all day reading all the related posts on the forums, and I must say that I have learnt so much from you all! this really is a fantastic place. What does concern me though is working knowing that the commpositer is very inexperienced with this being an almost no budget production.

keeping the camera locked off when shooting, I guess, would make things alot easier for the digital artists and compossiters as they dont have to work so hard with creating background movement to match that of the cameras. However, if the story so requires a fluid style of camera, then how do I make that compromise? Is there anyway of making the compositers job easier if I am required to use movement? I know im pretty inexperienced my self, but I do understand the importance of communication between all areas of a production. Ive noticed people mentioning shooting empty "plates" for the compositers etc. Im afraid I dont understand this, so some explanation would be wonderfull.

Aso, there is the possibility of shooting on HDV. does anyone know how easy it is working with HDV from a compositers possition? I read that people experience problems because its mpeg2. is this true?
[/quote]
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#3 Adam Frisch FSF

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Posted 23 January 2006 - 06:39 PM

Adding dots or points to the background that a software can track is standard procedure today. But if your post guy is inexperienced, I'd advise against it. You need to kind of have done it before.
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#4 Rolfe Klement

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Posted 24 January 2006 - 05:45 AM

Make sure your Director actually knows what he wants and why

Greenscreen can be used in 2 simple ways - either a locked off shot where you use plates for the background and it is a simple composite action in post (this can "kind of" be extended to car and plane sequences easily)

OR

You are going to get an exctensive 3d world created and your actors are going to act in this world - plates can also be incorportated for 3d planes - such as view through a window etc

If you are going for option 2 and if you trust him | her

you need to make sure you have track marks in each of the three axises (XYZ)
The tracking software needs to understand distance to all axis and XYZ planes and any obstacles
the camera needs to understand and view the tracking points so we tend to use red x points - since it is the opposite of G on a RGB scale....
So in simple non technical terms it is best to place a grid of red x marks at uniform distances apart on the floor, walls and ceiling and a different grid size on any obstacles (boxes etc the talent must react with)


Here is a great 1 min demo of what is needed

http://www.claudiomi...obreakdown.html

thanks

Rolfe
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#5 stephen lamb

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Posted 25 January 2006 - 11:18 PM

Major B,

just to let you know, i left a long post about this very same topic that you posted under the lighting forum, you can check out my response there, i posted a few links to a few stills from a VFX film i shot this past fall. Cheers!
Steve
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#6 Mark Allen

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Posted 26 January 2006 - 04:42 AM

Major B... unless they are going to be in some super crazy world... don't shoot this greenscreen.

Greenscreen and DV don't mix. Greenscreen and HDV don't mix. Not for long extended dramatic shots. Seriously consider all options prior to greenscreen unless you have some experienced artists who are putting this together in post... and not shooting on DV/HDV. BetacamSP is better than DV for greenscreen imho.

The concept of shooting "plates" is very much an ultimatte requirement, not so much for keylight - but either way only matters if the cmera is locked or motion controlled. The plates are used to help let the software find the screen. (So it can go "oh - anything that looks like this blank plate here should go away.)

For tracking - you need markers, preferably multiple layered ones for any 3D tracking needs. Tracking isn't rocket science as much as keying, frankly.

Here are some links:

http://www.fxguide.com/fxtips.html

http://www.fxguide.com/fxtips-273.html
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