Green or Blue screen

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vincent6805

Member
I am getting ready to shoot a short film borrowing from Dickens', A Christmas Carol. What would be the best way to go; As the various ghost take the protagonist through his life, should I shoot what they, and my audience sees, on Green or Blue screen. When it comes to Visual Effects, I bow to the experts. I know the look I want, all my camera angles, etc.. should I shoot my scenes in front of the screen, then add what they are viewing? Any help will be greatly appreciated.
 
Will you be shooting a predominately blue color palette or green color palette against the chroma key? That's the only question you need to answer, really.
 
in video you need to choose green unless you got a specific camera shooting uncompressed video (4:4:4).
in current codecs, the video is 4:2:2 or 4:2:0 . That means the RGB channel (same applies for YUV) is splitted in Luminance and chrominance signal.
Then Luma is compressed at unity (the 4) and the Chroma is compressed twice more (the :2:2 or 2:0) .
Most camera are using the green as LUMA reference (that why sensors have more green pixel than blue or red), blue and red being derived from the difference with green.
So at the end, you got the full res (720x480) in green (and luma) and a quarter of resolution in R and B (360x240) .
This can be easily seen by extracting a frame from video and then splitting by channel.
The same problem exit when shooting scenes where the main color is blue or red (like music stage) . The picture looks ugly for that reason.

So if you plan some keying with video, you will get a better result using green than other color.
That could be pretty diffcult (X-mas tree is green !) and so you would need to use another color.
in that case make sure you can record at least 4:2:2, but again most mpeg2 and mpeg4 codecs are 4:2:0

an other trick is to shoot and edit high resolution(full HD, 2k, 4K) , so if your product is going to DVD, you are still good
 
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You'll get a cleaner key with green for the reasons stated above;

If you have time to test the screen and lighting you will use, shoot some footage and check the RGB levels for (optimally) 255 green and 0 blue and red. Keeping the green to one color channel is really helpful.

You can pull good keys with AVCHD cams (like HMC150) - you can pull decent keys with a DSLR, but the aliasing make them difficult.

If there's a lot of motion in the subject, consider upping your shutter speed and adding some motion blur when you comp. This will require more light though.

Get Christiansen's After Effects book for some great keying methodology and tips. Many of them are AE & Keylight centric, but overall, his approach to masking and layering is fairly universal.
 
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