[Class Assembling]
Hi! John Hess from FilmmakerIQ.com. In the greenscreen history lesson I talked about
the processes filmmakers have used to make traveling mattes - in this lesson I'll dive
into the necessary elements you need to pull off a great chromakey.
The first element in pulling off a key is the type of space you have to work with. Your
space will determine what kind of chromakey shot is possible. When shooting against a
greenscreen or a blue screen, you'll want to pull you subject away from the background
far enough so that shadows don't fall on the screen and you'll want to minimize the
reflection of the screen on your subject.
With smaller spaces you should be able to pull off a reasonable talking head shot - that's
where the shot is just the head and shoulders of the subject speaking. For a full body shot,
you're not only going to need a larger space for a screen, but some distance to place the
camera so that your subject looks natural and isn't distorted by a wide angle lens.
Shooting outdoors is also a possibility especially for certain shots, just keep in mind you'll
have to deal with all the issues that come shooting outdoors including wind, noise, and shadows.
The first question you're going to ask is green or blue? Blue screen was a traditional
color in the film days and is still used today for many productions, but green is the preferred
color for digital keying. Why? Because many digital cameras use a Bayer pattern of Red
Green Blue photosites where there are twice as many green photosites as there are red
and blue. This makes digital cameras much more sensitive to green coloring. Green screen
also requires a lot less light than blue screen and is less likely to match the clothing of your actors.
Still you may want to use blue screen in certain cases - say you're shooting a green monster.
In fact on the Sam Raimi's Spiderman, The Green Goblin had to be photographed on a bluescreen
because the suit would have been lost on a greenscreen. Spiderman on the other hand had
to be shot on a Greenscreen, because his suit was red and blue..
When it comes to the material of your screen you have several options. The first is to
paint your background using a chroma key paint. This is the most permanent although labor
intensive way to create a chromakey background though certainly necessary if you are planning
on installing a cyclorama.
The other option is to hang your screen You'll need background stands and clamps to hang
your screen on. The screen itself can be made out of paper, or muslin background clothe,
but I prefer foam backed cloth because it scatters the light more evenly so you can
avoid hotspots and the foam keeps the screen from wrinkling when not in use.
For small setups, foldable chorma key screens are available. These kits are really handy
for quick portable setups or outdoor use and built in frame keeps the screen from getting too wrinkled.
When lighting for chroma key you have to think of lighting first for the screen itself and
then for the subject. In tight situations you can combine the lighting but you could
end up with shadows on your screen and that will make for a much more difficult task of pulling a good key.
Start by focusing on getting a clean and even light on your screen without your subject.
Here I'm using 2 ikan IDMX1500B dual color fixtures - part of the ikan 5 piece dual color
chroma key kit. These are hung in front of the screen using a truss system. These are
awesome LED lights that put out a lot of soft even light which is an absolute must properly
lighting a greenscreen.
Now if you're an avid DIYer you may try to use long fluorescent tubes to light the
screen. Another DIY option, is the Hollywood strip lighting fixture - this one I found
on a street curb being thrown out - and it made a nice shadowless screen light.
To check to see the quality of our lighting on the greenscreen, I'm using the ikan MD7's
built-in waveform monitor. Notice the tight band.. This means that are screen is evenly
lit and there's not much variation on the screen itself. You want to get this band as
tight as possible, not slanted or have spikes which are hotspots..
If you can't get your hands on a waveform monitor, you can use your camera's zebras
settings. Zebras tell you what part of your image is overexposed, if you lit your screen
properly you should see the entire screen turning into zebra stripes at once as you
open your iris to over expose the screen.
Once we have a well lit screen it's time to put our subject in place and work on the
subject's lighting. Keep in mind the final composition and try to light your subject
that will match the scene you're attempting to composit..
Here I'm using the ikan IB1000 LED lights - part of their chroma key kit. These lights
can be set to 3200k which will match great with the tungsten lights I already own. Just
take care when lighting your subject to avoid casting shadows on your screen.
When I first started experimenting with greenscreen, I was working with DV in standard definition.
You had to be exact in order to get an acceptable key, and even then there were some funky artifacts.
But I'm happy to say that with HD, getting a reasonably acceptable key is much easier
because the pixels are so much smaller. But the type of compression can be a detriment
to quality greenscreen.
Most consumer cameras use a 4:2:0 compression for handling color - I talk more what color
compression is in the lesson on Non-Linear editing if you want a refresher. 4:2:0 throws
away a lot of color data that can be very useful for achieving a quality key. In this
demonstration, I'm using the Blackmagic Cinema Camera decked out with ikan's DSLR
Rig. The Blackmagic Cinema Camera is capable of recording in 4:2:2 compression using ProRes
or DNxHD formats or even recording 4:4:4 uncompressed 12 bit raw.
My tests have shown that the compressed 4:2:2 is pretty good for pulling a good chroma key
but the RAW is simply unbeatable in terms of ease and quality. If you want the best
chroma key, by all means shoot RAW - just know that it is expensive in terms of the
amount of memory it eats up. I'm not saying you can't pull a decent key using anything
less than 4:2:2 color compression or RAW as in the Blackmagic Cinema Camera, but a camera
like this with it's compression or lack thereof sure makes a the key a heck of a lot easier.
Most of the basic keyers you find inside NLEs behave like hardware chroma keyers in that
you'll most likely be let down by the results. But there are a lot of professional keying
software solutions available out there that can help you pull pretty good keys even out
of so-so footage. My favorite. happens to be one that comes bundled inside of Adobe
After Effects called "Keylight" made by The Foundry. This plugin is incredibly robust
with all kinds of features like matte choking, spill removal, de-spotting- and even masks
that you can apply to fix little chroma key mistakes here and there. Plus After Effects
is just a good platform to be working doing your compositing in the first place. Keylight
is also available for purchase for other editing platforms.
Now if you can't pull a perfect key off the entire frame - don't worry as there
is a technique called garbage matting. Using After Effects, you can create a rough mask
around your subject and throw away all the other junk in your frame thats unimportant.
This is a great way to work in smaller spaces - just make sure you subject does cross into the garbage matte
Once you have a good clean key, it's a matter of compositing the subject into your background
plate adjusting the color to match, and compositing foreground elements to really sell the effect.
By considering your working space, using a quality screen, lighting it evenly, using
a camera with as little compression as you can get, and finally using a good software
keyer, you should be able to pull off a great key. Good chroma keying is a skill, and it
will take a little practice, but the reward for your patience and experimentation can
be quite liberating. - Another tool for you use in your quest to make something great.
I'm John Hess, I'll see you at Filmmaker IQ.com