How to Shoot Chroma Keys

Shooting Chroma Keys?

How to Shoot Chroma Keys – This article will cover the essentials for shooting a good chroma key using a blue or a green screen.

What is a chroma key?

A chroma key is basically a selection of color pixels you would like to make transparent, otherwise known as key pixels. Chroma keying is normally used to place a subject in front of any desired background and is popular when it comes to special effects.

Shooting digital video for green screen and keying purposes has traditionally been a bit of an intimidating affair.

Thankfully with new developments in cameras, keying software and even lighting its become a lot easier to achieve a great key with a simple setup, but you still need to keep your wits about you.

How to set up a good chroma key on a blue or green screen?

Screen Choice.
Generally there are two choices for backdrop colours, while technically speaking you can key any colour there is a good reason to either use chroma blue or chroma green.

These colours have the least in common with colours found in skin tones so when you key them you are least likely to clip into skin tones on your subjects.

When budgets don’t allow for a chroma infinity curve Our screen of choice for small keyed shots is a simple colorama backdrop available from most pro-photographic stores. Other alternatives are the more traditional stretched fabric style ones.

Either way a good Matte background is what you are looking for.

Firstly, you must decide whether you want to shoot using a blue or a green screen. This will be the colour or chroma that you wish to key out. You can essentially use any uniform color as the key color but green and blue have become the accepted.

Your choice of screen will depend largely on the camera and the setting. Film tends to work better with blue as the light emission crystals are finest on the blue layer and therefore give better detail with minimal grain.

Green screens seem to be favored when it comes to digital video production as it retains more detail in the green channel, and also has a higher luminance value than the color blue.?

Of course if your subject needs to wear green clothing then you would have to shoot on blue, and vice versa.

Lighting.
Good lighting is crucial when it comes to a well balanced chroma key. When I say well balanced I mean that the light must fall across the screen as evenly as possible eliminating all shadows and hot spots.

Make sure that all the light bulbs are the same make and wattage as any variance will effect the color temperature. Do not trust your eye when it comes to lighting a chroma screen, rather use the zebra or histogram function on your professional camcorder to measure luma values.

The general rule for lighting for green screen is to keep it clean and simple. Don’t go for heavy mood lighting, keep your lighting crisp and clean.?? Lighting wise there are two aspects one generally has to consider.

(a) lighting your green/blue back drop, and
(b) lighting your subject.

For your backdrop lighting you want as soft and as uniform a light as possible, think diffusion, poly bounce, soft kino lights, or over head lantern lights. At all cost avoid direct light sources. Your eyes might not be able to pick up the unevenness in you lighting but as you’ll see later on there is some what of an art to getting the perfectly uniform light on your back drop.

It is also good to light the screen and subject separately as shadows will naturally fall after lighting the subject. The further the subject is away from the screen the better as this will help reduce any spill and wrap that may occur from light’s ability to bounce, 8 – 10 feet should do it. This space also helps when it comes to shooting as it allows for a lower depth of field once the camera is positioned and the shot is framed.

When framing for the shot you want to lower your depth of field by using a bit of zoom. Allow for some space between the subject and the lens. Pull a sharp focus on the eyes of the subject then pull out to the suited frame size.

If you have allowed for some space between the subject and the lens, the zoom will lower the depth of field leaving the subject in focus and the screen slightly off focus. This helps to creating more contrast between the subject and the background as well as soften the luma spread and even out the key.

[somewhere here I think we should do a diagram (over head layout of lights, subject and screen)

Remember to have the overall composition in mind when lighting, you do not want the background sunset on the left hand side to be lighting the subject from the right. Most HD cameras have an unsharpen mode, make sure that that is the setting, with it on, the camera will compress edges giving your footage a pixilated jagged edge (so it must be off).

Lastly, make sure you get at least a few seconds of the fully lit screen being used for the key, no subject, no props, just the screen. This can be used as a backup difference matte if the key just isn’t working for some reason, it’s a great reference for your keying regardless of what application or plug-in you are using.

After lighting the screen, turn it off and light your artist. Try and keep a minimum of 10 feet between the green screen and your artist. This will help minimize spill, giving a cleaner matte, but a well lit matte will not reflect green like a mirror. Using a longer lens will also help alleviate this problem.

Where is your subject going to appear, how, what environment? Know if the artist enters or exits on or off stage. What is the lighting, day night, interior or exterior?

Good planning beforehand can save you a fortune in post and any other extras that might be needed, from possible shadows from leaves rustling, flashing lights to reflections from a water scene, these can all be enhanced with a little thought, bouncing lights on rippled water or a small broken branch swayed in front of a 300w fresnel.

Check your background footage, note the shadow and where the light is coming from and mimic it with a back light from that side. Possibly help eliminate any edging with a straw gel for blonde hair if it is a problem.
With a full length shot any back lighting might be too harsh and give the effect of a halo! Try using a magenta gel which will help rectify this.

Try and avoid unnecessary areas of green spillage, cover any green that is not needed, even with a light pink sheet. Try not to use a fluorescent tube on your artist but rather to light your background.

And lastly if you are shooting a full shot you need to make sure the floor is also lit correctly. The more time spent lighting before saves time in post!

Histogram.
One way to read your lighting and how balanced it is, is to connect your camcorder through your Apple Mac and using Final Cut Pro select “Log and Capture” here you have access to the Video Scopes under the Clip Setting Tab.

In Final Cut Pro we now realize that what is shown is a combination of RGB values. Showing the darkest black and brightest white values in an image with corresponding spikes in the histogram which can clearly be seen in the first two histogram images below, the graduated tint of black to white and the graduated wave created by two greens.

You can now check your lighting for balance and hopefully have a very even histogram running right to left in the mid 50’s. Below is an image of a histogram that was produced using a Sony Z1 connected to an Apple Mac and aimed directly at a green colorama.

Any random scattering of the graph can mean several things, creases, bad curves, kinks in the colorama, to uneven lighting. So, a histogram which has lots of dark pixels will be skewed to the bottom and one with lots of lighter tones will be skewed to the top, (as displayed in the first image above).

To give you a better understanding of how the waveform interprets the view, I have placed several photoshop images in a Final Cut Pro timeline and taken screen shots of the wave forms produced by these flats, nested images of the photoshop documents have been superimposed which created the waveforms. The narrower the histogram the less your application has to sample to key out.

Below are 2 photographic stills taken of an VDU of a Sony EX3, displaying zebras where the luminance is too high.

In the bottom right hand corner of each of these images you will see how the histogram is also reflecting the light and contrast of the given shot (exactly like Photoshop histogram or level display palette). Notice that none of the settings for the camera have change in any instance, white balance, F Stop, or shutter speed, only the lighting has changed!

Here a histogram which has lots of dark pixels will be skewed to the left and one with lots of lighter tones will be skewed to the right, (as displayed in the images below).

Post Production.
Invest in a decent matte program there are a lot out there (Boris Effects, Ultra2, Ultimatte, Red Giant’s Key Correct Pro, Primatte Keyer Pro), download a demo version if need be and see what works for you in budget or the complexities of using the package.??With a little experience beforehand using test footage will give you a better understanding of shooting your chroma key and how to deal with the final footage. Regardless of what application you are using, Final Cut Pro, Premier to After Effects. Though sound in AF can be a bit of a problem at times.?When you start to matte your footage always use your highest or earliest generation as with each step you lose a generation.

Below are before and after images from our last green screen production for HuntaLive.