Keying

Keying is the process of separating and isolating elements of an image by their color or brightness. It’s often done for visual effects (such as to remove green screens), or in color correction (to add warmth just to skin tones).

It begins with selecting a color or brightness value in a shot, which will be rendered transparent. This is called pulling a key. If a shot is planned and executed correctly, pulling a key may be straightforward, but it usually requires a bit of finesse.

There are two primary types of keys: luma keys and chroma keys.

Luma keys operate on the brightness levels, or luminance values, of an image. You can generate a matte by applying a luma key to your shot, then designating which brightness level is transparent and which is opaque.

Chroma keys let you select a specific color in an image (i.e. the chrominance value), designating anything with that color transparent. Performances shot in green or blue physical environments were executed with chroma keying in mind.

Why Green?

Green is a color that isn’t commonly found in, or on, humans. In instances where green is an integral part of a character’s wardrobe or costume, shooting on blue is the next choice.

From a camera standpoint, most digital camera sensors use a Bayer Pattern.

Bayer Pattern sensors have twice as many green photosites than red or blue. More photosites means more information to work with in the captured image, so green is the default choice for digital cameras.

Here’s a 3-step approach to keying shots:

1. 1
   Outer
2. 2
   Inner (Core)
3. 3
   Edge

Outer Key

Once you apply a chroma key, focus on cleaning up the outer portions of the shot. Start by selecting the main hue of green that will become your transparent background. Then refine the key, perhaps with some Grow or Shrink controls, to encompass all the detail you want to keep. Even if it’s hair blowing, make sure you get it all. You’ll want a wider band around the subject you’re keying.

Shooting for the keying process is rarely perfect. Uneven light levels, shadows, wrinkles, seams, other objects or people in the shot – all of these exist, even on big-budget feature films.

The solution? Before you pull the key, create a garbage matte. Garbage mattes are quickly drawn masks that eliminate larger, unwanted elements in the shot.

Inner (Core) Key

Focus on shrinking this inner, or core, band. This may eat into your image for now, but that’s OK. Again, refine the key, perhaps with some Shrink controls, encompassing all the detail you want to keep. That green band should be gone, with a bit of loss around the edge of the image.

Edge

Now it’s time to finesse the edge. Feather and erode controls, tuning black levels and white levels, and edge color correction will restore any imaging lost.

If a series of shots were performed on the same day under the same conditions with the same camera, you could save a preset from one shot to reduce work on the rest of the shots. More than likely, though, each shot will have its own quirks to address. Have the mindset: a preset’s a good start, but more work is likely to get it right.

Like any process, there’s bound to be trouble along the way. Here are some common problems you’ll discover while keying.

Spill Suppression

Spill occurs when the color of the background is reflected on the subject. Depending on how much color spills onto the subject, unwanted image loss may occur while keying the shot. Remember that a green-screen chroma key works by removing everything that’s green. If the green background is reflecting off of your subject’s sweaty arm, then the chroma is going to remove part of their arm as well.

Thankfully, many keyers come with some controllable form of spill suppression. This can be addressed during the Inner (Core) stage.

Also, applying a holdout matte can restore any unwanted image loss from tuning spill suppression. Or a holdout matte alone may solve the problem.

Color Correction

Your subject’s color may shift during the course of keying, so you may need to compensate for that afterward. Also, you might perform color correction to match the brightness levels or lighting angles of other shots you’ll composite. Time and budget constraints will dictate how much can or will be corrected.

Multiple Keys & Corrections

It’s also possible that a shot will require multiple keys and corrections before it’s released. It’s sometimes necessary to layer several different keys together, especially if the greenscreen wasn’t evenly lit.

Rotoscope

When all else fails, it’s sometimes necessary to resort to rotoscoping (manually cutting out the subject from the background) if they key really doesn’t cooperate. Sometimes, in a tricky shot, the left of an actor’s body may separate nicely from the greenscreen with a good chroma key, but the right side of the key may fail. In those cases, it’s common to accept the key where it works and then use roto to fix the areas where it doesn’t.