Overview of Compositing Using the Chroma Keyer Filter
Although you can use one of several different filters for keying, you often use more
than one filter, depending on the qualities of the video clip. In general, the process of
compositing two shots together by keying consists of seven main steps, using several
different types of filters. For more details, see “
Example: Using the Chroma Keyer Filter
on page 434.
For best quality, start with the Color Smoothing filter
Apply the Color Smoothing filter to the clip that you want to key the background out
of. This filter improves the quality of chroma keys and reduces the diagonal “stair-step”
look that occurs in video clips with areas of high-contrast color.
Use 4:1:1 Color Smoothing with NTSC or PAL DV-25 video sources. (The exception is
PAL mini-DV/DVCAM, which uses 4:2:0 color sampling.) Use 4:2:2 Color Smoothing for
DVCPRO50 and 8- and 10-bit uncompressed video.
As you add additional key filters, make sure that the Color Smoothing filter remains the
first one listed in the video section of the Filters tab.
Apply the Chroma Keyer filter
Now you can apply the Chroma Keyer filter to the clip. Choose a color or level of
brightness to key on, and then make adjustments to select the range of color or
brightness that most effectively keys out the background, without eliminating the
details of your foreground subject, such as hair, fingers, or the edges of clothing. You
can also use the Thin/Spread slider to adjust the fringing that appears around your
foreground subject, but don’t use it too aggressively.
Tip: Although the Chroma Keyer filter is the fastest and easiest to use, you may find
that the Blue and Green Screen filter, in conjunction with the Matte Choker filter, can
perform a closer key on certain clips that have more subtle detail around the edges of
the foreground subject. For more information on the controls of the Blue and Green
Screen filter, see “
” on page 246.
Eliminate fringing with the Matte Choker filter
After keying out as much of the background as you can without touching the
foreground subject, apply the Matte Choker filter to eliminate any faintly remaining
blue or green fringing or pixels surrounding the edge of your foreground subject.
Using the Matte Choker filter to eliminate this fringing works similarly to using the
Thin/Spread slider in the Chroma Keyer. You may find that, for some clips, the Matte
Choker filter works better than using more aggressive settings in the Chroma Keyer,
giving you a better chance of preserving as much fine detail around the edges of your
foreground subject as possible. Drag the Edge Thin slider to the right to remove faint
areas of the key color around your foreground subject and to smooth out the rough
edges of your key.
A second Matte Choker filter can also be applied to fill holes in the foreground subject
that appear as a result of aggressive settings applied to key out the background. By
dragging the Edge Thin slider to the right, you can fill in semitranslucent areas in your
foreground subject, without changing the background areas you’ve already keyed out.
For more information about the Matte Choker filter controls, see “
Readjust the Chroma Keyer filter’s settings
When keying, additional filters you add usually affect the overall results of previously
applied filters, so after applying the Matte Choker, you’ll probably want to readjust the
Chroma Keyer filter’s settings to take into account the effect the Matte Choker is
having. Changes you make to the Chroma Keyer filter’s settings affect what the Matte
Choker does, so go back and forth between the Chroma Keyer and Matte Choker filters
until you find a balance of settings that effectively removes the background without
eating into your foreground subject.
Desaturate the key with spill suppressor filters
If you have some slight color spill from the background around the edge of your
foreground subject, you can use the Enhance control of the Chroma Keyer to
desaturate the color spill so that it’s not noticeable.
If you have other regions of color spill that appear within your foreground subject—
showing through a sheer dress, for instance—you may want to use the Spill Suppressor
- Blue or Spill Suppressor - Green filter to selectively desaturate just the key color so
that it’s not noticeable. The spill suppressor filters may affect the overall color of the
foreground subject, however, so you may need to use a color correction filter to
compensate for this effect.
Keying, Mattes, and Masks
Crop out elements using the Garbage Matte filter
If there are “unkeyable elements” other than your foreground subject that you want to
eliminate from the frame, such as props, lighting fixtures, or other undesirable objects,
you can use one of the Garbage Matte filters to remove those elements. For more
information on using Garbage Matte filters, see “
Using Mattes to Add or Modify Alpha
” on page 440.
Color correct the foreground and background clips to match
Even if you shot your background and foreground clips to match one another, it’s
unlikely the lighting you used matches perfectly. For this reason, it’s usually necessary
to color correct either the foreground subject or the background to make sure the two
match. For more information on color correction in Final Cut Pro, see Chapter 27, “
,” on page 545.
Tip: When shooting video you intend to composite together using key filters, it’s
important to make sure that the direction of the lighting matches in both the
foreground and background shots. You can color correct for color temperature, relative
brightness levels, and contrast, but lighting direction cannot be altered.
Perform additional adjustments to the background layer
Finally, you should spend some time working on the appearance of the background
layer. Editing a foreground clip in front of a background clip is just the beginning. There
are numerous details you must now consider to make the shot look convincing. For
example, the foreground and background of video you shoot in the field are seldom
both in focus, so the shot may look more realistic if you put the background out of
focus with a blur filter.
You may also need to consider other strategies for making the background look
suitably distant, such as adding a translucent gradient layer to create haze over a
landscape or adjusting the appearance of the sky. Adding other keyed foreground
elements can also make your shot look more interesting and add depth to the shot