May 30, 2012
by Larry Jordan
In looking over my library, I realized that the last time I wrote about chroma-key, also called green-screen, was four years ago. So this is the first of two articles on chroma-key. This article shows you how to chroma-key using Final Cut Pro 7. In this article, I show you how to chroma-key using Final Cut Pro X.
First, the best thing you can do to improve the quality of your keys is to improve how you shoot them. Here are seven basic production rules:
- Actors should be at least 10 feet in front of the green screen. This avoids light from the background “spilling” around their body or shoulders.
- In general, don’t cast shadows on the green screen. Be very careful shooting feet.
- The green background should be as smooth as possible. Paint is always better than fabric; avoid wrinkles and folds.
- The green background should be lit smoothly, both from side to side and top to bottom. I try to have the green background display between 40-50% level on the waveform monitor.
- There is NO relationship between how the background is lit and how your actors are lit. This example will illustrate that.
- Light your background for smoothness. Light your actors for drama.
- Don’t worry about having the green background fill the frame. It only needs to completely surround the edges of your actors. Garbage mattes are used to get rid of the junk.
SETTING UP THE KEY
Here are my two elements – a dramatically lit Lisa in front of a green-screen background, and the night scene I want to key her into.
Just to illustrate, look at how flat the background is on the scope – right around the 40% level. Notice that there is no relationship between how Lisa is lit and how the background is lit. (This was a painted background. She is about 9 feet in front of it.)
FCP 7 now includes the Primatte RT filter – which is the preferred built-in keyer to use for green screen work. However, it has problems with translucent objects – such as water glasses and gauze – and shadows cast on the background. If you need more powerful keying, consider buying:
CREATING THE KEY
1. In Final Cut Pro, put the background clip in the Timeline on V1 and the foreground (green-screen) clip on V2. Put your playhead on top of the two clips and select the V2 clip.
2. Apply Effects > Video Filters > Key > Primatte RT. Don’t panic, your image will look terrible at this point.
3. Double-click the V2 clip to load it into the Viewer.
4. Click the Filters tab and set the Output Type popup to Foreground, then use the eyedropper next to Backing Color to select a representative color of your chroma-key background. Pick a portion of the background that is near to the face, but be CAREFUL not to get too close. (You don’t want to accidentally select loose hair or an edge of skin.) Selecting a color makes sure you are selecting the shade of green that matches your background, rather than using a default green setting.
5. Now, set the Output Type popup to Matte. The goal to creating a clean key is to make the foreground solid white and the background solid black; switching the view to Matte allows to you see how well you are doing.
6. Slide the Noise Removal slider all the way to 0.
7. Slowly slide the Matte Density slider until the foreground you want to retain goes solid white, indicating foreground opacity. Each key requires a different setting, however, you ALWAYS want the foreground to be solid white, with no shadows.
8. Next, slide the Noise Reduction slider slowly to the right until the background you want to make transparent becomes solid black; be careful to eliminate any light “dust.” The Noise Reduction value is ALWAYS less than the Matte Density slider. The better your background, the closer this setting will be to zero.
9. Set the top popup to Processed Foreground.
10. Adjust Spill Suppression until the color looks normal. Use the Vectorscope to accurately set skin tones. Here’s an article on reading scopes.
Look at how the lighting on Lisa is designed to reflect the night feeling of the background. This reinforces what I said earlier that as long as you separate the actor from the background, you light the background for smoothness and the actor for drama.
NOTE: If your green-screen doesn’t cover the entire background, you use a garbage matte to remove the portions of the image you need to hide. Here’s an article that describes how.
LEARN MORE: Want to learn more about keying in FCP 7 – or want to see it in action? Check out this webinar that covers this subject in more detail.
- Thoughts on Improving Green Screen Results
- Chroma-keying using Motion and Final Cut Pro
- Thoughts on A New Way to Do Green-Screen Graphics
- Using Nests to Clean Up Dissolves Between Chroma-keys
- Color Correcting Using Limit Effect