FCP 6: Chroma-keying using Motion
[ This article was first published in the August, 2008, issue of
Larry's Final Cut Pro Newsletter. Click here to subscribe.
Updated November 2010. ]
Recently, I went looking in the Editing Library on my website for an article on chroma-keying, only to discover that I have never written on the subject.
Since the keyer in Motion can do some really nice work, I figured it was about time I wrote about how to use it, so here it is.
While you can do the entire key in Motion, that would mean that you need to really understand the Motion interface and its tools. For some, that’s easy, for many others, Motion remains an unknown application.
So, in this exercise, I set myself the goal of showing how to create a great looking key using the combined features in Final Cut Pro and Motion that only requires learning one new keyboard shortcut and one dialog panel. And the results look really nice.
For this tutorial, I’m using some footage of me that I shot for my video podcasts. This was shot in DVCPROHD 720p60 and 720p24, using a rented HVX-200 camera.
Important note: If you plan to do a lot of green screen work, DON’T shoot in 24 fps. The motion blur it causes is significant and makes keys much tougher to do well. Shoot at a higher frame rate, do your keys, then convert the resulting video down to 24 fps for final output. Here’s a recent article on shooting 24 fps.
SETTING UP THE KEY IN FINAL CUT PRO
Let’s get started: Build your key in Final Cut Pro. Put your background image on V1 and your green/blue screen image on V2. (In this example, I’m using green screen material – however, the procedure is the same for blue-screen as well.)
As we will do the final composite in Final Cut, all we need to send to Motion is the top clip. We’ll use Motion to remove the green background and make it transparent.
Select the V2 clip and go to File > Send to > Motion project. In the dialog that follows, give your key a name, and store the file in your FCP Projects folder, along with the other elements you are using for this project. This file will become a permanent part of this project.
(In my case, since I only need this file to write this tutorial, I’m storing it to my Desktop to make it easy to erase.)
Once you click OK, Motion starts up and your V2 clip appears in it automatically.
CHROMA-KEYING IN MOTION
Once your clip appears in Motion, select the clip by clicking it once. (Just as in Final Cut, the key interface rule for Motion is to “Select something, and do something to it.”)
A thin white bounding box appears around the edges of your clip. This tells you that it is selected.
Go to the Toolbar at the top and click the Add Filter button. From the pop-up menu, select Keying > Primatte RT. This applies the filter to your selected clip.
Everything will look worse. DON’T PANIC.
Once you apply the filter, the floating gray palette called the HUD should display with a variety of controls in it. If it doesn’t press F7. If there’s nothing in the HUD, be sure the clip is selected the clip and reapply the filter.
With your clip selected, and the Primatte RT controls visible in the HUD, make the following changes:
* Click the Green Auto Sample button if your background is green, or the Blue Auto Sample button if your background is blue.
* Set the Output Type pop-up menu to Matte.
Slide the Noise Removal slider all the way to the left.
Adjust the Matte Density slider until the foreground is solid white. The background will go hazy, or show white dust. This is normal, all you are looking at in this step is to make the foreground opaque.
Adjust the Noise Removal slider so the background is solid black. Its setting will always be less than the Matte Density slider.
Switch the Output Type pop-up back to Processed Foreground.
Adjust the Spill Suppression slider until the image color looks normal. (You can color-correct this in Final Cut if you need more control.)
Save your work in Motion and switch back to Final Cut Pro.
Final Cut will automatically update the sequence with the newly-keyed clip from Motion and, poof! You have a great looking chroma key.
DEALING WITH ROUGH EDGES
Sometimes you’ll end up with heavy stair-stepping around the edge of a foreground object. The best way to fix this is to:
- Switch the Output type menu back to Matte
- Slide the Noise Removal slider all the way to the left
- Slide the Matte density slider to the middle of the scale
- Control-click the green color chip in the top third of the HUD and manually adjust the green selection by dragging the pointer until the matte is solid white. Again, remember that the background will go gray/hazy, this is normal.
- Adjust the Noise Removal slider until the background is solid black. This may take a bit of tweaking between these settings to look right.
The nice part about this procedure is that it is fast and works great — without having to learn a lot about Motion. And, I’ve found, I get better keys this way than in Final Cut Pro by itself.
Rick LeCompte sent this in:
I was working with some 1920 x 1080, Pro Res 442, 23.98 footage shot on the Canon 5D, and I’m comping some green screen shots. Everything looks OK as I set it up, but when I render the shot it ends up with thin vertical lines visible. The lines are not just over the green screen areas but over the foreground talent. I had never seen this before.
Then, I solved this problem. I originally set the key up with Color Smoothing 4:2:2 and Chroma Keyer. It turned out that by removing the Color Smoothing 4:2:2 filter from the key, the lines went away. Something in the 5D doesn’t work with the color smoothing because I had the lines return with Color Smoothing 4:1:1 also. Anyway, it’s fixed, and thought you might be interested.
[To give credit where credit is due, I should mention that my assistant, Jesse Kerman, did the dirty work experimenting with the clips.]
Larry replies: Cool, thanks for letting me know.