OK, I’ll just say this right at the top – this article has a lot of heart in it.
So, here’s the question: When do you use a Luma key? And the answer is: When you don’t have any color to work with. For instance, here is an animated dancing heart (courtesy of Pond5.com). Unlike a green-screen key, the background here is pure white. A chroma-key won’t work. Time to use a Luma key.
I want to composite this dancing heart over the lights of a stage concert. To do this, we will use a Luma Key to make the white background disappear. And, with the recent 10.3 update to Final Cut Pro X, the Luma Key has even more adjustments than it did before.
In the Timeline, stack the two clips so the foreground clip – the heart – is above the background clip.
From the Effects browser, drag Keying > Luma key onto the foreground clip.
Things immediately look worse. Don’t panic. By default, the Luma key removes the darkest portions of an image; which is the opposite of what we want.
Open the Inspector (Command+4), click the Video tab, and check the Invert checkbox.
This removes the white background, but it also removes a lot of the heart, itself. Time to tweak.
Click the middle View icon to switch to Matte view. It is ALWAYS best to adjust a key while watching the matte.
Our goal is to make the foreground completely white, which means it is totally opaque, and the background completely black, which means it is totally transparent. Currently, the dancing heart is gray, which means it is translucent.
Normally, we’d use the Fill Holes setting, but this only makes things worse.
Instead, twirl down Matte Tools, then twirl down Levels. This allows you to adjust how Final Cut Pro X perceives the levels of the image, without actually changing the picture quality itself.
Because there are so many reflections on the heart, along with shadows under its feet, I’ll tweak the Levels to get the cleanest key that I can. Always by looking at the matte and trying to get the foreground solid white, with no shadows in the middle of the image.
Next, at the top, is the Luma adjustment. Again, we are making these adjustments to get as close as we can to pure white/pure black and no shadows or shades of gray.
Move gently when making these adjustments, keep checking how the image changes.
Next, to blend some of the background color with the foreground image, twirl down Light Wrap and slide the Amount up. This adds background color to the edges of the foreground, which makes the two layers blend more seamlessly. Try changing the Mode, while watching the edges of the foreground image. Pick the blend mode that provides the cleanest results. Light Wrap is very subtle, so look carefully.
Finally, I flipped the background image upside-down, and positioned the heart lower in the frame to make it look like he is standing on the stage.
The Luma key makes all this possible. All it takes is a little tweaking and a lotta heart.
(Sigh… I couldn’t resist.)
NEW & Updated!
Edit smarter with Larry’s latest training, all available in our store.