7. Creating a Luma Key
A key is a special effect that removes portions of an image based upon certain values. In the case of a luma key, those values are based on luminance, or grayscale. In the case of a chroma key, those values are based on color. In this exercise, you will learn how to do a luma key.In Chapter 9, "Text, Titles, and Graphics," you learned how to key Photoshop graphics, which have built-in alpha channels. Luma keys are used when you have a graphic against a black or white background without an alpha, or transparency, channel. They are most often found when you download JPEG graphics off the Web.
| 1. | If Final Cut is not running, start it and open Chapter 11 Lesson. Double-click Seq Key Luma to load it into the Timeline. | | 2. | This is a color graphic of a snowboarder, Snowboard Luma.tif, which you need to key into the background shot on V1. To do so, select the clip on V2. (Remember, effects are always applied to the top clip.) Choose Effects > Video Filters > Key > Luma Key.Double-click the Snowboard Luma clip to load it into the Viewer, then click the Filters tab. | | 3. | Because you want to remove the black background, choose Key Out Darker from the Key Mode pop-up menu. | | 4. | Since this black background was computer-generated, you can reduce the Tolerance to 0. If you shot this graphic with a camera, you would decrease the Tolerance from 100 until the black background completely disappears.To minimize the black lines that appear around the image, decrease the Threshold to 65. | | 5. | Play the sequence. Because video is bitmapped, some stair-stepping on smooth curves is unavoidablethat's the nature of video. You can minimize these jagged edges by avoiding lines or edges of objects that are almost, but not quite, vertical or horizontal (a rule that I violated in this instance with the edges of the snowboard). | | 6. | That's it. A quick exercise. You don't need to save your work. Quit Final Cut if you want to take a break; otherwise, it's on to chroma keying. |
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