Tutorial The ABCs of Chromakeys (page 2)

Virtual Worlds

The Ultra CS3 software includes a nice bonus in the form of assorted virtual sets. These sets let you place your foreground subject in virtual settings, such as a newsroom, museum, library, talk show stage or other cyber-locations. The package also provides a generous assortment of diverse stock background images. Additional virtual set libraries are available for separate purchase.

Of course, you can use your own image or video footage for background, if you wish. And you can combine it with the virtual sets, so the combinations and creative flexibility at your disposal here are quite extensive. Some of the virtual sets also have inset and overlay capabilities. These added features enable you to add a desk, podium or other prop device to the composite - even in front of the foreground image - to further push the illusory envelope.

The Whole Ball of Wax

I like to experiment with stuff and do what-if-I-try-this scenarios to see what it will look like, and I did so with Ultra CS3. I have a wealth of my own existing images and footage shot over the years (some of it in VHS-C and Hi-8 - you remember those video formats, don't you?). So I decided to use them to see how convincing I could make things look. I was pleasantly surprised at the results, and I had a lot of fun playing with the possibilities, too. Before you look at the screen captures, here's a quick run-through of what the chromakey process entails.

Some Caveats

It is of paramount importance that your subject is not wearing anything that is the same color as that of the key! Why? Because it will also become invisible when the key is active and will disappear from the footage when you composite it, allowing the background footage to show through this area. Avoid jewelry or other objects that may reflect the greenscreen background color for the same reason.

Put as much distance between the subject and the key background as possible; this lessens the chance of casting shadows on it. I like to shoot outdoors on overcast days when possible, as I did with the foreground footage of my examples. This light is ideal, because it is diffused, it casts no shadows and it provides uniform illumination. What's not to love about it?

You can use a live foreground subject, rather than one you've taped already, à la live TV news broadcasts. However, using a taped subject makes things a bit easier. Live subjects get understandably bored and fidgety while waiting around between takes. This is especially true if you're experimenting or adjusting variables in the production environment.

Keying It Up

First, shoot your foreground subject against a uniform green background. For my chromakey backgrounds, I use a 4'x8' piece of bright green polyester fabric I purchased for less than $20 at a local sewing and craft store. I like using the polyester for my greenscreen background because it folds compactly and any wrinkles smooth out quickly when it's stretched and hung. Fabric backgrounds are also easy to mount, using push-pins or masking tape.

You can also use a painted wall or painted plywood sheets for your background. As I noted earlier, the backdrop doesn't have to be green - it can be just about any solid color you want, although green works exceptionally well. The important thing here is to key a solid color that's as uniform as possible.

Once the foreground subject is ready, select several regions in the background to key on. The reason is to create a color range that compensates for any shadows, wrinkles or other small variances in the background.

Next, select the background to use, whether it is a still image or moving video footage. Check the preview screen to see how the keyed foreground image looks against the background when composited.

Make any required adjustments to the keyed image, such as smoothing or sharpening the edges; adjusting the transparency, color, brightness and contrast; adding shadows or changing the overall position with regard to the background. If you're using a virtual set capable of accepting an inset, drop in a still image or another video clip, and again check the preview. When everything meshes and looks right, you just select the desired output parameters, and export the composited piece. Then, you can do any additional post-production work, such as adding additional sound, titles and more, as you integrate the composited clip into your completed production. So that's basically how you do it. Now that you have a general idea of how the chromakey process works, go back to page 42 and look at actual screen captures on each page that show the various elements of the process, using Ultra CS3. Good luck!