Basic Keying Techniques

There are as many keying techniques as there are compositers, and it takes some practice to find the best approach for any given footage. The amount of finessing required depends on where the key will fit into the overall composite as well as the format of the final output. This section gives an overview of one of these techniques. You will find detailed procedures in subsequent sections.

The first step is to place the front clip over the back clip and apply a Keyer operator to the front clip.

Now that the front and back clips are in place, you pick the color to be keyed out of the front clip. This is called the key color. All pixels with that color are made transparent. In the matte, they are black. Depending on the keying technique you are using, you can extract a single color, a range of colors, or a luminance range.

Now increase the range of colors that are keyed out. To do so, you can sample a few more pixels in the image and add them to the tolerance range. To create a natural looking composite, add softness—making the pixels around the edge of the matte partially transparent so that they blend into the background.

One way to get just the right amount of softness is to add too much softness, and then remove some. You know that the softness range is too large when gray appears in the white part of the matte.

How you proceed from here depends on the image you are working with. You can adjust the softness range, increase the tolerance, adjust the matte using the histogram, or even use garbage masks to work around problem areas.

The final steps involve refining the edge. You can use the Shrink, Erode, or Blur tools, or use the Color controls to remove color spill. You can also adjust the front and back mattes using the Curves controls.