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The Software Side
In many ways, the software side is a
Compositing works differently in all programs, but
Many programs offer a specific blue-screen or
Choosing the Color
Using generic controls, the first step is almost always to choose the background color, typically with an eyedropper or similar control. This is shown in
. Try to choose a spot
Figure 6.4. Choosing the chromakey color with an eyedropper.
When you choose a pixel with the eyedropper, you choose
color value out of 16.7 million available colors. As you can see in Figure 6.4, this is enough to eliminate some of the small
However, most of the background doesn't match the exact color value
Basically, as you increase the similarity value, you're telling the editor to expand the search from the original color value chosen to additional colors adjacent to that value on the color chart. The effect is
Figure 6.5. Going, going, gone! Expanding the similarity value eliminates more and more of the background color, and—if you go too far—the foreground image.
When adjusting the similarity value (and all chromakey values for that matter), use the program's zooming controls to focus in on problem areas like hair and the edges between the foreground image and the background.
Don't be shy with the similarity control; increase it dramatically until you see it eat into the subject itself, as shown in the image on the far right. Then you can back off and find a value that
If you can't find a similarity value that works well, try choosing a different starting color with the eyedropper. In addition, don't rely solely on any single frame to produce the final values. Instead, move from frame to frame to test your values, and use the program's zooming controls (see Figure 6.6 ) to analyze the detail. Compositing is more art than science, and is usually an iterative process.
Figure 6.6. Smoothing or softness controls smooth the edges between the subject and the background image, making the overlay appear more natural.
Even the middle image of Figure 6.5 looks pretty clean, we're not done. Your
Most programs include a smoothing control (also known as feathering or softness) to smooth the edges, which achieves the effect shown in Figure 6.6. On the left is the original, unsmoothed image; on the right, the same image, post-smoothing. As you can see, these controls help the background and overlay
Sometimes the edges of the overlay video are the toughest
Figure 6.7. Garbage mattes are very effective when you're having problems eliminating the edges of the overlay video.
One other feature worth mentioning is Spill Suppression, which is generally available only on higher-end software programs. Operationally, spill suppression identifies background coloring that isn't eliminated during the chromakey process, and converts it to gray, making it much less visible.
Spill suppression is useful when you can't increase the similarity value without eliminating portions of the subject, so visible residue of the background color remains. The feature isn't widely available in the sub-$1,000 class of editors, but if you run across it, now you know what it does.
Whenever you composite one video over another, there's a risk the overlay process will change the background image in some
Figure 6.8. Note that the region on the extreme right of the video looks brighter than the rest. That's compositing residue.
As you can see, the overlay darkened the background video slightly, which is visible in both the sky and the parking lot. Since the effect is subtle, you probably wouldn't notice it unless you actually shifted the overlay video and
In fact, this is my main concern with programs or plug-ins that "automatically" calculate the optimal chromakey settings for you; almost invariably, they leave some residue. It's easy enough to fix, but unless you
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