Moving on to Keying

The opacity rubberband works great with text, but when using two full-screen clips or graphics it can be an inexact science. You can get more precise compositing results using keys.

In this hour I'll go over Color and Luminance keys. In Hour 15 I'll explain Matte and Alpha Channel keys.

Using the RGB Difference Key

First up is the simplest key?the RGB Difference Key.

Use this key when you have a brightly lit scene with no shadows, a solid-color background, and a subject with a color that's distinctly different from the background. Not many scenes will qualify. Almost all have some shadows, especially when the subject has texture. But it's a good way to see how keying works?or frequently does not work.

Task: Use the RGB Difference Key

To use the RGB Difference key, follow these steps:

  1. Place the clip from the first item in the assignment list?the inanimate object shot in front of a solid color?on Video 2.

  2. Drag the location clip, the fifth item in the assignment list, to Video 1.

  3. Open the Transparency Settings dialog box for the clip on Video 2.

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    As a reminder, to open the Transparency Settings dialog box, either right/Option-click the clip and select Video Options, Transparency or open the Effect Controls palette and click Setup next to Transparency.


  4. In the drop-down menu, highlighted in Figure 14.5, select RGB Difference.

    Figure 14.5. You can access all the Color and Luminance keys from within the Transparency Settings dialog box.

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  5. Place your cursor over the clip image in the Color window (the middle screen). It turns into an eyedropper. Use it to select the background color that you will "key out."

  6. Click the Page Peel icon below the Sample window on the right. In a moment this will display how the keyed clip and the one below it will look when composited together.

  7. Drag the Similarity slider to the right and watch the Sample window as your background disappears?becomes transparent?to reveal the clip on Video 1. Similarity expands or reduces the range of background color keyed out. However, the more you increase Similarity, the more likely you are to key out other colors and create transparencies in both the background and the subject itself.

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Your subject will probably have "aliasing" or "jaggies," those stair-step edges common to diagonal lines in computer monitors and TV sets. Just as you did with Transitions, you can turn on anti-aliasing to fix that. It blends the pixels around the edges of your object. Use the Smoothing drop-down menu, choose Low or High, and check your results in the Sample screen.


Before moving on, check out the other controls in the Transparency Settings dialog box. Below the Sample window, as highlighted in Figure 14.6, from left to right, they are as follows:

  • The Black/White icon. Clicking this replaces the keyed-out background with white. Clicking it again displays a black background.

  • The Checkerboard icon. This uses a black and white pattern to replace the keyed-out background. Click it again to reverse the checkerboard pattern.

  • The Page Peel icon. You've seen already that this tool reveals the clip below the keyed image.

  • The magnifying glass icon is on by default. Move the cursor over the Sample screen and it turns into a plus sign. Click to zoom in on the image. Alt/Option-click to zoom out.

  • While zoomed in you can use the hand icon to drag the image to check other portions of the clip.

Figure 14.6. Use the first three icons to view your "keyed" clip over a white or black background, a checkerboard, or the clip below it on the timeline. The second two icons zoom in and let you drag a zoomed image.

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Using the zoom and hand icon tools, take a close look at how well the RGB Difference key worked. It probably looks mediocre. As you move the slider to make the solid color transparent, you may notice that subtle color differences within that "solid" color mean you have to move the slider so far that part of the subject becomes transparent. I've demonstrated this in Figure 14.7.

Figure 14.7. Too much "similarity" can create transparent holes in the subject you want to key over another clip.

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Welcome to keying. It takes some trial and error to find an approach that works.

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You may notice something unexpected when you apply even the smallest bit of transparency to a clip on a superimposing video track. You no longer see it play in the Program Monitor screen when you play your project or drag the edit line along the timeline. Normally, Premiere displays the clip on the highest track during standard project playback, as opposed to Real-time Preview or rendering. But if you reduce opacity by a mere 1% or apply any transparency, Premiere behaves as if that clip is not there.

Therefore, if you want to view that clip?to look for a logical edit point or some other purpose?you need to turn off transparency. That means either clicking the f check box in the Effect Controls palette to temporarily switch off transparency (while retaining the settings) or raising the opacity back to 100%.


Before using other keying types, try the RGB Difference key on the person you taped in front of a solid color. That, too, likely will be less than satisfactory. Because the person you taped probably moves around a bit that makes finding the right "similarity" around the edges of that person that much more difficult.

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RGB Difference does have an option that only one other key (Difference Matte) offers: a drop shadow. This shadow falls only down and to the right. There are no options for opacity, distance, and so on. But it is a nice touch. Problems arise when your background scene has shadows going some other direction. If that's the case, you might consider using the Camera View video effect on the background to flip the image right to left to swap shadow directions.


Using the Chroma Key

Because RGB Difference works only for very limited, well-planned shots, you'll likely rely on other color or transparency keying techniques.

It's very easy to try them out. Chroma-keying is your best all-around method, but it's not as accurate as the more narrowly defined green/blue screens I'll cover later. It has more options than RGB Difference and therefore gives you a better chance to create a decent-looking key.

Within the same Transparency Settings dialog box, choose Chroma from the Key type drop-down list. As I've highlighted in Figure 14.8, in addition to Similarity, you have sliders for Blend, Threshold, and Cutoff:

Blend? Softens the transition edges between the keyed subject and the clip below it. Zoom in on the Sample screen and gradually slide Blend to the right. Watch as the image on Video 1 begins to replace the few remaining bits of your object's single-color background. Typically those snippets of background color fall right around the subjects outline.

Threshold? Controls the extent of shadowing thrown by the subject that will show up in the keyed clip. To see this in action, zoom in on any shadow on the solid background color and see how this looks in the Sample screen.

Cutoff? Darkens or lightens shadows. Its value must be less than Threshold; otherwise, it'll invert gray and transparent pixels and your screen will become black.

Figure 14.8. Chroma-keying offers more options that increase the likelihood of a successful key.

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A good way to use Threshold and Cutoff to your best advantage is to click the check box next to Mask Only, which displays only the silhouette of your subject and the keyed-out color/background. What you want is a black background and a white subject. What you'll probably start with is a dark gray background and a light gray subject. As you slide Threshold to the left, the background darkens. Try to turn it black. Then slide Cutoff to the right and try to get the subject as white as possible. Once you've found the right combination, uncheck Mask Only and use the magnifying glass to check your results.


Despite your best efforts, even the Chroma key may not make the entire background color transparent without punching a few transparent holes in your subject.

Try the Chroma key on the person you taped. As with the RGB Difference key, you probably will find it difficult to make a clean key, especially in and around the subject's hair.

Using the Luminance Key

To test this key, use the images from the third item in the assignment list?the dark object/light background and vice versa.

Use the Luminance key on those shots. The default Threshold setting is 100%, meaning the widest range of dark values will become transparent. Reduce it and you tighten that range. Cutoff sets just how transparent those dark areas will become. Higher values increase transparency.

As my subjects demonstrate in Figure 14.9 and 14.10, if you taped very contrasting scenes, the Luminance key should work smoothly. It's similar to the Image Mask transition. Whatever is dark gets keyed out (that is, it becomes transparent, letting images below it show through), and light areas remain opaque, displaying whatever is on the upper clip.

Figure 14.9. The Luminance key works well if you have a highly contrasting subject and background and little texture to your subject.

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Figure 14.10. Use the Luminance key on a dark object like this clock shot against a light background to create this unique visual combination.

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To add a realistic feel to any keyed shot, make the background blurry. Typically, you want to make the subject, which you've shot with a key in mind, the focal point of your composited clip. By using a background that's a bit out of focus, the subject stands out even more. To create that illusion, simply use the Fast Blur video effect on the background clip. I used that effect with the sand dollar clip in Figure 14.11.

Figure 14.11. Using the Fast Blur video effect on the background clip makes the keyed image stand out.

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Again, this key takes some planning. When you're shooting, it's best to illuminate the light object or background and work to make the dark area as dark as possible. Objects with fine edge detail such as hair are very hard to key under any circumstances, including when you use luminance.

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You can add motion to Luminance and other keys. Simply open the Motion Settings dialog box from the Effect Controls palette. As shown in Figure 14.12, when you open the Motion Settings dialog box, the entire clip?as opposed to only the keyed subject?moves through the frame, leaving large white gaps on the sides. You can fix this by using the Fill Color eyedropper on the dark background or subject. As I've highlighted in Figure 14.13, this will fill the background with the color you've told Transparency to make transparent. So now only the keyed object will move through the scene. You can create any path you'd like plus add distortion and rotation.

Figure 14.12. A Luminance-keyed image in the Motion Settings dialog box without a color fill applied.

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Figure 14.13. The same Luminance-keyed image in the Motion Settings dialog box with a color fill applied.

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Using the Multiply and Screen Keys

The Multiply and Screen keys are like the Luminance key, but they create more subtle superimpositions. They both examine the clip below the superimposed clip on the time line for dark and light areas. Then they make portions of the superimposed clip transparent to match those areas. The explanation for this in the Adobe manual is counterintuitive. I'll try to clarify it.

As I've illustrated in Figure 14.14, the Multiply key looks for dark areas in the lower clip and then creates transparencies above them, letting those dark areas on the lower clip play through. The superimposed clip displays its image only above the lower clip's light areas. It does so with some opacity, giving the combination of the two images that subtlety I referred to.

Figure 14.14. The Multiply key creates transparencies in the superimposed track over dark areas in the Video 1 clip.

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The Screen key, shown in Figure 14.15, looks for light areas in the lower clip and makes transparencies above them, letting those lighter areas of the lower clip play through the superimposed clip. The superimposed clip displays its image generally only above the dark areas of the lower clip. It is less effective than Multiply and looks more like simply reduced opacity.

Figure 14.15. The Screen key reverses the Multiply key's process and looks for light areas in the lower clip.

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Using the Blue Screen and Green Screen Keys

The Blue Screen and Green Screen keys are your best bets for accurate, relatively low-budget keying. To use them well, you'll not only need what's called "chroma blue" and "chroma green" backdrops but you'll need to follow a few procedures as well. Because this is sort of an involved process, I've included a sidebar on the subject.

The Blue and Green Screen keys' options work like other Premiere keys, only they're simpler because Premiere is looking for very specific chroma blue and green background colors.

There are only two principle slider controls?Threshold and Cutoff. Again, drag Threshold to the left to make the entire blue/green screen transparent. Drag Cutoff to the right to ensure the opaque areas look satisfactory. I asked Matt Zaffino, my favorite weatherman, to demonstrate in Figures 14.16 and 14.17 how things look before and after applying keying effects.

Figure 14.16. KGW-TV's Matt Zaffino in front of a green screen before the key controls are applied.

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Figure 14.17. How things look after keying out chroma green and keying in the weather graphic.

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If at first your key does not succeed, try again. Sometimes try as you might, you cannot remove all the jaggies from the edges of your green- or blue-screened actors. This is endemic to consumer/prosumer DV (technically, DV25) camcorders. To possibly remedy that, key them twice.

However, that won't work for the original clip on the timeline. If you go back to Transparency Settings to tweak the existing blue/green screen settings, that only changes them. It doesn't apply the settings twice.

To do that requires creating a virtual clip. I'll explain virtual clips in Hour 16, "Tips, Tricks, and Techniques: Part 1."


Making Blue and Green Screens Work

Setting up chroma green or blue backdrops can be a royal pain?especially on location. About 10 years ago I hired one of Portland's top production companies to do a fairly involved blue screen shoot at a local college. They lit the heck out of that blue screen, got the dolly rails and the trucking move down perfectly, and rolled and recorded.

When we got back to the studio, try as we might, we could not completely key out the blue screen without creating some transparency in the actor. We ended up building an elaborate moving matte (it had to fit the actor's shifting silhouette). What a time-consuming hassle!

As someone once said, "It's not easy being green" (or blue). Here are some tips:

  • Blue and green screens require "flat" lighting?no hot spots. If you can set up your screen outdoors, sunlight or cloudy days work best. No need to overdo the lighting. Simply make it even.

  • The actor's lighting does not have to be flat. Controlled spot lights or lights with "barn doors" work well. Using soft lighting with umbrellas and reflectors is less dramatic but also is effective. A so-called "key" backlight aimed at the actor's head helps more clearly illuminate hair to eliminate or at least minimize blue or green screen "halos."

  • If you plan to key in an outdoor background, try to re-create outdoor lighting on your subject. If you're working with live actors, further enhance the illusion by using a fan to blow their hair around a bit.

  • Avoid the dreaded blue or green spill. Actors' skin will pick up the reflected color of the backdrop if they're too close to it. Move them at least a few feet away. One other way to minimize this is to use a backlight.

  • Tight shots work better than full-body shots. The closer you are to your subject, the more realistic the finished product.

  • If there is fast-paced action in your shot, you may have trouble keying right to the edges of your subjects.

  • Set your camcorder to Manual and open the iris (you'll probably need to increase shutter speed to avoid overexposure). A wide-open iris?1.8 or so?limits the focal plane to your subject and throws the green screen a bit out of focus, making it easier to key out.

  • To build a backdrop on a budget, consider using unofficial chroma blue or green paint (the real stuff retails for about $40 a gallon). Grab a paint sample collection, videotape it, and then use Premiere's blue screen or green screen transparency on it. Find a color that keys well and buy a gallon. Paint that on a large piece of plywood and you have a portable studio.

    Alternatively, you can buy Chroma key fabric or wide rolls of Chroma key paper. Both run about $10 a square yard. One source is www.Filmtools.com.

  • Which color to use? With chroma green you have reasonable assurance no one will have clothing that matches and therefore will key out. Chroma blue works well because it's complementary to skin tones. The kind of scene you key in may be the determining factor. If you will have your actors keyed in to a scene with a blue sky, use a blue screen. In this case, the dreaded blue spill could be a nice feature.

  • Consumer and prosumer camcorders do not key as well as professional camcorders. The 4:1:1 color sampling compression leads to some quality loss. Because the green portion of an RGB signal receives extra weight to correspond to the sensitivity of human eyes to different colors, green screens key more cleanly than blue. A mathematical analysis done a few years ago showed that using a green screen with a DV camcorder keys only 15% less cleanly than a broadcast-quality Betacam SP camcorder.

Using the Non-Red Key

The Non-Red key is your blue/green screen fall back. Both Blue and Green Screen keys look for very specific colors. They offer no "similarity" controls to select a color range. If you just can't quite dial in a Blue/Green Screen key because somehow the color is slightly off or there is too much fringing around the edges of nontransparent subjects, try the Non-Red key. It's a little more forgiving. As you move the Threshold slider, this key looks for non-red colors (blue/green) to key out.

As I've demonstrated in two additional "before and after" images with Matt Zaffino (Figures 14.18 and 14.19), the Non-Red key offers one other control: Blend. This ostensibly lets you smoothly blend two scenes together. I've found that it doesn't make much difference if you already have a reasonably good blue/green screen shot.

Figure 14.18. Sometimes the Chroma or Green/Blue Screen keys fail to fulfill their promise.

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Figure 14.19. The Non-Red key tends to clear up the "jaggies" along the edges of your opaque subjects.

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    Part II: Enhancing Your Video
     
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