Adobe Premiere Pro 2.0 [Electronic resources] نسخه متنی

1.

Drag Photo 13a to Video 1 and Photo 13b to Video 2 (place them after the clips you worked on earlier).

This is the reverse order you used in lesson 13-1. You will

key out the blue sky (make it transparent) to display the sunset clip below it in the sequence.

2.

Apply Chroma Key to the clip on Video 2.

Take a look at its parameters in the Effect Controls panel.

• Similarity The range of the target color that will be made transparent.

• Blend How much of the clip that you are keying out blends with the underlying clip.

• Threshold Shadow amounts of objects not keyed out that are retained in the keyed-out color.

• Cutoff Darkens or lightens shadows. Dragging too far to the rightbeyond the Threshold slidereffectively switches off the Chroma Key effect.

• Smoothing The amount of anti-aliasingedge softeningapplied to the boundary between transparent and opaque regions.

• Mask Only Displays a white silhouette of the opaque areas in the keyed clip. Use this to fine-tune the parameters to avoid creating transparent holes in areas you don't want to key out.

  
  

3.

Drag the Color Eyedropper into the clip in the Program Monitor and click somewhere in the sky to select its blue color.

The Eyedropper selects a color from a single pixel. Frequently that single pixel does not represent the average color of the region you want to key out, leading to keying results that are less than satisfactory. When using the Eyedropper to get a color sample for a key, Ctrl+click to get a subsample, a 5x5 pixel area.

4.

Slide Similarity to the right to about 25% until all the blue disappears and some of the sunset starts showing through sections of the cathedral.

If you have not seen a chroma key at work before, that little parameter change is sure to get your attention. But you've keyed out too wide a selection of color. You'll fine-tune that in the next steps.

5.

Click Mask Only.

Your keyed shot should look like the image on the left in the next figure.

6.

Adjust the Similarity slider until there are no more holes in the silhouetteto about 13%.

Use the View Zoom Level in the Program Monitor to zoom in on the image and get a closer look at the boundary between the silhouette and the sunset. That will help you fine-tune Similarity and other parameters. You'll note that it's darned hard to get rid of all the artifacts near the edges without cutting some small holes in the building. You need to find a reasonable compromise such that, at regular magnification, your viewers won't notice.

7.

Uncheck Mask Only.

Your keyed shot should look like the next figure on the right. Because the sky is so uniformly blue (that's why I selected this shot), there's no real need for you to adjust the other parameters.

8.

Delete the sunset clip on Video 1, move the clip on Video 2 to Video 3, and place Stained Glass 13 on Video 4, above the cathedral photo on Video 3.

You placed the stained glass image above the cathedral clip to help position it (you'll move it to Video 2 in a moment) and you moved the cathedral clip up because you're going to put another instance of it below it on the timeline.

9.

Drag and slightly expand the stained glass clip in the Program Monitor to position it so it completely covers the window (see next figure, left image).

10.

Drag that stained glass clip from Video 4 to Video 2, below the clip on Video 3.

11.

Select the clip on Video 3 to display its parameters in the Effect Controls panel (Chroma Key is still applied to it), drag the Eyedropper tool to the cathedral window and Ctrl+click there to get a 5 x 5 subsample or average color value.

12.

Use Mask Only, Similarity and Blend to attempt to key out the window without keying out edges of the cathedral (refer to the next figure, center image).

You will not be able to do it. But, because this graphic has a transparent alpha channel, you can put another instance of the cathedral image below it on the sequence to fill the holes left by the chroma key, in effect creating a three-layer image sandwich.

13.

Drag Photo 13b to Video 1.

14.

Uncheck Mask Only for the clip on Video 2.

Your image should look like the image on the right in the next figure.

1.

Drag Photo 13a to Video 1 and Lesson 13 Nested Sequence 2 to Video 2 (place them after the clips already on your sequence).

That nested sequence is the same three-layer image sandwich you just created.

2.

Apply Chroma Key to the Nested Sequence clip, select the sky color, and adjust Similarity. Your clip should look like the next figure.

Tips for effective chroma key shots

Chroma key video shoots don't always go smoothly. For the Blue and Green Screen keys to work effectively, you should follow these tips:

• Use flat lightingtwo lights at 45° angles to the screento avoid creating hot spots. No need to overdo the lighting. Simply make it even.

• The actor's lighting does not have to be flat. Controlled spotlights or lights with "barn doors" work well.

• If you're going to key-in an outdoor background, use daylight-balanced blue gels over your lights to re-create outdoor lighting (or shoot your chroma key shots outdoors). If you're working with live actors, use a fan to blow their hair around to enhance the illusion.

• Avoid chroma key spillkeep actors at least four feet away from the backdrop to avoid picking up its reflected color. A back light on the actors minimizes spill.

• The tighter the shot, the more realistic the finished look will be.

• Fast-paced action is harder to key right to the edges of your subjects.

• Use a wide-open iris on your camcorder to limit the depth of field and to throw the green screen a bit out of focus, making it easier to key out.

• Chroma key fabric and paper costs about $8 a square yard and paint about $60 a gallon. You can find many dealers online.

• Which color to use? With chroma green, you have a reasonable assurance that no one will have clothing that matches it and therefore will key out. Chroma blue works well because it's complementary to skin tones.

• Consumer and prosumer camcorders do not key as well as professional camcorders because they record less color information. But, because they give more weight to green colors to correspond to the color sensitivity of human eyes, green screens key more cleanly than blue.

Note

I made Background 13 using a third-party collection of customizable animated backgrounds. It's a looping videomeaning the last frame flows smoothly into the first frame such that I could string together several of these clips in a row and they would play seamlessly. Several companies make products like this (along with animated borders and lower and upper third backgrounds for supers), with varying levels of customizability and pricing. An Internet search on "animated video backgrounds" will give you an idea of what's out there.

Zoom in to adjust parameters

Sometimes blurring a keyed shot's background can give it a realistic look. 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. You can try it on Background 13 but it's generally more effective when the background is a set as opposed to an animation.

As I mentioned in Lesson 9, clip-based effect order goes from top to bottom in the Effect Controls panel with the two fixed effects, Opacity and Motion, applied last.

As shown in the image on the left in the next figure, placing Brightness & Contrast below the Green Screen Key means it's applied

after keying, so only the model becomes brighter. Placing it above the Green Screen Key (the image on the right in the next figure) means it's applied to the clip

before keying, which brightens both the model and the green screen. That causes the animation inserted into the green screen to become brighter.

Green Screen Key composite with Brightness & Contrast added after applying the key (left) and before (right).

Using the Luma Key

Luminance keysLuma, Multiply, and Screencreate transparencies using clip brightness values. Luma is the catch-all of this category. Multiply creates transparencies in bright areas of the clip and Screen creates transparencies in dark areas.

Extra credit tasks

Take this key a few steps further.