Adobe Premiere Pro CC (2016)

15. Exploring Compositing Techniques

Lesson overview

In this lesson, you’ll learn about the following:

• Using the alpha channel

• Using compositing techniques

• Working with opacity

• Working with a greenscreen

• Using mattes

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This lesson will take approximately 50 minutes.

Premiere Pro has powerful tools that enable you to combine layers of video in your sequences.

In this lesson, you’ll learn about the key technologies that make compositing work and about approaches to preparing for compositing, adjusting the opacity of clips, and keying greenscreen shots with chromakey and mattes.

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Compositing comprises blending, combining, layering, keying, masking, and cropping, in any combination. Anything that combines two images is compositing.

Getting started

Until now, you have been mainly working with single, whole-frame images. You have created edits where you have transitioned between one image and another or edited clips onto upper video tracks to have them appear in front of clips on lower video tracks.

In this lesson, you’ll learn about ways to combine those layers of video. You’ll still use clips on upper and lower tracks, but now they will become foreground and background elements in one blended composition.

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This title...

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...combines with this video...

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...to produce this composite image.

The blend might come from cropping part of the foreground image or from keying—selecting a specific color to become transparent—but whatever the method, the way you edit clips onto a sequence is the same as ever.

You’ll begin by learning about the important concept of alpha, which explains the way pixels are displayed, and then try several techniques.

1. Open Lesson 15.prproj in the Lesson 15 folder.

2. Switch to the Effects workspace by clicking Effects on the Workspaces panel or by choosing Window > Workspaces > Effects.

3. Reset the workspace by clicking the Effects menu on the Workspace panel and choosing Reset to Saved Layout or by choosing Window > Workspaces > Reset to Saved Layout.

What is an alpha channel?

Everything begins with cameras selectively recording the red, green, and blue parts of the light spectrum as separate color channels. Because each channel is monochrome (just one of the three colors), they are commonly described as grayscale.

Adobe Premiere Pro CC uses these three grayscale channels to produce the corresponding primary color channels. They are combined using what’s called primary additive color to create a complete RGB image. You see the three channels combined as full-color video.

Finally, there is a fourth grayscale channel: alpha.

The fourth channel defines no colors at all. Instead, it defines opacity—how visible the pixel is. Several different words are used in the world of post-production to describe this fourth channel, including visibility, transparency, mixer, and opacity. The name is not particularly important. What matters is that you can adjust the opacity of each pixel independently of its color.

Just as you might use color correction to adjust the amount of red in a clip, you can use Opacity controls to adjust the amount of alpha.

By default, the alpha channel, or opacity, of a typical camera footage clip is at 100%, or fully visible. On the 8-bit video scale of 0 to 255, this means it will be at 255. Clips that are animations or text or logo graphics will often have alpha channels that control which parts of an image are opaque or transparent.

You can set the Source Monitor and Program Monitor to display transparent pixels as a checkerboard, just as in Adobe Photoshop.

1. Open the file Theft_Unexpected.psd in the Source Monitor.

It looks as if the graphic has a black background, but those black pixels are displayed in place of transparency. Think of them as the background of the Source Monitor.

2. Click the Source Monitor Settings menu (Image) and choose Transparency Grid.

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Now you can clearly see which pixels are transparent. However, for some kinds of media, the transparency grid is an imperfect solution. In this case, for example, it can be a little difficult to see the edges of the text.

3. Click the Source Monitor Settings menu button and select Transparency Grid again to deselect it.

Making compositing part of your projects

The use of compositing effects and controls can take your post-production work to a whole new level. Compositing means creating new image compositions from existing ones. Once you begin working with the compositing effects available in Premiere Pro, you’ll find yourself discovering new ways of filming and new ways of structuring your edit to make it easier to blend images together.

It’s the combination of filming techniques and dedicated effects that produces the most powerful results when compositing. You can combine simple images of environments with complex, interesting patterns to produce extraordinary textured moods. Or, you can cut out parts of an image that don’t fit and replace them with something else.

Compositing is one of the most creative parts of nonlinear editing with Premiere Pro.

Shooting videos with compositing in mind

Much of the most effective compositing work begins when you are planning your production. Right at the start, you can think about how you can help Premiere Pro identify the parts of the image you’d like to be transparent. Premiere Pro has a limited number of ways of identifying which pixels you’d like to make transparent. Consider chromakey, for example, the standard special effect used to allow weather reporters to appear in front of a map.

The weather reporter is actually standing in front of a screen that is solid green. Special-effects technology uses the green color to identify which pixels should be transparent. The video image of the weather reporter is used as the foreground of a composition, with some visible pixels (the reporter) and some transparent pixels (the green background).

Next, it’s just a question of putting the foreground video image in front of another background image. In a weather report, it’s a map, but it could just as easily be any other image.

Planning ahead can make a big difference to the quality of your compositing. For that greenscreen to work well, it needs to be a consistent color. It also needs to be a color that does not appear anywhere on your subject. Green-colored jewelry, for example, might turn transparent when the key effect is applied.

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This...

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...combined with this...

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...becomes this.

If you’re shooting greenscreen footage, the way you film can make a big difference to the finished result. Be sure to capture the background with soft light and try to avoid spill, where light reflected from the greenscreen bounces onto your subject. If this happens, you’ll be in danger of keying out, or making transparent, parts of your subject.

Essential terminology

In this lesson, you’ll encounter some terms that might be new to you. Let’s run through the important ones.

• Alpha/alpha channel: The fourth channel of information for each pixel. An alpha channel defines transparency for a pixel. It’s a separate grayscale channel, and it can be created entirely independently of the content of the image.

• Key/keying: The process of selectively making pixels transparent based on their color or brightness. The Chromakey effect uses color to generate transparency (that is, to change the alpha channel), and the LumaKey effect uses brightness.

• Opacity: The word used to describe the overall alpha channel value for clips in a sequence in Premiere Pro. You can adjust the opacity for a clip over time using keyframes.

• Blend mode: A technology originally seen in Adobe Photoshop. Rather than simply placing foreground images in front of background images, you can select one of several different blend modes that cause the foreground to interact with the background. You might, for example, choose to view only pixels that are brighter than the background or to apply only the color information from the foreground clip to the background. Experimentation is often the best way to learn about blend modes.

• Greenscreen: The common term that describes the overall process of filming a subject in front of a screen that is solid green and then using a special effect to selectively turn green pixels transparent by creating an alpha matte based on the color background. The clip is then composited over a background image. A weather report is a good example of greenscreen.

• Matte: An image, shape, or video clip used to identify a region of your image that should be transparent or semitransparent. Premiere Pro allows multiple types of mattes, and you’ll work with them in this lesson.

Working with the Opacity effect

You can adjust the overall opacity of a clip using keyframes on the Timeline or in the Effect Controls panel.

1. Open the sequence Desert Jacket. This sequence has a foreground image of a man in a jacket, with a background image of a desert.

2. Increase the height of the Video 2 track a little by hovering the mouse cursor over the track header and scrolling or by dragging the top of the track header upward.

3. Click the Timeline Settings menu and make sure the option to display video keyframes is enabled.

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4. Now you can use the clip rubber band to adjust the settings and keyframe any effect you apply to a clip. Since the fixed effects include Opacity, this option is automatically available. In fact, it’s the default option, which means that the rubber band already represents clip opacity. Try dragging the rubber band up and down using the Selection tool on the clip on Video 2.

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In this example, the foreground is set to 63% opacity.


Image Tip

When adjusting the rubber band, after you begin dragging, you can hold Control (Windows) or Command (Mac OS) for fine control. Be careful not to hold the modifier key before clicking or you’ll add a keyframe.


When you use the Selection tool in this way, the rubber band is moved without additional keyframes being added.

Keyframing opacity

Keyframing opacity on the Timeline is almost the same as keyframing volume. You use the same tools and keyboard shortcuts, and the results are likely to be exactly what you expect: The higher the rubber band, the more visible a clip will be.

1. Open the Theft Unexpected sequence in the Sequences bin.

This sequence has a title in the foreground, on track Video 2. It’s common to fade titles up and down at different times and with different durations. You can do so using a transition effect, just as you would add a transition to a video clip; or, for more control, you can use keyframes to adjust the opacity.

2. Make sure track Video 2 is expanded so you can see the rubber band for the foreground title, Theft_Unexpected.psd.

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3. Control-click (Windows) or Command-click (Mac OS) the rubber band for the title graphic to add four keyframes—two near the beginning and two near the end.

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Image Tip

It’s often easier to add the keyframe markers to the rubber band first and then drag them to adjust them.


4. Adjust the keyframes so they represent a fade-up and a fade-down in the same way that you would adjust audio keyframes to adjust volume. Play the sequence, and watch the results of your keyframing.

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Image Tip

Once you’ve added a keyframe with Control (Windows) or Command (Mac OS), you can release the key and start dragging with the mouse to set the keyframe position.


You can also use the Effect Controls panel to add keyframes to the opacity for a clip. Like the audio volume keyframes controls, the Opacity setting has keyframing turned on by default in the Effect Controls panel. For this reason, if you want to make a flat level adjustment to the overall opacity of a clip, it’s sometimes quicker to do it on the Timeline rather than in the Effect Controls panel.

Combining layers based on a blend mode

Blend modes are special ways for foreground pixels to combine with background pixels. Each blend mode applies a different calculation to combine the foreground red, green, blue, and alpha (RGBA) values with those of the background. Each pixel is calculated in combination with the pixel directly behind it.

The default blend mode is called Normal. In this mode, the foreground image has a uniform alpha channel value across the entire image. The more opacity the foreground image has, the more strongly you will see those pixels in front of the pixels in the background.

The best way to find out how blend modes work is to try them.

1. Replace the current title in the Theft Unexpected sequence with the more complex title Theft_Unexpected_Layered.psd in the Graphics bin.

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You can replace the existing title by dragging and dropping the new item onto it while holding Alt (Windows) or Option (Mac OS). Replacing a clip this way retains the Timeline clip’s keyframes.

2. Select the new title on the Timeline and take a look at the Effect Controls panel.

3. In the Effect Controls panel, expand the Opacity controls and browse through the Blend Mode option.

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4. Right now, the blend mode is set to Normal. Try a few different options to see the results. Each blend mode calculates the relationship between the foreground layer pixels and the background pixels differently. See Premiere Pro Help for a description of the blend modes.

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In this example, the graphic has the Screen blend mode.


Image Tip

Hover the mouse cursor over the Blend Mode menu and scroll to quickly browse through the modes.


Working with alpha-channel transparencies

Many types of media will already have varying alpha channel levels for pixels. A title is an obvious example: Where text exists, pixels have 100% opacity, and where there is no text, pixels usually have 0% opacity. Elements such as drop shadows behind text typically have a value somewhere in between. Keeping some transparency in a drop shadow helps it look a bit more realistic.

Premiere Pro sees pixels with higher values in the alpha channel as being more visible. This is the most common way to interpret alpha channels, but occasionally you might come across media that is configured in the opposite way. You will immediately recognize the problem because you’ll see a cutout in an otherwise black image. This is easy to address because, just as Premiere Pro can interpret the audio channels on a clip, it’s also possible to choose a different interpretation of an alpha channel.

You can see the results using a title in the Theft Unexpected sequence.

1. Locate Theft_Unexpected_Layered.psd in your project.

2. Right-click the clip and choose Modify > Interpret Footage. At the bottom of the Modify Clip dialog, you’ll find the Alpha Channel interpretation options.

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3. The first options, relating to premultiplication, relate to the way semitransparent areas are interpreted. If you find that soft semitransparent image areas are blocky or poorly rendered, try selecting this option and view the results.

4. Try ignoring the alpha channel and then inverting the alpha channel. Observe the results in the Program Monitor. You will need to click OK before the display will update.

The options are as follows:

• Ignore Alpha Channel: Treats all pixels as having 100% alpha. This can be useful if you don’t intend to use a background layer in your sequence.

• Invert Alpha Channel: Reverses the alpha channel for every pixel in the clip. This means that pixels that were fully opaque will become fully transparent, and pixels that were transparent will become opaque.

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It’s easy to spot when there’s an issue with the alpha channel.

Color keying a greenscreen shot

When you change the opacity level of a clip using the rubber band or the Effect Controls panel, you adjust the alpha for every pixel in the image by the same amount. There are also ways to selectively adjust the alpha for pixels, based on their position on the screen, their brightness, or their color.

Chromakey effects adjust the opacity for a range of pixels based on their specific luminance, hue, and saturation values. The principle is quite simple: You select a color or range of colors, and the more similar a pixel is to the selection, the more transparent it becomes. The more closely a pixel matches the selection, the more its alpha channel value is lowered, until it becomes fully transparent.

Let’s make a chromakey composition.

1. Drag the clip Timekeeping.mov, in the Greenscreen bin, onto the New Item button menu in the Project panel. This creates a sequence that matches the media perfectly, with the clip on Video 1.

2. In the sequence, drag the Timekeeping.mov clip up to Video 2—this will be the foreground.

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3. Drag the clip Seattle_Skyline_Still.tga directly from the Shots bin to track Video 1, under the Timekeeping.mov clip on the Timeline.

Because this is a single-frame graphic, its default duration is too short.

4. Trim the Seattle_Skyline_Still.tga clip so that it’s long enough to be a background for the full duration of the foreground clip on Video 2.

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Image Note

There’s no special secret to creating multilayered compositions in Premiere Pro. Place clips on multiple tracks, knowing that clips on upper tracks will appear in front of clips on lower tracks.


5. In the Project panel, your sequence is still called after Timekeeping.mov and is stored in the same Greenscreen bin. Rename the sequence Seattle Skyline and move it into the Sequences bin.

You now have foreground and background clips. All that remains is to make the green pixels transparent.

Preprocessing the footage

In a perfect world, every greenscreen clip you work with would have a flawless green background and nice, clean edges on your foreground elements. In reality, there are lots of reasons why you might be faced with less than perfect material.

Of course, there are always potential problems caused by poor lighting when the video is created. However, there’s a further problem caused by the way many video cameras store image information.

Because your eyes do not register color as accurately as they do brightness information, it’s common for cameras to reduce the amount of color information stored.

Camera systems achieve reductions in file size using this system of reduced color capture, and the approach varies from system to system. Sometimes color information is stored for every other pixel; other times it might be recorded for every other pixel on every second line. Whatever the system, it’s going to make keying more difficult because there simply isn’t as much color detail as you’d like.

If you find that your footage is not keying well, try the following:

• Consider applying a light blur effect before keying. This blends pixel detail, softening the edges and often giving a smoother-looking result. If the amount of blur is light, it should not dramatically reduce the quality of your image. You can simply apply a blur effect to the clip, adjust the settings, and then apply a chromakey effect on top. The chromakey effect will work on the clip because it appears after being blurred.

• Consider color correcting your shot before you key it. If your shot lacks good contrast between your foreground and background, you can sometimes help the key by adjusting the picture first with an effect like the Three-Way Color Corrector or the Fast Color Corrector.

Using the Ultra Key effect

Premiere Pro has a powerful, fast, and intuitive chromakey effect called Ultra Key. The workflow is simple: Choose a color you want to become transparent and then adjust settings to suit. The Ultra Key effect, like every greenscreen keyer, dynamically generates a matte (defining which pixels should be transparent) based on the color selection. The matte is adjustable using the detailed settings of the Ultra Key effect.

1. Apply the Ultra Key effect to the Timekeeping.mov clip in the new Seattle Skyline sequence. You can find the effect easily by typing Ultra in the Effects panel search box.

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2. In the Effect Controls panel, select the Key Color eyedropper. Use the Key Color eyedropper (Image) to click a green area in the Program Monitor. This clip has a consistent green background, so it’s not too important where you click. With other footage, you may need to experiment to find the right spot.

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Image Tip

If you hold Control (Windows) or Command (Mac OS) when you click with the eyedropper, Premiere Pro takes a 5×5 pixel sample average, rather than a single-pixel selection. This often captures a better color for keying.


The Ultra Key effect identifies all pixels that have the green you selected and sets their alpha to 0%.

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3. In the Effect Controls panel, change the Output setting for the Ultra Key effect to Alpha Channel. In this mode, the Ultra Key effect displays the alpha channel as a grayscale image, where dark pixels will be transparent and light pixels will be opaque.

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It’s a pretty good key, but there are a few areas of gray where the pixels will be partially transparent, which you don’t want. The right and left sides don’t have any green, so none of those pixels can be keyed. You’ll fix that later.

4. In the Effect Controls panel, change the Setting menu for the Ultra Key effect to Aggressive. This cleans up the selection a little. Scrub through the shot to see whether it has clean black areas and white areas. If you see gray pixels in this view where there should not be, the result will be partially transparent parts in the picture.

5. Switch the Output setting back to Composite to see the result.

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The Aggressive mode works better for this clip. The Default, Relaxed, and Aggressive modes modify the Matte Generation, Matte Cleanup, and Spill Suppression settings. You can also modify manually to get a better key with more challenging footage.

Here’s an overview of the settings:

• Matte Generation: Once you’ve chosen your key color, the Matte Generation category of controls change the way it’s interpreted. You’ll often get positive results with more challenging footage just by adjusting these settings.

• Matte Cleanup: Once your matte is defined, you can use these controls to adjust it. Choke shrinks the matte, which is helpful if your key selection misses some edges. Be careful not to choke the matte too much because you’ll begin to lose edge detail in the foreground image, often supplying a “digital haircut” in the vernacular of the visual-effects industry. The Soften setting applies a blur to the matte, which often improves the apparent “blending” of the foreground and background images for a more convincing composite. Contrast increases the contrast of the alpha channel, making that black-and-white image a stronger black-and-white version and more clearly defining the key. You will often get cleaner keys by increasing the contrast.

• Spill Suppression: Spill suppression compensates for color that bounces from the green background onto the subject. When this happens, the combination of the green background and the subject’s own colors are usually different enough that it does not cause parts of the subject to be keyed transparent. However, it does not look good when the edges of your subject are green. Spill suppression automatically compensates by adding color to the foreground element edges that are positioned opposite, on a color wheel, to the key color. For example, magenta is added when greenscreen keying, or yellow is added when bluescreen keying. This neutralizes the color “spill” in the same way that you’d fix a color cast.

For more information about each of these controls, see Premiere Pro Help.

The built-in Color Correction controls give you a quick and easy way to adjust the appearance of your foreground video to help it blend in with your background.

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Image Note

In this example, you’re using footage with a green background. It is also possible you’ll have footage with a blue background for keying. The workflow is the same.


Often, these three controls are enough to make a more natural match. Note that these adjustments are applied after the key, so you won’t cause problems for your key by adjusting the colors with these controls.

Masking clips

The Ultra Key effect generates a matte dynamically, based on the colors in your shot. You can also create your own custom matte or use another clip as the basis for a matte.

When you create your own matte, you’ll use the mask feature applied to the Opacity settings for your clip. Let’s create a matte to remove the edges from the Timekeeping.mov clip.

1. Return to the Seattle Skyline sequence.

The foreground clip has an actor standing in front of a greenscreen, but the screen does not reach the edge of the picture. It’s common to shoot greenscreen footage this way, with a background that does not reach the edges of the shot, particularly when filming on location where full studio facilities may not be available.

2. Disable the Ultra Key effect, without removing it, by clicking the Toggle Effect button (Image) in the Effect Controls panel. This allows you to clearly see the green areas of the picture again.

3. Still in the Effect Controls panel, expand the Opacity controls and click the Create 4-point Polygon Mask button (Image).

A mask is applied to the clip, making most of the image transparent.

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4. Resize the mask so that it reveals the central area of the shot but hides the black edges. You will almost certainly need to reduce the Program Monitor zoom to 25% to see beyond the edges of the image. You can click directly in the Program Monitor to reposition the corner control points for the mask.

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Image Note

The mask in the example extends beyond the edge of the image. This is fine—the main goal is to choose what you will exclude. In this case, the curtain is successfully excluded.



Image Tip

If you deselect the mask, the control points displayed in the Program Monitor will disappear. Select the mask in the Effect Controls panel to re-activate them.


5. Set the Program Monitor zoom option to Fit.

6. Toggle the Ultra Key effect back on in the Effect Controls panel, and deselect the clip to remove the visible garbage mask handles.

The result is a clean key.

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Using mattes that use graphics or other clips

Adding a mask to the Opacity settings in the Effect Controls panel sets user-defined regions that should be visible or transparent. Premiere Pro can also use another clip as a reference for a matte.

Using the Track Matte Key effect, Premiere Pro uses the luminance information or alpha channel information from one clip to define a transparency matte for another clip. With a little planning and preparation, this simple effect can produce powerful results.


Image Note

There’s no Video 4 track at the moment, but that doesn’t matter. You can drag the clip from the Project panel onto the black space in the Timeline above the Video 3 track, and Premiere Pro will automatically create a new Video track for the clip.


Using the Track Matte Key effect

Let’s use the Track Matte Key effect to add a layered title to the Seattle Skyline sequence.

1. Edit the clip Laura_06.mp4, from the Shots bin, onto the V3 track.

2. Drag the title clip SEATTLE from the Graphics bin onto the Timeline V4 track, directly above the Laura_06.mp4 clip.

3. Trim the Seattle graphic clip to match the duration of the Laura_06.mp4 clip.

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4. Apply the Track Matte Key effect to the Laura_06.mp4 clip on the V3 track.

5. In the Effect Controls panel, set the Track Matte Key Matte menu to Video 4.

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Scrub through the sequence to see the result. The top clip is no longer visible. It’s being used as a guide to define the visible and transparent regions of the clip on V3.

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The Track Matte Key effect is an unusual effect because most other effects exclusively change the clip they are applied to. The Track Matte Key effect changes both the clip it’s applied to and the clip used as a reference.

The colors in the Laura_06.mp4 clip work well against the blue in the background clip, but they could be more vivid. Experiment with color correction tools to make the red stronger and brighter so it’s a more compelling composition.

You might also want to try adding a blur effect and changing the playback speed to create a softer, slower-moving texture.


Image Tip

In this example, you’re using a still image as a reference for the Track Matte Key effect. You can use any clip, though, including other video clips.



Using the After Effects Roto Brush tool

If you are feeling bold, here is an advanced workflow using Adobe After Effects to extend the compositing capabilities of Premiere Pro.

Premiere Pro allows complex mask shapes that will track clip contents. This is useful but not as powerful as Adobe After Effects, which can apply even more precisely designed masks, with advanced controls for positioning and tracking. The process of precisely selecting a foreground asset with a mask is called rotoscoping.

Adobe After Effects has a special Roto Brush tool that dramatically reduces the amount of work required to rotoscope a foreground image.

To send a sequence clip to After Effects and use the Roto Brush tool, follow these steps:

1. Right-click the clip you want to send to After Effects and choose Replace With After Effects Composition. Premiere Pro passes the clip to After Effects, where an After Effects composition is automatically created.

2. You will need to give the new After Effects project a name and location. Consider saving the project in a subfolder with your Premiere Pro project to make relocating it easier in the future. If you already have an After Effects project open, this will be used instead.

3. The clip is automatically added to a new composition in After Effects. Double-click the clip inside the Timeline to open it, ready for editing.

4. Like Premiere Pro, After Effects has a series of tools for different tasks. In the case of After Effects, they are positioned at the top of the screen by default. Select the Roto Brush tool.

5. Using the Roto Brush tool, draw over the foreground subject. There’s no need to carefully draw close to the edges. The Roto Brush tool will automatically seek and snap to the edges of your subject.

6. Click repeatedly with the Roto Brush tool to add to your selection. Alt-click (Windows) or Option-click (Mac OS) to remove image areas from your selection.

7. Just below the time ruler in the layer panel, there’s a range selection for the Roto Brush tool, shown as lighter gray. Drag the end of the range as if you were trimming to select the total duration you would like the Roto Brush tool to work on.

8. Press the spacebar. The Roto Brush tool will track the edges you selected, creating a mask that sets the pixels outside of the selection as transparent and the pixels inside the selection as visible.

If the Roto Brush tool loses the edge of the selection, press the spacebar to stop the analysis, use the Roto Brush tool to adjust the selection, and press the spacebar again to continue.

9. When the Roto Brush tool has finished analyzing the clip, you can save the After Effects project and return to Premiere Pro. The clip will now have transparent pixels outside the selected area, and you can use it as a foreground element in a composition.

The Roto Brush tool has effect controls, and you can access them in the Adobe After Effects Effect Controls panel that appears automatically when you use the tool.

Be sure to save the After Effects project when you have finished because Premiere Pro will need it to display the results of your mask.

After Effects includes an addition to the Roto Brush tool: the Refine Edge tool. Having created a mask with the Roto Brush tool, draw over the edge of the mask on difficult areas of the image for a finely calculated result.


Review questions

1. What is the difference between the RGB channels and the alpha channel?

2. How do you apply a blend mode to a clip?

3. How do you keyframe clip opacity?

4. How do you change the way a media file’s alpha channel is interpreted?

5. What does it mean to key a clip?

6. Are there any limits to the kinds of clips you can use as a reference for the Track Matte Key effect?

Review answers

1. The difference is that the RGB channels describe color information, whereas the alpha channel describes opacity.

2. Blend modes are under the Opacity category in the Effect Controls panel.

3. You adjust clip opacity in the same way you adjust clip volume, on the Timeline or in the Effect Controls panel. To make an adjustment on the Timeline, make sure you’re viewing the rubber band for the clip you want to adjust and then drag with the Selection tool. If you hold Control (Windows) or Command (Mac OS) while clicking, you’ll add keyframes. You can work with keyframes using the Pen tool.

4. Right-click the file and choose Modify > Interpret Footage. The Alpha Channel options are at the bottom of the panel.

5. A key is usually a special effect where the color or brightness of pixels is used to define which part of the image should be transparent and which part should be visible.

6. No. You can use just about anything to create your key with the Track Matte Key effect. In fact, you can even apply special effects to the reference clip, and the results of those effects will be reflected in the matte.