14 Exploring Compositing Techniques
In this lesson, you’ll learn how to do the following:
Use the alpha channel.
Use compositing techniques.
Work with opacity.
Work with a greenscreen.
This lesson will take about 60 minutes to complete. To get the lesson files used in this chapter, download them from the web page for this book at www.adobepress.com/PremiereCIB2020. For more information, see “Accessing the lesson files and Web Edition” in the Getting Started section at the beginning of this book.
Starting the lesson
Until now, you have been mainly working with single, whole-frame images. You have created edits in which 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 combined composition.
The combination of images might come from cropping part of the foreground image, masking, or keying—setting pixels with a specific color to become transparent—but whatever the method, the way you edit clips onto a sequence is the same as ever.
Let’s begin by learning about the important concept of alpha, which explains the way pixels are displayed, and then try several compositing techniques.
Open Lesson 14.prproj in the Lessons folder.
Save the project as Lesson 14 Working.prproj.
Switch to the Effects workspace by clicking Effects in the Workspaces panel or by choosing Window > Workspaces > Effects.
Reset the workspace by opening the Effects menu in the Workspaces panel and choosing Reset To Saved Layout, by choosing Window > Workspaces > Reset To Saved Layout, or by double-clicking the Effects workspace name in the Workspaces panel.
What is an alpha channel?
Cameras record the red, green, and blue parts of the light spectrum as separate color channels. Because each channel has a single color, the channels are commonly described as monochrome.
Adobe Premiere Pro uses these three monochromatic (single-color) channels to produce the corresponding primary color channels. They are combined using what’s called additive color to create a complete RGB image. You see the three channels combined as full-color video.
This approach of combining multiple monochromatic colors to produce a full color range is a little like combining two mono audio channels to produce a stereo mix.
Finally, there is a fourth monochromatic channel: alpha. The fourth channel defines no colors at all. Instead, it defines opacity—how visible a pixel is. Several different words are used 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 pixels independently of their colors because the alpha channel is separate from the color channels.
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 clips is 100%, or fully visible. On the 8-bit video scale of 0 to 255, this means it will be at 255. Not all media will include an alpha channel. Video cameras do not generally record an alpha channel, for example. Most codecs cannot store an alpha channel.
Original animation clips, text, or logo graphics will often include alpha channels to define 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 instead of black, just as in Adobe Photoshop. Let’s compare views:
From the Graphics bin, open the clip Theft_Unexpected.png in the Source Monitor (be sure to open the PNG clip).
Open the Source Monitor Settings menu , and make sure Transparency Grid is not selected.
It looks as if the graphic has a black background, but those black pixels are actually the background of the Source Monitor. If you were to export this as a file to a codec that does not support an alpha channel, the new file would indeed have a black background.
Open the Source Monitor Settings menu again and choose Transparency Grid to turn it on.
Now you can clearly see which pixels are transparent. However, for some kinds of media, the transparency grid is an imperfect solution, and this is a good example. It’s a little difficult to see the edges of the text against the grid.
Open the Source Monitor Settings menu, and choose Transparency Grid again to turn it off.
Media files that are created using a codec that does not support an alpha channel will always have a black background rather than transparent areas.
Making compositing part of your project
Using compositing effects and controls can take your post-production work to a whole new level. 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.
A combination of preproduction planning, filming techniques, and precise effects setup will produce the most powerful results when compositing. You can combine still 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, flexible, and fun parts of nonlinear editing with Premiere Pro.
Shooting videos with compositing in mind
Much of the most effective compositing work begins in pre-production, when you are planning your shoot. Right at the start, you can think about how to help Premiere Pro identify the parts of the image you’d like to be transparent, and there are a number of ways to do this. Consider chroma keying, for example, a standard special effect used by major feature film productions to allow action to take place in environments that would otherwise be too dangerous or physically impossible—like the inside of a volcano!
The actors are actually standing in front of a screen that is solid green. The green color is used to identify which pixels should be transparent. The video image of the actors is used as the foreground of a composition, with some visible pixels (the actors) 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 an epic action feature film, it’s the prebuilt set, a real-world location, or a composite created by visual effects artists; it could be anything.
Planning ahead makes a big difference to the quality of your compositing. For the greenscreen effect to work well, the background 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 a chroma key effect is applied.
When shooting greenscreen footage, the way you film can make a big difference to the finished result. Try to match the lighting for your subject to the replacement background you intend to use, in particular when considering the direction of shadows.
Capture the greenscreen background with soft, evenly distributed light, and try to avoid spill, where light reflected from the greenscreen in the background bounces onto your subject. If this happens, you’ll be in danger of keying out, or making transparent, parts of your subject because they will be in the same green color that you are removing.
Understanding 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 channel, and it can be created entirely independently of the content of the image. Whether or not your original media includes an alpha channel, you can work with it in sequences in Premiere Pro.
Key/keying: The process of selectively making pixels transparent based on their color or brightness. Chroma key effects use color as a reference to generate transparency (that is, to modify the alpha channel). Luma key effects use brightness.
Opacity: The overall alpha channel value for clips in a sequence in Premiere Pro. The higher the value, the more opaque the clip is—the inverse of transparency. You can adjust the opacity for a clip over time using keyframes, just as you have adjusted audio level in a previous lesson.
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 blend modes that cause the foreground to interact with the background in various ways. 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. You used a blend mode in Lesson 12, “Adding Video Effects.” Experimentation is a good way to learn about the many blend modes available. You’ll find them on the Effect Controls panel, under the Opacity effect.
For more information on blend modes in Adobe software, see The Hidden Power of Adobe Photoshop, by Scott Valentine (Adobe Press).
Greenscreening: The 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. The clip is then combined with a background image. An old-style TV weather report with a meteorologist in front of an animated map is a good example of a greenscreen effect.
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. You can use an image, another video clip, or a visual effect like a chroma key to generate a matte dynamically based on pixel colors.
When you made a secondary color adjustment in Lesson 13, “Applying Color Correction and Grading,” Premiere Pro dynamically generated a matte that was applied to the color adjustment based on the selection you made.
This limited the pixels that are changed by the color adjustment. A chroma key effect applies its matte to the alpha channel, selectively making pixels transparent.
Working with the Opacity effect
You can adjust the overall opacity of a clip using keyframes on the Timeline panel or in the Effect Controls panel.
If it’s not already open in the Timeline panel, open the sequence Desert Jacket. This sequence has a foreground image of a man in a jacket, with a background image of a desert.
You can do this by dragging the dividing line in the track header (at the far left of the Timeline panel) between Video 2 and Video 3. You can also hover the pointer over the Video 2 track header, hold Option (macOS) or Alt (Windows), and scroll your mouse wheel.
Open the Timeline panel Display Settings menu, and make sure Show Video Keyframes is enabled.
Now you can use the rubber band (the thin horizontal black-and-white line on clips) to adjust the settings and keyframe effects. There is only ever one rubber band per clip.
By default, the rubber band on visual clips controls the Opacity setting. The default for audio clips is the Volume setting.
If you want to specify that the rubber band adjust a different effect parameter, right-click the small fx badge in the upper-left corner of a clip, and choose an effect and parameter from the menu.
The fx badge changes color if an effect is applied to a clip or if a fixed effect is adjusted.
Try dragging the rubber band up and down using the Selection tool on the clip on Video 2. Try to set it to exactly 50%.
When adjusting the rubber band, after you begin dragging, you can hold Command (macOS) or Ctrl (Windows) for fine control. Be careful not to press the modifier key before clicking or you’ll add a keyframe instead.
When you use the Selection tool in this way, the rubber band is moved without additional keyframes being added.
Keyframing opacity in the Timeline panel is similar to keyframing volume. You use the same tools and keyboard shortcuts, and the results are likely to be exactly as you expect: The higher the rubber band, the more visible a clip will be.
Open the Theft Unexpected sequence in the Sequences bin.
This sequence has a title in the foreground, on the V2 track. 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.
Make sure track Video 2 is expanded so you can see the rubber band for the foreground title, Theft_Unexpected.png.
Command-click (macOS) or Ctrl-click (Windows) the rubber band for the title graphic four times to add four keyframes—two near the beginning and two near the end. Don’t worry about the precise positions of the new keyframes, you’ll adjust them in a moment.
It’s often easier to add the keyframe markers to the rubber band to set the timing first and then drag them vertically to adjust them.
Adjust the keyframes so the title fades up and down, just as you would with audio keyframes.
Once you’ve added a keyframe by Command-clicking (macOS) or Ctrl-clicking (Windows), you can release the key and start dragging to set the keyframe position.
Play the sequence, and watch the results of your keyframing.
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. If you select the title clip in the Timeline panel, you’ll see that the keyframes you just added are also displayed in the Effect Controls panel.
Combining tracks using a blend mode
Blend modes are special ways for foreground pixels (in clips on upper tracks) to combine with background pixels (in clips below them). 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.
Replace the current title in the Theft Unexpected sequence with the more complex title Theft_Unexpected_Layered.psd in the Graphics bin.
You can replace the existing title by dragging the new item onto it while holding Option (macOS) or Alt (Windows). Notice that replacing a clip this way retains the sequence clip keyframes you added.
Select the new title in the sequence and take a look at the Effect Controls panel.
In the Effect Controls panel, expand the Opacity controls, and browse through the Blend Mode options.
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.
To quickly browse through the modes, hover the pointer over the Blend Mode menu, without clicking to open it, and scroll.
Try the Lighten blend mode. In this mode, a pixel is visible only if it is lighter than the pixel behind it.
Choose the Normal blend mode when you have finished experimenting.
Adjusting alpha channel transparencies
Many types of media will already have an alpha channel. A title graphic is an obvious example: Where text exists, pixels generally have 100% opacity, and where there is no text, pixels usually have 0% opacity. Elements such as drop shadows around text typically have a value somewhere in between. Keeping some transparency in a drop shadow helps it look a little 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.
Locate Theft_Unexpected_Layered.psd in the Project panel.
Right-click the clip and choose Modify > Interpret Footage. In the lower half of the Modify Clip dialog box, you’ll find the Alpha Channel interpretation options.
Alpha Premultiplication options: The Alpha Channel Premultiplication options relate to the way semitransparent areas are interpreted. If you find that soft semitransparent image areas are blocky or poorly rendered, try selecting Premultiplied Alpha and compare the results.
Ignore Alpha Channel: Treats all pixels as having 100% alpha, with no transparency. This can be useful if you don’t intend to use a background clip in your sequence and would prefer black pixels.
Invert Alpha Channel: Reverses the alpha channel. This means that pixels that were fully opaque will become fully transparent, and pixels that were transparent will become opaque.
Try selecting Ignore Alpha Channel, and click OK; then open the Modify Clip dialog box again, select Ignore Alpha Channel, and then choose Invert Alpha Channel. Compare the results in the Program Monitor.
Blend modes still apply when changing the interpretation of the alpha channel. For example, if you invert the alpha channel and use a blend mode like Lighten, the black background won’t be visible.
When you have finished experimenting, make sure the Alpha Channel option is set to Use Alpha Premultiplication From File. You can use Undo to restore the clip interpretation.
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.
Highly compressed media files will generally not give the same high-quality result as a RAW or lightly compressed file (such as ProRes 4:4:4:4) captured using a high-end camera.
Chroma key effects adjust the opacity for specific pixels, selected based on their luminance, hue, and saturation level. The principle is straightforward: Select a color or range of colors, and the more similar a pixel is to the selection, the more transparent it becomes. That is, the more closely a pixel matches the selection, the more its alpha channel value is lowered, until it becomes fully transparent.
Let’s create a chroma key composition.
Drag the clip Timekeeping.mov, in the Greenscreen bin, onto the New Item button in the Project panel. This creates a sequence that matches the media perfectly. The clip will be added to the Video 1 track.
To create a new sequence with matching settings, you can also right-click a clip in the Project panel and choose New Sequence From Clip. This works with multiple clips selected too.
In the sequence, drag the Timekeeping.mov clip up to Video 2—this will be the foreground.
Drag the clip Seattle_Skyline_Still.tga from the Shots bin to track Video 1, under the Timekeeping.mov clip on the Timeline panel.
Because this is a single-frame graphic, its default duration is too short.
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.
There’s no special secret to creating multi layered compositions in Premiere Pro. Place clips on multiple tracks, knowing that clips on upper tracks will appear in front of clips on lower tracks.
In the Project panel, your sequence is still named after the Timekeeping.mov clip, and it’s stored in the same Greenscreen bin. Rename the sequence Seattle Skyline, and drag it into the Sequences bin.
This kind of on-the-fly organization is worth the extra effort, as it helps you stay in control of your project.
You now have foreground and background clips. All that remains is to make the green pixels transparent.
Using the Ultra Key effect
Premiere Pro includes a powerful, fast, and intuitive chroma key effect called Ultra Key. To apply a key, you’ll choose a color that should be transparent and then adjust settings to improve the color selection.
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. Let’s try using the effect on the Timekeeping clip.
Apply the Ultra Key effect to the Timekeeping.mov clip in the new Seattle Skyline sequence. You can find the effect by typing Ultra in the Effects panel search box.
In the Effect Controls panel, click the Key Color eyedropper (not the color swatch) to select it.
Holding the Command key (macOS) or Ctrl key (Windows), use the eyedropper 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 best spot.
If you hold Command (macOS) or Ctrl (Windows) when you click with the eyedropper, Premiere Pro takes the average of a 5×5-pixel sample, 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%.
In the Effect Controls panel, under the Ultra Key effect, choose Alpha Channel from the Output menu. In this mode, the Ultra Key effect displays the alpha channel that has been generated as a grayscale image, where dark pixels will be transparent and light pixels will be opaque.
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. Where there’s naturally semitransparent detail, like hair or the soft edges of clothing, there should be some gray, and the right and left sides of the image don’t have any green, so none of those pixels can be keyed. We’ll fix that later. Still, the main areas of the alpha channel should be solid black or white.
In the Effect Controls panel, under the Ultra Key effect, choose Aggressive from the Setting menu. 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, the result will be partially transparent parts in the picture. This might be desirable if the image area should be partially transparent. Expect to switch back and forth between viewing the composite image and the alpha channel often when setting up the Ultra Key effect.
Choose Composite from the Output menu to see the result.
The Aggressive setting works better for this clip. The Default, Relaxed, and Aggressive settings modify the Matte Generation, Matte Cleanup, and Spill Suppression settings. You can also modify these manually to get a better key with more challenging footage.
Here’s an overview of those settings:
Matte Generation: Once you’ve chosen your key color, the controls in the Matte Generation category change the way it’s interpreted. You’ll often get positive results with more challenging footage just by adjusting these settings.
In the example you are working on, there are issues with the original media, particularly around the edge of the subject. These are more visible during fast motion, such as when the jacket is moving quickly. Try matching the settings in the following figure.
When adjusting settings, try dragging each control to its extremes to discover the best configuration possible. You will often find you will need to re-adjust a parameter as you finesse the 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.
Soften 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.
Mid Point is the level from which contrast is adjusted—a kind of anchor point for contrast. Adjust contrast around a different level for more subtle control.
Spill Suppression: Spill Suppression compensates for color that bounces from the green background onto the subject. When this happens, the reflected green color does not affect the subject’s own colors enough to cause them to be keyed out (made 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.
Color Correction: 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 more naturally. For more comprehensive color adjustment controls, use the Lumetri Color panel.
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.
These three color adjustment controls are often enough to make a more natural match. The adjustments are applied after the key, so you won’t cause problems for the key by adjusting the colors with these controls. You can use any color adjustment tools in Premiere Pro, including the Lumetri Color panel.
Partially 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.
Earlier, you used a mask to constrain an effect to a region in the image. The Opacity setting can be combined with a mask in the same way, allowing you to set regions in an image that should be transparent with great precision.
Let’s create a matte to remove the edges from the Timekeeping.mov clip.
Return to the Seattle Skyline sequence.
As you discovered earlier, 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, particularly when filming on location, where full studio facilities may not be available.
Disable the Ultra Key effect, without removing it, by clicking the Toggle Effect button in the Effect Controls panel. This allows you to clearly see the green areas of the picture again.
Still in the Effect Controls panel, expand the Opacity controls and click the Create 4-Point Polygon Mask button just under that heading.
A mask is applied to the clip’s opacity setting, making most of the image transparent.
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 50% or 25% to see beyond the edges of the image.
As long as you have the mask selected in the Effect Controls panel, you can click directly in the Program Monitor to reposition the corner control points for the mask. Don’t worry about precisely matching the edges of the frame. Instead, focus on selecting the areas in the picture the subject moves into.
If you deselect the mask, the control points displayed in the Program Monitor will disappear. Select the mask in the Effect Controls panel to reactivate them.
Set the Program Monitor zoom option to Fit.
A rough mask of this kind, used to remove unwanted image elements, is often referred to as a garbage matte.
Toggle the Ultra Key effect back on in the Effect Controls panel, and deselect the clip to hide the visible mask handles.
This is particularly challenging footage to key because the original camera recording includes adjustments to the edges of the subject, and it uses a color compression system that reduces color fidelity. With patience and precise adjustments, you can still achieve a reasonable result.
Using the Track Matte key effect
Adding a mask to the Opacity effect 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.
The Track Matte Key effect uses the luminance information or alpha channel information from any clips on a selected track to define a transparency matte for a clip on another track. With a little planning and preparation, this simple effect can produce powerful results because you can use any clips as a reference and even apply effects to them, changing the resulting matte.
Let’s use the Track Matte Key effect to add a layered title to the Seattle Skyline sequence.
Trim the Seattle_Skyline_Still.tga clip longer so it can be used as the background for another foreground clip.
Edit the clip Laura_06.mp4, from the Shots bin, onto the Video 2 track, toward the end of the background clip.
Drag the graphic clip SEATTLE from the Graphics bin onto the Timeline V3 track, directly above the Laura_06.mp4 clip.
Trim the SEATTLE graphic clip to match the duration of the Laura_06.mp4 clip.
Find the Track Matte Key effect in the Effects panel, and apply it to the Laura_06.mp4 clip on the Video 2 track.
In the Effect Controls panel, choose Video 3 from the matte menu in the Track Matte Key controls. Any clips on the selected track will be used as the reference for the newly generated matte.
Scrub through the sequence to see the result. The white text in the title clip on Video 3 is no longer visible. Instead, it’s being used as a guide to define the visible and transparent regions of the clip on V3.
By default, the Track Matte Key effect uses the alpha channel from clips on the selected track to generate a key. If your reference clips don’t use an alpha channel, change the Composite Using menu to Matte Luma, and the brightness of the reference clips will be used instead.
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, which is made transparent.
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.
The colors in the Laura_06.mp4 clip work well against the blue in the background clip, but they could be more vivid. You might want to experiment with color correction tools to make the red stronger and brighter so it’s a more compelling composition.
You could animate the SEATTLE title, having it move on-screen or gradually increase in size.
You could also add a blur effect to the Laura_06.mp4 clip and change the playback speed to create a softer, slower-moving texture.
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?
1. RGB channels contain color information; the alpha channel describes opacity.
2. Choose a blend mode from the Blend Mode menu in the Opacity category in the Effect Controls panel.
3. You adjust clip opacity in the same way you adjust clip volume, in the Timeline panel or in the Effect Controls panel. To make an adjustment in the Timeline panel, make sure you’re viewing the Opacity rubber band for the clip you want to adjust and then drag with the Selection tool. If you hold Command (macOS) or Ctrl (Windows) while clicking, you’ll add keyframes. You can also work with keyframes using the Pen tool.
4. Right-click the file in the Project panel and choose Modify > Interpret Footage.
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. You can use just about anything to create your key with the Track Matte Key effect, as long as it is positioned on a track above the clip you apply the effect to. You can apply special effects to the reference clip, and the results of those effects will be reflected in the matte. You can even use multiple clips because the setting is based on the whole track, rather than a particular clip.