Creating and Working with Mattes

On the surface, the matte is a straightforward concept. But confusing terms and applications can make using one counterintuitive.

By definition, mattes are color or grayscale images used to create transparent, opaque, and translucent areas in clips.

But that definition doesn't intuitively apply to the two mattes you've already encountered: the black alpha and white alpha mattes. They fall into the alpha channel transparency side of the compositing equation.

Then there's the difference matte, which I'll get to in a few minutes. It acts more like a Chroma key.

When is a matte really a matte? It can be bewildering.

Understanding Matte Basics

Mattes follow standard transparency rules: Black areas are transparent, white areas are opaque, and gray areas create varying levels of opacity.

If the matte is a color image, then Premiere removes the same level of color from the clip you are keying, thus creating an inverse image. So if you use a matte with green in it, the displayed image will look purple (red and blue).

You can create basic mattes using simple paint programs or you can use clips or images as mattes. You can use a Premiere video effect to convert a color image to a grayscale matte to avoid dealing with color inversions.

Most of the time mattes are simply black and white graphics. They work like scissors used on white paper. Any (black) holes cut into the paper lets that portion of the clip on Video 1 show through and combine with the clip above it on the timeline.

Task: Combine Clips with an Image Matte

Here's an easy way to see how mattes work:

  1. Either open a simple art program such as MS Paint or use Premiere's Title Designer.

  2. As I've demonstrated in Figure 15.23, create a simple shape and fill it in with solid black. Create another shape and fill it with gray.

    Figure 15.23. Use a basic paint program or Premiere's Title Designer to create simple shapes filled with black and gray

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  3. Save your matte or Title Designer file to the Scratch Disk file folder where you keep all your original video clips and give it a name such as Matte Test.

  4. Return to the timeline and place two different video clips or still images on Video 1 and Video 2.

  5. Open the Transparency Settings dialog box for the clip on Video 2 (remember, you can't apply transparency to a clip on Video 1).

  6. Select Image Matte in the Key Type drop-down list.

  7. Note the new button that appears with Image Matte selected?Choose. In Figure 15.24, I've highlighted it in the Transparency Settings dialog box's upper-left corner. That button becomes available for only Image Matte and Difference Matte.

    Figure 15.24. Combine your newly created matte with your superimposed image by selecting Image Matte in the Transparency Settings dialog box.

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  8. Click Choose. Then locate and select the graphic you just created. This combines that matte with the clip on Video 2, thus "carving" those holes, I mentioned earlier, into it.

  9. Preview the transparency by clicking the page peel icon. The black areas of your graphic should show the corresponding portion of the clip on Video 1. The white areas should show those portions of the clip on Video 2. The gray area should mix both clips.

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You can electronically swap black with white by selecting Reverse Key. That will swap transparent and opaque areas of the clip on Video 2, letting different portions of the clip on Video 1 show through.


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You can use image mattes to isolate portions of a clip as a means to highlight that section or to change its characteristics for special purposes. I'll explain that technique in Hour 16, "Tips, Tricks, and Techniques: Part 1."


Putting a Matte in Motion Using the Track Matte

This works much like an image matte but at first seems counterintuitive. It's called a track matte because you place the matte on a video track rather than apply it directly to a clip. It's counterintuitive because you place that matte above a clip on the timeline, so you'd think white areas in the matte would be opaque and block all lower tracks. But they don't because you tell the clip right below it that there's a matte on the next higher track and to treat that track matte as an image matte.

You place the matte on a separate video track to apply motion to it?that is, to move the matte (and its associated transparency regions) around the screen. You use a track matte to make a moving or traveling matte. It's a very cool way to follow action in a clip.

Just about any movie involving "impossible" motion?spaceships, Superman in flight, or giant spiders (such as the 1955 cult classic Tarantula, with Clint Eastwood in a minor supporting role as a pilot)?use traveling mattes.

Task: Work with a Track Matte

Try this:

  1. Create text using Title Designer or some other graphic tool. Make the text very large?80 points or so. You will use it to display part of a clip, so you want it large enough to make sure viewers see what you're up to.

  2. Save it and call it something like Track Matte Text.

  3. Use the same two clips from the previous task. Place the Track Matte Text file on Video 3 and drag its edges to fit the other two clips.

  4. Change Transparency Settings on the clip on Video 2 to Track Matte. As shown in Figure 15.25, you should see the text from Video 3 show up in the Sample window.

    Figure 15.25. Track Matte uses a matte (in this case the matte is text) on a higher video track and applies it to the clip below it.

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  5. Apply motion settings to the Track Matte Text file on Video 3. Keep it simple.

Preview your work. You should see the clip from Video 2 with text floating across it. Images from Video 1 should show through the text. You can reverse this process?that is, have Video 1 play with portions of Video 2 showing through the text?by going back to Transparency Settings for the clip on Video 2 and selecting Reverse Key.

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You can use a track/moving/traveling matte to highlight something moving within an image. For instance, you can single out a racecar from group of cars by placing a moving matte over it and changing its color. I'll show you how to do this in Hour 16.


Using a Split-Screen Matte

You've seen movies with the same actor playing multiple characters in the same scene. There are several ways to accomplish this. Using a split-screen matte is one. This is where assignment number five from Hour 14, "Compositing Part 1: Layering Images and Clips," comes into play.

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As a reminder, for this assignment you were to tape a distinct background with nothing moving in it?that is, no waving palm trees or soaring birds. Then, without turning off or moving the camera (this is one exercise where you need a rock-steady shot), have someone walk into the left side of the scene without crossing the center line, stand around for a while, then walk back out the left side. Have that person do the same thing entering from the right side and walking out to the right. For a bit of comic relief, have your "actor" wave toward the center of the scene before walking off camera.


Task: Create a Split-Screen Effect

To create a split-screen effect using your video track from Hour 14, follow these steps:

  1. Go back to your paint program. Build a black rectangle that covers the left or right half the screen. It doesn't have to be exactly half and half. Save it and call it something like Split Screen Matte.

  2. Place the assignment clip on Video 1.

  3. Trim the left side of the clip to a point just before your actor enters the scene. Drag the trimmed clip to the start of the timeline.

  4. Place the assignment clip on Video 2 and trim it to a point just before your actor enters the right side. Drag it to the start of the timeline.

  5. Open Transparency for the clip on Video 2 and select Image Matte.

  6. Click Choose and locate and select Split Screen Matte.

  7. Preview your split screen by click ing the page peel icon and dragging the slider. As shown in Figure 15.26, your actor should enter the scene from both sides, hang around, wave to his/her other self, and then walk off.

    Figure 15.26. Using a split-screen image matte lets you use the same actor playing two parts at once.

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There's a 50% chance that when you preview your split screen you will not see your actor on either side of the screen. That's because the black and white halves of your split screen are on the wrong sides. No worries. Either fix it the hard way by swapping clips (move the clip on Video 1 to Video 2 and vice versa) and then reapply the image matte or take the easy way out and click the Reverse Key check box.


Obviously this takes some planning. The actor can't cross the centerline, for instance. The lighting can't change from one shot to the next, and there can't be any movement in the middle of the scene that might fall on the centerline.

Using a Difference Matte

This "matte" behaves more like a key. As with a split-screen matte, the difference matte lets you tape an actor working in one spot in the scene and then tape the same or another actor working in another section at a different time. It doesn't have to be distinctly left and right sides. Also, you can have multiple actors.

Here's how it works: Using a locked-down camera to avoid any movement, you tape a scene with a static background and later create a still image?the difference matte?from a segment when no actors are in the scene. Then you apply that difference matte to video clips with action and use it to look for matching areas in order to remove the background. When completed, you'll have a collection of clips of just actors over black (transparent) backgrounds. Then you place the static background image on Video 1 and composite all the actors onto it.

If you've seen the Eddy Murphy film Dr. Doolittle 2 (hey, I'm the father of a seven year old), you've seen technology like this in action. In one scene they had a couple dozen animals all paying rapt attention to another animal. Those animals weren't all there at the same time. The production crew filmed them in several takes at different times and then combined them using technology like the difference matte.

Task: Work with a Difference Matte

Try it out by following these steps:

  1. You could use the clips already on the timeline, but it's easier to start fresh. Therefore, clear the timeline and place the assignment clip on Video 2 (not Video 1).

  2. Move the edit line to a section of the clip on Video 2 where there is no action taking place. You'll use that scene for your still image.

  3. To create a still image, go to the main menu and select File, Export Timeline, Frame. Navigate to the Scratch Disk file folder, name the image something such as Difference Matte, and click Save. The default settings should work fine.

  4. This should pop up a clip view window in your workspace. You don't want to place that image on your timeline just yet, so just close that window.

  5. Open the Transparency Settings dialog box for the clip on Video 2 and select Difference Matte.

  6. Click Choose and find and select your difference matte still image.

  7. You're going to use that still image to key out the static background. Check the Reverse Key box. Use the slider below the Sample screen to move into the clip far enough to see your actor enter the scene; then use the Similarity slider to fine-tune the removal of the background. In a perfect world the background would disappear, leaving only the actor. As you can see in Figure 15.27, glitches are common.

    Figure 15.27. Using a difference matte on two clips frequently creates jagged edges in the actor(s).

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  8. Place your assignment clip on Video 3 and trim it to the point where your actor enters from the other side of the scene.

  9. Apply the difference matte to it, use the slider beneath the Sample screen to show both actors at once, and use the Similarity slider to fine-tune the keying.

  10. Place the difference matte still image on Video 1 and drag its edges to make it long enough to match the action on the two clips above it.

Preview your project. You should see something akin to the split screen you worked on earlier. But you'll probably notice that this is far from a flawless process. Working with two clips means experimenting to find a compromise Similarity setting that allows the actors in both clips to look reasonably sharp.

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You can use a difference matte to place an actor or object over a different background. But that's really the wrong technology for that effect. If you tape actors in front of a static setting, you probably want to use that setting in the final cut. If you want to place your actors into a different background, you might as well tape your actors in front of a blue or green screen.


Difference mattes take a very controlled and simple background to pull off. In most cases you might want to stick to split screens or blue/green screens.

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One way to see a more accurate preview of any matte or key is to create a brightly colored matte (yes, the terms can be confusing) and temporarily place it on Video 1 instead of on your planned background clip. As a reminder, to create a solid-color matte, right/Option-click inside the Project window and select New, Color Matte. Select a color, name it, and save it. It'll appear in the Project window.


Using a Garbage Matte

The garbage matte is so named because you use it to get rid of garbage in your image. This is a very simple matte. You won't find it in the Transparency Settings drop-down menu. You create it by moving the four corners in the Transparency Settings dialog box's Sample screen.

I'll just show you an example. In one of my Chroma key shots there is some video noise around the edges of the frame. You can see it in Figure 15.28.

Figure 15.28. Use the garbage matte to remove noise or other unwanted items from the edges of your keyed clips.

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To replace that noise with a transparency, I simply drag those four screen corners to cover it up?or to cover up at least part of it. This is an inexact process because you can work only with four straight lines. You can't fine-tune it by grabbing a line midway between two points and adding a new "handle."

You also can't use keyframes to adjust the size and shape of this box over time. It works well with a blue/green screen shot where you have a boom microphone off to one side and you want to remove it from the scene.



    Part II: Enhancing Your Video
     
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