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Most of the time, the alpha channel will be stored in your movie document. You won’t have to think about where it is or go through an extra step to import it into your project. However, there might be times when you want to create a separate alpha channel movie for use in more complicated special effects.
There are a number of ways to create an alpha channel. Many programs—such as 3D rendering and animation programs—can create alpha channels automatically when they generate an image. Some editing and effects programs let you generate alpha channels from a key. For example, you could use a chroma key to pull an alpha channel matte from blue-screen footage. For certain effects, working with an alpha channel is easier than working with a key, and alpha channel mattes are fully editable—that is, you can go in and reshape them if you need to, unlike key effects.
Mixing SD and HD Footage
One of the most basic uses of mattes is letterboxing. Letterboxing is simply a black matte that covers part of the frame so that a widescreen 16:9 image fits onto an SD 4:3 monitor.
But combining SD & HD footage in the same project can be a little more complicated than just adding a matte. For starters, SD video is much smaller than HD video. The primary effect you’ll use to convert SD to HD or vice versa is the Resize effect (Avid) or the Motion effect (Premiere and Final Cut).
You have two choices: make the SD video bigger or make the HD video smaller. Most editors will opt for the first choice, but it really depends on how you shot your video. If your project is mostly HD, you should enlarge the SD video. If your project is mostly SD, you should scale down the HD video.
Sometimes, your editing software will make this choice for you. If you set up your project for widescreen HD video (like the Adobe Premiere project settings in Figure 17.3), when you import SD footage, it will retain its true size and appear very small in the monitor.
You have two choices for enlarging SD video (see Figure 17.23):
nEnlarge it so that the height fills the frame, leaving two black bars on the right and left of the frame. (Some choose to fill these empty parts of the screen with a background plate or graphic.)
nEnlarge it so that the width fills the frame, which means the top and bottom of the image is cropped. This might be less distracting but the SD image may not be well suited to widescreen framing and also you’ll have to enlarge it even more, resulting in an even more degraded image.
If you decide to scale down your HD video, you also have two options (see Figure 17.24):
nAdd a matte to letterbox it so that the whole HD image fits into the width of the SD frame.
nCrop the HD image so that it fills the full SD frame. Sometimes a technique called pan and scan is used to make sure that the most important parts of the image are visible.
None of these choices is ideal, but because so much of the media created in the last century has a 4:3 aspect ratio, the need to mix the two is not going away in the near future.
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Figure 17.23
A frame of SD video is much smaller than a frame of HD video (top). You can either enlarge it to fill the height (middle) or enlarge it to fit the width and crop it (bottom). Note that both options will induce video artifacts due to the extreme amount of size increase.
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Figure 17.24
A frame of HD video can be reduced to fit the width of the SD frame, aka letterboxing (top), or to fit the height of the SD frame and cropped (bottom).
In addition to resizing your video, you may need to change the frame rate and de-interlace your video.
Modifier Keys
Adobe Systems does a great job of creating identical keyboard commands and interfaces for their separate Mac and Windows products. However, to better understand the following tutorial, note that Cmd on the Mac is synonymous with Ctrl on a Windows computer, and Option on the Mac is synonymous with Alt on a Windows computer.
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Tutorial
Adding Camera Shake
In Chapter 4, “Choosing a Camera,” we told you to buy a camera with good optical image stabilization. Then we nagged you to use a tripod during your shoot. Now we’re going to risk sounding hypocritical and show you how to add a shaky camera effect to your footage. Why on earth would we want to do this?
Next time you see a big action/adventure movie, pay attention to how much the camera shakes when something explodes or when cars crash into each other. Adding a camera shake can greatly increase the sense of impact in a special effect or stunt shot.
Earlier in this chapter, we created a video of a finger firing a gunshot. We’re going to return to this clip now and add a tiny bit of camera shake to make the image more compelling. We’ll do this by animating the position of the clip to make it jitter up and down.
We recommend using Adobe After Effects for these types of effects, as its motion features are much easier to use, as well as more powerful, than many editing programs. A demo version of After Effects is available for download at the Adobe Web site. However, although After Effects is the ideal tool, you can use any editor or effects package that allows you to change and animate the position of a layer (see Figure 17.25).
STEP 1: CREATE A PROJECT
In After Effects, create a project, and import the hand-gun.mov file from the Camera Shake Tutorial folder located on the Chapter 17 page on the companion Web site. Create a new composition with the same size and duration as the movie. Place the movie in the composition.
STEP 2: ENABLE ANIMATION OF MOTION
In many programs, you must specify that you want to animate a particular property before you can set any keyframes for that property. This ensures that you don’t accidentally set keyframes for things that you don’t want to animate. In the After Effects Time Layout window, open the arrow next to the hand-gun.mov file. Now, open the Transform arrow. Next to each property is a small Stopwatch icon. Click the Stopwatch icon next to the Position property to tell After Effects that you will be setting keyframes for the position of this layer.
STEP 3: SET THE FIRST KEYFRAME
In the Time Layout window, scrub forward until you find the frame where the first muzzle flash occurs. When we clicked the Stopwatch icon in Step 2, After Effects automatically set a position keyframe at frame one. The current time marker is now at the first frame of muzzle flash. If we reposition the frame here, After Effects will automatically interpolate between the first frame and the current frame, creating a very slow movement of the image between those two frames. That’s not what we want. We want a sharp jolt from the first position to a new position.
Back up one frame and set a new keyframe by clicking in the check box at the extreme left of the Time Layout window, in the same row as the Position property. This will set a keyframe at our current position—one frame before the muzzle flash—and serve to “lock down” the position of the image until we want it to move.
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Figure 17.25
By setting keyframes for the “hand-gun” layer’s position, you can make it jitter up and down, resulting in the appearance of a shaky camera.
STEP 4: SET THE NEXT KEYFRAME
Now, move forward one frame. You should be back on the first frame of the first muzzle flash. Click on the image in the Comp window. You should see a bounding box and handles appear over the image. Press the Up arrow key three times to move the image up three pixels. Now click the Left arrow key twice to move the image two pixels to the left. Notice that After Effects automatically sets a keyframe for you—be sure to thank it.
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STEP 5: BRING THE IMAGE BACK DOWN
Think about how you might shake a camera when you are startled. The initial movement will be sharp and extreme. Going back to your original position will be slower. We don’t want a huge shake in this case because the gunfire is not a huge event. However, we still want a slower return to our original position.
Move forward one frame. Press the Down arrow twice and the Right arrow once. This will move your image most of the way back to its original position.
Now, move forward one more frame. Press the Down arrow once and the Right arrow once. This will return your image to its original location.
STEP 6: NOW DO THE REST OF THE FLASHES
Use the same technique to shake the camera during the rest of the flashes. The flashes have different intensities, so not all of the shakes need to be the same. In addition, move the frame in different directions and in different amounts so that each flash doesn’t look identical. You might also find that you want to put a second, smaller movement immediately after your first one. This will create a “bounce” as the camera returns to its original position.
When you’re done, render your movie and look at it. You might find that some shakes are too quick and need to be slowed down. You can click on a particular keyframe and reposition your image to adjust the degree of shake, and slide the keyframes around to adjust the timing of your camera shakes.
You’ll probably also notice that when we moved the frame, we exposed the empty black space that’s lying beneath it. If your final destination is video, this is not a problem, as we only moved the frame a few pixels. These black areas are at the far extreme of the action-safe area, so any monitor will crop out these areas.
If you’re going out to film, or distributing this movie on the Web, DVD, or film, then you’ll have a couple of options:
nPlace another copy of the original footage beneath the shaking copy. When the top copy moves, it will reveal the lower copy. Although the pixels aren’t necessarily accurate, they’re close enough that no one will notice.
nShrink your image size or enlarge your footage. Instead of the previous option, you can always just crop your frame to eliminate the black areas. If you’re going to the Web, you’re probably going to be reducing the size of your image anyway (usually to something like 320 240 or smaller). Therefore, even if you crop your image now, you’ll have plenty of pixels left to do a good resizing later. We’ll discuss resizing for Web output in detail in Chapter 18, “Finishing.” The other option is to enlarge your footage. For the small screen, you can probably get away with enlarging it 90–95% without noticeably degrading the image. For the big screen, enlarging more than a tiny amount is not recommended. For the Web, you can get away with enlarging it by 80% or more, depending on your material. T
Automatic Camera Shake
In addition to superior keying functions, After Effects comes with a number of powerful extra plugins, including special motion controls such as motion tracking (which lets you match the position of one layer to the movement of an image inside another layer). DigiEffects Damage also includes a special Destabilizer plug-in that can greatly ease the creation of camera shake effects.