Part Three

An Introduction to Adobe After Effects

Video Basics

by Ben Bardens

Materials for This Lesson | Complete Tutorial

In the last issue, I explored some basic animation capabilities of After Effects by moving still images to make an airplane seem to fly. That exercise introduced the concept of working with a layer-based timeline, where each piece of footage is in its own layer.

In this lesson, I’ll introduce the basics of working with video footage in the AE timeline: how to use in and out points for trimming, how to speed up and slow down footage and how to play material backwards. These are familiar tasks for editors, but AE implements them differently than some editing applications. Rather than offering a step-by-step tutorial as I did last time, I’ll explain some basic techniques for manipulating video and leave it to you to practice them with materials I’ve posted on the Guild web site. Click here to find a sample QuickTime movie of swimming fish and a project file in which the movie is manipulated —run backwards, sped up and slowed down.

Importing and Interpreting Footage

We import video footage exactly as we import still images, either by selecting Import from the

Figure 1. To import an image sequence using the Import/Footage File dialog box, put a check in the “Sequence” box under the File Type pop-up.
Figure 2. Check the specifications of a clip at the top of Project Window to make sure that AE is interpreting frame rate and other settings correctly.

File menu, or by simply dragging-and-dropping from the desktop. AE supports just about any video format. These can include ordinary video clips such as QuickTime movies, in which a single file contains all the frames of video, or image sequences such as Targa or SGI, in which each frame of video is a separate file. To import an image sequence, select Import Footage from the File menu and navigate to the file representing the first frame of the sequence. Select an image sequence format from the File Type pop-up, and select the “Sequence” check box (Figure 1). Note that for After Effects to import an image sequence properly, the images have to be numbered sequentially. An easier way to import image sequence files is by simply dragging the folder containing the sequence into the project window.

Regardless of what format or type of material you import into a project, it’s a good idea to check how AE is reading it before you start to work with it and place it into a composition. While AE is a smart program, it doesn’t always interpret frame rate and alpha channels correctly. You can check the settings assigned to your clip by selecting it in the Project Window and looking at the specifications next to the displayed thumbnail (see Figure 2).

If AE is interpreting the footage incorrectly, you can change its settings by selecting Interpret Footage from the File menu, and then choosing Main. You can then specify the frame rate of the footage, its field dominance and its type of alpha channel. You can even remove 3:2 pull-down from film transfers here. I’ll cover pulled-down and interlaced footage in more detail in a future column.

Creating a Composition and Choosing Settings

You are now ready to drop the footage into a timeline and start working with it. You can use your footage in any type of composition. The frame rate of the source clip and its resolution in pixels do not necessarily have to match the settings of the comp you create.

To create a new timeline, select New Composition from the Composition menu. When the Composition Settings dialog box appears, specify the editing time base (frame rate) and resolution (in pixel dimensions) for your composition. You’ll notice a pop-up menu of various

Figure 3. In the Composition Settings dialog box, you can set the frame size, pixel aspect ratio, frame rate, duration and resolution for your composition.

preset frame sizes, starting with the smallest (160x120) at the top, and going all the way to the largest (Cineon Full) at the bottom. You can also specify frame size manually by typing the dimensions numerically (see Figure 3).

After Effects makes a distinction between a native composition and the files that are rendered from it. You can render a file as many times as you want, at a different resolution each time, from film to video to web. But remember that while you can safely render down without losing quality, you cannot render a small comp up to a higher resolution without introducing artifacts and degrading the image. So if you plan to output your project in several different resolutions, make sure your comp is set to the highest resolution you will need.

Contrary to what you might think, all pixels are not created equal, and different output and tape formats employ different pixel aspect ratios. When you’re working in AE, I recommend that you always use a square pixel ratio in your comp settings. If you work with rectangular pixels, the image on your computer will appear squashed. If you work with this distorted image in the Comp Window, the effects you apply and animated elements you add might look different when the movie is rendered and displayed on a video screen, and features like motion sketch and motion tracking (which we’ll cover later in this series) can be affected as well. When working for television, I usually set up my comps to 768 x 576 with square pixels (Figure 3). This is the largest size in the TV and video settings group, and it will allow for the slightly higher-resolution European market and also facilitate rendering down for NTSC. If you are working exclusively for NTSC delivery, then I suggest 720x540 with square pixels. Either way, you’ll create rectangular pixels only when you do your final render.

For projects that will finish on NTSC video, I recommend working at 29.97 frames per second, drop frame. But because your comp settings don’t have to match your render settings, you can always change your frame rate when you render out the project. Once again, though, it’s always better to render down, to avoid artifacts, so set your comp frame rate to the highest possible setting you’ll need.

Adding Video Footage to the Composition

To add video to your comp, simply drag the footage from the Project Window into the Timeline or Comp Window. The footage will appear as a timeline layer, just like the still images we used in the last exercise. You can add the same piece of footage to the comp several times, and it will appear as a new layer each time it is added. Keep in mind that each layer has its own set of transform properties that you can animate. So if your clip is just a little shy of filling the screen, use the Scale property to size it up, or if you want to position it so that it shows only in the top right corner, simply drag the layer in the comp window to move it. To fade footage in or out, or create cross dissolves between layers, simply set some key frames for the Opacity property.

Figure 4. The Time Layout Window offers numerous ways to control the length and speed of a clip.

Set In and Out Points

When you drop footage into a comp, it appears in the Time Layout Window as a yellow bar with a little gray handle at each end representing the clip’s in and out points. This is different from still art, which appears as a pink bar that runs the full length of the comp. You can, of course, change the in and out points for both still image layers and video footage layers by sliding the little gray handles along the timeline (see Figure 4). You can also change in and out points in the Layer Window, which works like a source window in an editing environment. Double click on the layer name and your layer opens in the Layer Window. Move the blue time marker as needed and click the in or out point icons to set new start and end frames (see Figure 5). Or use keyboard shortcuts: Option-[ for set-in and Option-] for set-out.

Looping

AE offers several ways to loop a clip, with varying degrees of control. If all you want to do is loop the same clip over and over, select the material in the Project Window and then specify a

Figure 5. Double click on a layer name to open the Layer Window. Set new start and end frames by moving the time marker and clicking the in or out point icons.

loop value in the Interpret Footage dialog box (File/Interpret Footage/Main). You can extend the layer’s out point in the timeline, and the clip will repeat over and over until that out point, or until you have used up the number of loops specified. However, since you’ve applied looping to your original source clip, it will now loop wherever it is used in the composition. If you want continuous looping but also want the freedom to use the clip normally elsewhere, then import another copy of the clip, and turn on looping in the second copy.

Another way to loop footage is to place the same clip in the Timeline several times and offset each one so the footage loops. The advantage to this technique is that each iteration of the loop is on its own layer, giving you the ability to control the speed and direction individually. In the sample project provided, the clip loops four times (Figure 4).

Time Stretch and Frame Blending

In the sample project, the clip plays sped up, then plays backwards fast, then forward even faster, then backwards faster yet. To control the speed at which the layer plays, select the layer name, then from the Layer menu, choose Time Stretch. Input a value as a percentage. A smaller number will shorten the clip, speeding it up. A larger number will extend the clip, slowing it down. You can also type in a new duration in timecode. Of course, if you slow down footage too much, any motion in the clip will appear staggered. To reduce this, you can enable “frame blending” by checking the frame blending switch for the layer in question. This will cause AE to create new, interpolated frames and thus reduce motion artifacts. To see frame blending in previews you must also click the Frame Blending button at the top of the column, but this will slow your previews down (Figure 4).

Another way to access the Time Stretch dialog box is to expose the in, out, duration, and stretch columns that are hidden in the Timeline. Click the little triangle at the bottom of the window to show or hide these columns. If you click on a value for Time Stretch here, it will bring up the dialog box for that layer. (see Figure 6)

Figure 6. Click on the triangle at the bottom of the Time Layout Window to reveal in, out, duration, and stretch columns.

To make clips play backwards in your composition, simply enter a negative value for Time Stretch. When you give a layer a negative Time Stretch value, its in and out points reverse, and the layer changes position in the Timeline, so you’ll have to slide it over to reposition it in time. A useful keyboard shortcut when sliding layers or adjusting in/out points is to hold down the Shift key while dragging. This causes the layer to snap to in and out points in other layers. Layers playing in reverse have red candy-stripe lines through them to remind you that the in and out points have been reversed.

The Future...

Time Stretch is the most basic way to control video footage in the AE timeline. If you feel daring, you can also experiment with Time Remapping (also found under the Layer menu). This feature gives video layers a new property that allows you to set key frames representing frames in the footage clip. This allows you to slow down, gradually speed up, freeze frame, rapidly accelerate and reverse footage, all in a single layer. Those commercials where the cars go in slow motion toward a corner, then blast off on the way out are an example of what you can do with this function. I’ll cover it in depth in the future.

Next time I’ll explore the way AE “in-betweens” keyframes and show you how to create more sophisticated motion effects. In the next two issues, I’ll wrap up this introductory material and move into general tips and techniques. If you have questions that you’d like to see answered here, please let me know.