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Retiming Footage

Getting a shot to time out can be quite tricky. Sure, if your clients would pick the music and storyboard everything ahead of time, timing would be effortless. But that just never seems to happen. Even with good planning, there's always the human element, difference in performance, or even just gravity that seems to interfere. After Effects offers several ways to affect the timing and speed of clips. Knowing which one to choose goes a long way toward your success.

Frame Rate Conversion

The need to convert frame rates is quickly emerging as a top technical challenge. Gone are the simple days of Standard Definition video workflows. In their place are multiformat cameras and the many frame rates of High Definition (HD) video.

Conforming a Clip

An easy way to change the rate of a clip is to conform it. This can be done by changing how After Effects interprets the clip. Conforming changes the rate at which the frames are played back. The clip will be slowed down or sped up depending on the rate you choose. No frames are discarded with this method, nor are new ones created.

  1. Select the footage item in the Project panel and choose File > Interpret Footage > Main.
  2. Select the Conform To Frame Rate and enter a new frame rate for Frames Per Second (Figure 6.43). Double-check the rest of the settings. When you're satisfied, click OK.
    Figure 6.43

    Figure 6.43 The source footage has a frame rate of 23.976, whereas the intended frame rate is 29.97. This mismatch in footage versus composition frame rate can be resolved in several ways.

Resampling Footage

If you place a clip into a composition and their frame rates differ, After Effects will convert the footage to match. This means that the composition frame rate specifies how many times per second the original source clip is sampled. The difference between the source footage frame rate and the composition frame rate can affect how smoothly footage plays back.

A very common scenario is the need to convert 24p material to a different frame rate. For example, many people shoot 24p for the "film look." This rate is fine if you're outputting to the Web, DVD, Blu-ray, and of course film, but it doesn't work for broadcast. For television, you'll likely need to convert to 29.97 for NTSC or 25 for PAL.

Retiming is fairly straightforward. You can import individual clips, a self-contained movie of the finished edit, or a Premiere Pro sequence and place it into an After Effects composition. Set the composition frame rate to match your desired output (Figure 6.44).

Figure 6.44

Figure 6.44 You should try all three resampling methods to see the different effects on your clip. The method you choose may vary depending on the source material. Footage courtesy the National Foundation for Consumer Credit Counseling—www.DebtAdvice.org.

Once you have the footage placed into a composition, you can choose three different ways to process the speed change. These methods are listed from lowest to highest quality (as well as fastest to slowest render times). Click the Frame Blending switch in the Timeline to change methods (Figure 6.45):

Figure 6.45

Figure 6.45 The three frame blending methods are accessed with the Frame Blending switch.

  • No Frame Blending. If you leave the Frame Blending switch deselected, After Effects will simply repeat frames to pad out the clip (Figure 6.46).
    Figure 6.46

    Figure 6.46 Frames are duplicated to pad out the shot. Every fifth frame is repeated in a 24 to 29.97 frame rate conversion.

  • Frame Mix. The Frame Mix mode takes less time to render and produces results by mixing two frames together. The frames are overlaid on top of each other, and this creates a ghosted image but avoids the jerkiness caused by the No Frame Blending option (Figure 6.47).
    Figure 6.47

    Figure 6.47 You can see the two frames overlaid onto each other to create a new frame.

  • Pixel Motion. The Pixel Motion mode provides the best results. New frames are created by comparing surrounding frames and generating a new frame that essentially morphs in between (Figure 6.48).
    Figure 6.48

    Figure 6.48 Pixel Motion takes the longest, but the quality is worth it. Thanks to motion estimation, the new frames produced are very crisp.

Time Remapping

One of the most useful (and least used) commands in After Effects is Time Remapping. The command offers total control over playback speed—rewind, freeze, speed-up, and more. Time Remapping is very useful in motion graphics design because it lets you fluidly control the timing of elements so they can sync up to music or time-based cue points.

Many find Time Remapping difficult, so here's a simplified explanation. Imagine that you have a clip that is 10 seconds long. When you turn on Time Remapping for a layer, two keyframes are added. The first key frame has a value of 0;00;00;00 and the last is 0;00;09;29. If you were to drag the second keyframe closer to the first, the footage would play back quicker. If you stretched the two keyframes apart, it would play slower. Easy enough?

Here is where it gets a little tricky: You can add as many keyframes as you want. Those frames can have any value assigned to them that relates to a frame in the original clip. You can then create a variety of time-based effects.

Controlling Direction

With Time Remapping, you can use keyframes to create complex motion. With each keyframe you can move forward or backward through time (you can even repeat the value across two adjacent frames to create a freeze frame). The controls become fairly intuitive after you've spent about five minutes with them. The easiest way to understand how keyframes work is to drag left to right to scrub their values (Figure 6.49). Once the keyframes are set, experiment with expanding or contracting them to change speed.

Figure 6.49

Figure 6.49 Scrubbing the keyframe value makes it easy to choose the desired frame.

Let's give Time Remapping a try:

  1. Select a layer in the Timeline panel that you want to remap, and then choose Layer > Time > Enable Time Remapping.

    Two keyframes are added by default at the beginning of the layer and one at the end.

  2. Drag one of the existing frames to change the speed of a clip or add new keyframes and adjust to taste.
  3. To smooth out motion, enable frame blending for the composition, and then turn on Frame Mix or Pixel Motion for the layer.
  4. If needed, drag the layer's end point to extend (or trim) the layer. You can use the shortcut Option+] (Alt+]) to trim the Out point to the current time indicator.

Refining with the Graph Editor

Another way to work with Time Remapping is to use the Graph Editor. Add your keyframes in the standard Timeline view first, and then click the Graph Editor button to switch views (Figure 6.50).

Figure 6.50

Figure 6.50 The Graph Editor makes it easy to understand speed changes.

When you make adjustments in the Graph Editor, the change is represented in the steepness of the line. The steeper the incline, the faster the speed. If you reverse time, the graph will slope downhill. As you drag a keyframe, you'll see the Preview window and the time in the tool tip update.

Here are some general tips when working with the Graph Editor:

  • Dragging a keyframe down slows a layer's playback. (If the footage is reversed, drag up.)
  • Dragging a keyframe up speeds up the playback. (If the footage is reversed, drag down.)
  • To reverse playback, drag a keyframe lower than the previous keyframe.
  • To freeze playback, you can copy and paste a keyframe so it repeats. You can also choose Animation > Toggle Hold Keyframe.
  • To expand or contract keyframes, select all the time remapped keyframes in the Timeline. Hold down the Option (Alt) key and drag the last frame. The keyframes will expand or contract like an accordion.
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