by Ben Bardens

Materials for This Lesson | Complete Tutorial

If you’ve spent any time watching TV in recent years, you have undoubtedly seen commercials where a car comes tearing around a corner, freeze frames for a beat and then blasts off into the distance. If you’ve wondered how you can create that effect in After Effects, the answer is time-remapping.

Figure 1. When you turn on time-remapping for a layer, the light-colored bar that represents the layer’s potential length is extended to the end of the composition.

Time-remapping is one of the program’s most versatile and powerful features. It enables a user to loop a clip or vary its playback speed or direction, all within a single layer, without having to make cuts, change in and out points or duplicate layers. In time-remapping, keyframes are used to set reference points within a source clip. The time between those reference points can then be stretched or compressed, slowing down or speeding up that portion of the clip. Time-remapping can be applied to video footage layers (QuickTime, AVI or image sequence formats), as well as audio layers and pre-composed (nested composition) layers.

In this article, I’ll start by discussing how After Effects handles time-remapping and what the basic options are, and then I’ll walk you through a simple step-by-step tutorial.

Enabling Time-Remapping

Before you can use time-remapping, you must enable the feature for the layer you want to affect. First, click on the layer in the timeline to select it, then go to the Layer menu and choose Enable Time Remapping. This creates a new property called Time Remap under that layer in the timeline. You can display the Time Remap property by selecting the layer and hitting the U key on the keyboard.

When time-remapping is first enabled for a layer, the results are not very obvious. In the case of audio or video layers with a set duration, the most noticeable difference is that the layer’s potential length extends to the end of the composition. In the timeline, this is represented by the light-colored bar behind the layer (Figure 1).

Enabling time-remapping for a layer does not affect how the layer plays back within your composition. Until you set and change keyframes for the Time Remap property or alter the layer’s in or out points, the footage will continue to play exactly as it did before you enabled time-remapping.

How to Use Time-Remapping

Time-remapping alters the relationship between the source footage and the timeline. Each keyframe’s value is a timecode that represents the original timecode location of a frame in the source clip. The keyframe’s position in the timeline determines when that source clip frame will actually play. By changing the distance between key-frames, you can change the speed at which the source clip plays back within the composition.

Figure 2. Three layers affected by time-remapping: Layer One plays back faster than normal, Layer Two plays back at the actual speed of the source clip, and Layer Three plays back slower.

When time-remapping is applied to a layer, After Effects automatically sets two keyframes for the Time Remap property — one at the layer’s in point, and one at its original out point.

In the example pictured in Figure 2, the same video source clip appears in three layers, each of which has time-remapping enabled. In each case, the in-point keyframe (the first frame of the source clip) plays at 00:00:00 and the out-point keyframe represents frame 00:00:20 in the original clip. As the out-point keyframe is moved in the timeline, the last frame of the source clip is remapped to a new position on the timeline, and the footage in between is condensed or stretched to fit the space in between. Layer Two plays back at the actual speed of the source clip, because the in-point and out-point keyframes for Time Remap have not been moved. Layer One plays back faster than the original clip, because the two keyframes have been brought closer together — the out point is mapped to a point earlier in time than where it occurs in the original clip. Layer Three plays back slower, because the keyframes have been spaced further apart.

Using More Than Two Keyframes

While this is an easy way to speed up or slow down a clip, time-remapping can do much more sophisticated adjustments. When working with Time Remap keyframes, you are not limited to the in-point and out-point keyframes that are set when you enable the property. You can create new keyframes for Time Remap using the same techniques you would for any other property.

Figure 3. To set a Time Remap keyframe, you can either check the set-keyframe switch in the A/V panel of the timeline or click on the Time Remap value and input a new frame number or timecode value.

Scrub through the clip by dragging the time marker until you locate a frame for which you wish to set a keyframe. (As you scrub through the timeline, you’ll notice that the Time Remap value changes. It represents the source clip frame that is playing at that timeline position.) You can create a keyframe by checking the set-keyframe switch in the A/V panel of the timeline or by changing the Time Remap value (Figure 3).

Once you have created additional key-frames for the Time Remap property, you can then slide them around. When you change their position under the timeline, you change how the footage plays within the composition, because you are telling those frames to play at a different moments in time. You can also change the value of an existing keyframe. This changes the point in the source clip that the keyframe refers to and remaps the time values between keyframes as needed.

Time Remap Exercise

In this example, we’ll use a 20-frame loop of an animated character’s walk cycle to illustrate how to apply and use the Time Remap property. We’ll set Time Remap keyframes to make the character’s walk loop forwards and backwards at various speeds, all within the same layer. To follow along, you'll need to download the sample files from the Guild website: www.editorsguild.com/aftereffects.

	1.	Open After Effects and create a new project (File > New > New Project).
	2.	Import the walk_L.mov file into your project. Notice in the project window that the footage is saved with “millions+” colors. The “+” means the file contains an alpha channel for compositing.
	3.	Under the Composition menu, choose New Composition.
	4.	In the Composition Settings window, choose Medium 320x240 from the presets pop-up menu. Specify a duration of three seconds. Click OK.
	5.	Drag the walk_L.mov file from the project window into the Timeline window.
	6.	Click on the layer to select it, then choose Enable Time Remapping from the Layer window.
	7.	Push the U key on the keyboard to reveal the Time Remap property in the timeline. The two keyframes representing the in and out points of the source clip will be visible. The values for these keyframes will be 00:00 and 00:20 respectively.
	8.	Go to File>Project Settings to open the Project Settings window.
	9.	For simplicity, change the Display Style from Timecode to Frames. Change the “Start frame numbering at” option from 0 to 1. Click OK.
	10.	The time display in your Comp window and Timeline will now be in frames instead of timecode. Click on either time display and type in 90. This will change the current time to frame 90, the last frame of the composition.
	11.	With the walk_L layer still selected, change the layer’s out point to the end of the composition by pushing Option-] (Alt-] in Windows).
	12.	Do a RAM preview by hitting zero on the numeric keypad or by hitting the RAM Preview button in the Time Controls window. The footage will play normally once through, then freeze on frame 20 until the end of the comp.
	13.	To make the footage loop several times and change direction, we’ll set some more keyframes. Change the current time to frame 20 and click on the keyframe switch in the A/V panel of the timeline window.
	14	Change the current time to frame 21 and change the value for the Time Remap property from 21 to 1. Press Return on the keyboard. This creates a keyframe that points to the first frame in the sequence.
	15.	Change the current time to frame 40. Click on the Time Remap Value and change it to 20. Preview your work and you’ll see that the walk cycle now loops twice.
	16.	We’ll now make the clip briefly reverse direction, so that it appears to “rewind” for a moment before continuing on. Change the current time to frame 50. Click on the value and type in 11. Push Return.
	17.	Change the current time to frame 60. Click on the value and change it to 20.
	18.	Change the current time to frame 61. Click on the value and change it to 1.
	19.	To make the character appear to walk faster toward the end of the composition, we will gradually decrease the number of frames between keyframes for the last two loops. Change the current time to frame 75. Click on the Time Remap Value and change it to 20.
	20.	Change the current time to frame 76. Click on the value and change it to 1.
	21.	Change the current time to frame 90. Click on the value and change it to 20.
	22.	RAM preview the entire composition.

Additional Ideas

Figure 4. Frame Blending lessens the stuttering caused by stretching a series of frames over a longer duration.

Time-remapping opens up a host of possibilities — the exercise in this article only scratches the surface. Here are a few more tips:

	•	You can reverse the positions of two keyframes under the timeline, which will cause the portion of footage between them to play backwards. To do so, select the keyframes you want (drag a box around them), then under the Animation menu, go to Keyframe Assistant and select Time Reverse.
	•	To freeze on a specific frame in the composition, use the Hold Keyframe function. Select the keyframe that represents the frame you wish to freeze on. From the Animation menu, choose Toggle Hold Keyframe.
	•	For additional control over how AE plays back source footage within the composition, change the Interpolation type for the keyframes. Select your Time Remap keyframes, then use the Keyframe Assistant to change them from regular linear keyframes to Easy Ease In or Easy Ease Out keyframes. This will create the effect of the footage slowing down going into or out of freeze-frames.
	•	When slowing down footage in After Effects, use Frame Blending to lessen the stuttering you get when stretching a limited number of frames over a longer duration. Check the frame blending switch for the video layer in the Switches panel and then enable frame blending by pushing the frame blending button at the top of the column (Figure 4).

Final Word

Keep in mind that in those flashy car ads, video compositing and effects treatments are combined with production techniques. Footage is typically shot at a higher frame rate so that there are more frames to time-remap in the compositing and effects stage.