Time-stretching, time-remapping, and the Timewarp effect are all useful for creating slow motion, fast motion, freeze frame, or other retiming results.
For information on the Timewarp effect, see Timewarp effect.
Andrew Kramer provides a video tutorial on his Video Copilot website that demonstrates time-stretching, time-remapping, and frame blending.
Speeding up or slowing down an entire layer by the same factor throughout is known as time-stretching. When you time-stretch a layer, the audio and the original frames in the footage (and all keyframes that belong to the layer) are redistributed along the new duration. Use this command only when you want the layer and all layer keyframes to change to the new duration.
If you time-stretch a layer so that the resulting frame rate is very different from the original frame rate, the quality of motion within the layer may suffer. For best results when time-remapping a layer, use the Timewarp effect.
When you time-stretch a layer, the positions of its keyframes stretch with it by default. You can circumvent this behavior by cutting and pasting keyframes.
When you reverse the direction at which a layer plays back, all keyframes for all properties on the selected layer also reverse order. The layer itself maintains its original In and Out points relative to the composition.
Note:
For best results, precompose the layer and then reverse the layer inside the precomposition. For more information on this process, see About precomposing and nesting.
You can select and reverse keyframes across multiple layers and properties, but each set of keyframes for a property is reversed only within its original time range and not that of any other selected property. Markers in the Timeline panel are not reversed, so you may need to move markers after reversing keyframes.
You can expand, compress, play backward, or freeze a portion of the duration of a layer using a process known as time-remapping. For example, if you are using footage of a person walking, you can play footage of the person moving forward, and then play a few frames backward to make the person retreat, and then play forward again to have the person resume walking. Time-remapping is good for combinations of slow motion, fast motion, and reverse motion.
The Timewarp effect provides similar features with more control over some aspects of frame blending, but with additional limitations as a result of being applied as an effect.
When you apply time-remapping to a layer containing audio and video, the audio and video remain synchronized. You can remap audio files to gradually decrease or increase the pitch, play audio backward, or create a warbled or scratchy sound. Still-image layers cannot be time-remapped.
You can remap time in either the Layer panel or the Graph Editor. Remapping video in one panel displays the results in both. Each provides a different view of the layer duration:
The Layer panel provides a visual reference of the frames you change, as well as the frame number. The panel displays the current-time indicator and a remap-time marker, which you move to select the frame you want to play at the current time.
The Graph Editor provides a view of the changes you specify over time by marking your changes with keyframes and a graph like the one displayed for other layer properties.
When remapping time in the Graph Editor, use the values represented in the Time Remap graph to determine and control which frame of the movie plays at which point in time. Each Time Remap keyframe has a time value associated with it that corresponds to a specific frame in the layer; this value is represented vertically on the Time Remap value graph. When you enable time remapping for a layer, After Effects adds a Time Remap keyframe at the start and end points of the layer. These initial Time Remap keyframes have vertical time values equal to their horizontal position on the timeline.
By setting additional Time Remap keyframes, you can create complex motion results. Each time you add a Time Remap keyframe, you create another point at which you can change the speed or direction of playback. As you move the keyframe up or down in the value graph, you adjust which frame of the video is set to play at the current time. After Effects then interpolates intermediate frames and plays the footage forward or backward from that point to the next Time Remap keyframe. In the value graph, reading from left to right, an upward angle indicates forward playback, while a downward angle indicates reverse playback. The amount of the upward or downward angle corresponds to the speed of playback.
Similarly, the value that appears next to the Time Remap property name indicates which frame plays at the current time. As you drag a value graph marker up or down, this value changes accordingly and a Time Remap keyframe is set, if necessary. You can click this value and type a new one, or drag the value to adjust it.
The original duration of the source footage may no longer be valid when remapping time, because parts of the layer no longer play at the original rate. If necessary, set a new duration for the layer before you remap time.
As with other layer properties, you can view the values of the Time Remap graph as either a value graph or a speed graph.
If you remap time and the resulting frame rate is very different from the original, the quality of motion within the layer may suffer. Apply frame blending to improve time remapping for slow motion or fast motion.
Note:
Use the information shown in the Info panel to guide you as you work with time-remapping. The ratio given in the units of seconds/sec indicates the current speed of playback—the number of seconds of the original layer being played for each second after time-remapping.
You can time-remap all or part of a layer to create many different results, such as freeze-frame or slow-motion results. (See Time-remapping.)
Time-remapping is enabled, and After Effects places a Hold keyframe is at the position of the current-time indicator to freeze the frame. If you previously enabled time-remapping on the layer, any keyframes you created are deleted when you apply the Freeze Frame command.
The portion of the layer between the first and second keyframes plays at an unaltered rate (the same as for the non-time-remapped layer), as does the portion of the layer between the third and fourth keyframes. The second and third keyframes are identical, so a single frozen frame plays during the time between those two keyframes.
Note:
To switch between Graph Editor mode and layer bar mode, press Shift+F3.
-
In the Graph Editor, drag the keyframe marker up or down, watching the Time Remap value as you drag. To snap to other keyframes, Shift-drag.
Dragging the keyframe down slows down the layer.
To slow the layer down, drag the keyframe down. (If the layer is playing in reverse, drag up.)
To speed the layer up, drag the keyframe up. (If the layer is playing in reverse, drag down.)
To play frames backward, drag the keyframe down to a value below the previous keyframe value.
To play frames forward, drag the keyframe up to a value above the previous keyframe value.
To freeze the previous keyframe, drag the current keyframe marker to a value equal to the previous keyframe value so that the graph line is flat. Another method is to select the keyframe and choose Animation > Toggle Hold Keyframe, and then add another keyframe where you want the motion to start again.
in
the keyframe navigator to add a keyframe.Note:
Before you move a time-remap keyframe, it’s a good idea to select all subsequent time-remap keyframes in the layer first. This selection will preserve the timing of the rest of the layer when you remap time for the current keyframe.
-
On the upper time ruler, the remap-time marker indicates the frame currently mapped to the time indicated on the lower time ruler. To display a different frame at the time indicated on the lower time ruler, move the remap-time marker accordingly.
Drag the remap-time marker to replace the frame at the current time marker.
-
Move the remap-time marker on the upper time ruler to the frame you want to display at the time indicated on the lower time ruler:
To move the preceding portion of the layer forward, set the remap-time marker to a later time than the current-time indicator.
To move the preceding portion of the layer backward, set the remap-time marker to an earlier time than the current-time indicator.
To freeze a frame, set the remap-time marker to the frame you want frozen. Then, move the current-time indicator (lower ruler) to the last point in time where the frame will appear frozen and move the remap-time marker again to the frame you want frozen.
The speed graph of the Time Remap property directly relates to the pitch of an audio file. By making subtle changes to the speed graph, you can create a variety of interesting effects. To avoid screeching audio, you may want to keep the Speed value below 200%. When the speed is too high, use the Levels controls, located under the Audio property, to control the volume.
You may hear clicks at the beginning and end of an audio (or an audio and video) layer after setting new In and Out points in the Time Remap graph. Use the Levels controls to remove these clicks.
Aharon Rabinowitz provides a tutorial on the Creative COW website that shows how to use time-remapping to do lip-synching. This same basic concept can be used for many kinds of character animation.
Robert Powers provides a video tutorial on the Slippery Rock NYC website that demonstrates how to use time-remapping to animate a character to synchronize mouth movement with audio (lip synch).
Charles Bordenave provides a script on the After Effects Scripts website that automatically modulates time-remapping on a layer according to audio amplitude.
Sam Morris provides a tutorial that introduces time-remapping on his website.
Andrew Kramer provides a video tutorial on his Video Copilot website that demonstrates time-stretching, time-remapping, and frame blending.
When you time-stretch or time-remap a layer to a slower frame rate or to a rate lower than the frame rate of its composition, movement can appear jerky. This jerky appearance results because the layer now has fewer frames per second than the composition. Likewise, the same jerky appearance can occur when you time-stretch or time-remap a layer to a frame rate that is faster than the frame rate of its composition. To create smoother motion when you slow down or speed up a layer, use frame blending. Don’t apply frame blending unless the video of a layer has been re-timed—that is, the video is playing at a different frame rate than the frame rate of the source video.
After Effects provides two types of frame blending: Frame Mix and Pixel Motion. Frame Mix takes less time to render, but Pixel Motion provides much better results, especially for footage that has been drastically slowed down.
The Quality setting you select also affects frame blending. When the layer is set to Best quality, frame blending results in smoother motion but may take longer to render than when set to Draft quality.
Note:
When working with a frame-blended layer in Draft mode, After Effects always uses Frame Mix interpolation to increase rendering speed.
You can also enable frame blending for all compositions when you render a movie.
Use frame blending to enhance the quality of time-altered motion in a layer that contains live-action footage—video, for example. You can apply frame blending to a sequence of still images, but not to a single still image. If you are animating a layer—for example, moving a text layer across the screen—use motion blur.
Note:
You can't apply frame blending to a precomposition layer (a layer that uses a nested composition as its source footage item). You can, however, apply frame blending to the layers within the nested composition if those layers themselves are based on motion footage items, such as video or image sequences.
A check mark adjacent to the appropriate Frame Blending command (Frame Mix or Pixel Motion) indicates that it is applied to the selected layer. Also, the Frame Blending switch 
appears in the Switches column for the layer in the Timeline panel. Remove frame blending either by clicking the Frame Blending switch or by choosing the appropriate Frame Blending command again.
Regardless of the state of the layer switches, if frame blending is off for the composition, it is off for all layers in the composition. Set frame blending for the composition by choosing Enable Frame Blending from the Timeline panel menu, or by clicking the Enable Frame Blending button 
at the top of the Timeline panel.
Note:
Motion blur can make it harder for Pixel Motion to find discrete objects in each frame, which makes the calculation of motion vectors less reliable. For better results when using Pixel Motion to create slow motion, use footage with less motion blur.
Twitter™ and Facebook posts are not covered under the terms of Creative Commons.