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Adobe After Effects CS5 : The Timeline - Timing and Retiming

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6/6/2012 5:07:05 PM
After Effects is more flexible when working with time than most video applications. You can retime footage or mix and match speeds and timing using a variety of methods.

Absolute (Not Relative) Time

After Effects measures time in absolute seconds, rather than frames, whose timing and number are relative to the number per second. If frames instead of seconds were the measure of time, changing the frame rate on the fly would pose a much greater problem than it does.

Change the frame rate of a composition and the keyframes maintain their position in actual time, so the timing of an animation doesn’t change (Figure 1), only the position of the keyframes relative to frames. Here’s a haiku:

Figure 1. The bounce animation remains the same as the composition frame rate changes; keyframes now fall in between whole frames, the vertical lines on the grid.


keyframes realign
falling between retimed frames
timing is unchanged

Likewise, footage (or a nested composition) with a mismatched frame rate syncs up at least once a second, but the intervening source frames may fall in between composition frames. Think of a musician playing 3 against 4; one second in After Effects is something like the downbeat.

Time Stretch

Time Stretch lets you alter the duration (and thus the speed) of a source clip—but it doesn’t let you animate the retiming itself (for that, you need Time Remap or Timewarp). The third of the three icons at the lower left of the Timeline panel reveals the In/Out/Duration/Stretch columns.

I mostly change the Stretch value and find the inter-related settings of all four-columns redundant. I also never use a Time Stretch setting that is anything but an integer multiple or division by halves: 300%, 200%, 50%, or 25%. You can do without the columns altogether using the Time Stretch dialog (context-click > Time > Time Stretch). Ctrl+Alt+R (Cmd+Opt+R) or Layer > Time > Time-Reverse Layer sets the Stretch value to –100%. The layer’s appearance alters to remind you that it is reversed (Figure 2).

Figure 2. The candy striping along the bottom of the layer indicates that the Stretch value is negative and the footage will run in reverse.


Layer > Time > Freeze Frame applies the Time Remap effect with a single Hold keyframe at the current time.

Frame Blend

Suppose you retime a source clip with a Stretch value that doesn’t factor evenly into 100%; the result is likely to lurch in a distracting, inelegant fashion. Enable Frame Blend for the layer and the composition, and After Effects averages the adjacent frames together to create a new image on frames that fall in between the source frames. This also works when you’re adding footage to a composition with a mismatched frame rate. There are two modes:

  • Frame Mix mode overlays adjoining frames, essentially blurring them together.

  • Pixel Motion mode uses optical flow techniques to track the motion of actual pixels from frame to frame, creating new frames that are something like a morph of the adjoining frames.

Confusingly, the icons for these modes are the same as Draft and Best layer quality, respectively (Figure 3), yet there are cases where Frame Mix may be preferable instead of merely quicker. Pixel Motion can often appear too blurry, too distorted, or contain too many noticeable frame artifacts, in which case you can move back to Frame Mix, or move up to the Timewarp effect, with greater control of the same technology .

Figure 3. The Frame Blend switches for the composition and layer (the overlapping filmstrips to the right of frame). Just because Pixel Motion mode uses the same icon as Best in the Quality switch, to the left, doesn’t mean it’s guaranteed to be the best choice.


Notes

The optical flow in Pixel Motion and the Timewarp effect was licensed from the Foundry. The same underlying technology is also used in Furnace plug-ins for Shake, Flame, and Nuke.


Nested Compositions

Time Stretch (or Time Remap) applies the main composition’s frame rate to a nested composition; animations are not frame-blended; instead the keyframe interpolation is resliced to this new frame rate. If you put a composition with a lower frame rate into a master composition, the intention may be to keep the frame rate of the embedded composition. In such a case, go to the nested composition’s Composition Settings > Advanced panel and toggle Preserve Frame Rate When Nested or in Render Queue (Figure 4). This forces After Effects to use only whole frame increments in the underlying composition, just as if the composition were pre-rendered with that frame rate.

Figure 4. The highlighted setting causes the subcomposition to use its own frame rate instead of resampling to the rate of the master composition, if they are different from one another.


Notes

Effect > Time > Posterize Time can also force any layer to take on the specified frame rate, but effects in the Time category should be applied before all other effects in a given layer. Posterize Time often breaks preceding effects.


Tip

The final Time Remap keyframe is one greater than the total timing of the layer (in most cases a nonexistent frame) to guarantee that the final source frame is reached, even when frame rates don’t match. To get the last visible frame you must often add a keyframe on the penultimate frame.


Time Remap

For tricky timing, Time Remap trumps Time Stretch. The philosophy is elusively simple: A given point in time has a value, just like any other property, so it can be keyframed, including eases and even loops — it operates like any other animation data.

Ctrl+Alt+T (Cmd+Opt+T) or Layer > Time > Enable Time Remapping sets two Time Remap keyframes: at the beginning and one frame beyond the end of the layer. Time remapped layers have a theoretically infinite duration, so the final Time Remap frame effectively becomes a Hold keyframe; you can then freely scale the layer length beyond that last frame.

Beware when applying Time Remap to a layer whose first or last frame extends beyond the composition duration; there may be keyframes you cannot see. In such a case, I tend to add keyframes at the composition start and end points, click Time Remap to select all keyframes, Shift-deselect the ones I can see in the Timeline panel, and delete to get rid of the ones I can’t see.

Tip

There is also a Freeze Frame option in After Effects; context-click a layer, or from the Layer menu choose Time > Freeze Frame, which sets Time Remap (if not already set) with a single Hold keyframe.


Timewarp

The Foundry’s sophisticated retiming tool known as Kronos provides the technology used in Pixel Motion and Timewarp. Pixel Motion is an automated setting described earlier, and Timewarp builds this up by adding a set of effect controls that allow you to tweak the result. Timewarp uses optical flow technology to track any motion in the footage. Individual, automated motion vectors describe how each pixel moves from frame to frame. With this accurate analysis it is then possible to generate an image made up of those same pixels, interpolated along those vectors, with different timing. The result is new frames that appear as if in between the original frames. When it works, it has to be seen to be believed.

Figure 5. Frame Mix (left) simply cross-dissolves between adjacent whole frames, where as Pixel Motion (right) analyzes the actual pixels to create an entirely new in-between frame.



So flipping the Frame Blend toggle in the Timeline panel (Figure 5) to Pixel Motion with Time Stretch or Time Remapping gets you the same optical flow solution as Timewarp with the same Pixel Motion method. What’s the difference?
  • All methods can be used to speed up or slow down footage, but only Time Remapping and Timewarp dynamically animate the timing with keyframes.

  • All methods can access all three Frame Blending modes (Whole Frames, Frame Mix, and Pixel Motion).

  • Time Remapping keyframes can even be transferred directly to Timewarp, but it requires an expression (see note) because Timewarp uses frames and Time Remapping seconds.

Timewarp is worth any extra trouble in several ways:
  • It can be applied to a composition, not just footage.

  • It includes the option to add motion blur with the Enable Motion Blur toggle.

  • The Tuning section lets you refine the automated results of Pixel Motion.

To apply Timewarp to the footage, enable Time Remapping and extend the length of the layer when slowing footage down—otherwise you will run out of frames. Leave Time Remapping with keyframes at the default positions and Timewarp will override it.

The example footage has been pre-keyed, which provides the best result when anyone (or anything) in the foreground is moving separate from the background. Swap in the gs_timewarp_source footage and you’ll see some errors. Add the keyed_timewarp_source layer below as a reference, set it as the Matte Layer in Timewarp, and the errors should once again disappear, with the added benefit of working with the full unkeyed footage.


You can even further adjust the reference layer and precomp it (for example, enhancing contrast or luminance to give Timewarp a cleaner source), and then apply this precom-posed layer as a Warp Layer—it then analyzes with the adjustments but applies the result to the untouched source.

The Tuning section is where you trade render time and accuracy, but don’t assume that greater accuracy always yields a better result—it’s just not so. These tools make use of Local Motion Estimation (LME) technology, which is thoroughly documented in the Furnace User Guide, if you ever want to fully nerd out on the details.

Now try a shot that needs more tuning and shows more of the flaws of Pixel Motion, and how Timewarp can help solve them. The footage in the 02_rotoSetup_sk8rboi folder on the disc features several planes of motion—the wheels of the minivan, the van itself, the skater—and at the climatic moment where the skater pulls the 360 flip, the board utterly lacks continuity from one frame to the next, a classic case that will break any type of optical flow currently available (Figure 6).

Figure 6. Timewarp’s excellent super slow-mo capabilities work best with continuous motion, such as the torso and legs of the skater; the board itself and his hands move much more unpredictably from frame to frame, causing more difficulty. The best fix is to rotoscope to separate these areas from the background.

Here are a few tweaks you can try on this footage, or your own:

  • While raising Vector Detail would seem to increase accuracy, it’s hard to find anywhere in this clip where it helps. Not only does a higher number (100) drastically increase render time, it simply increases or at best shifts artifacts with fast motion. This is because it is analyzing too much detail with not enough areas to average in.

  • Smoothing relates directly to Vector Detail. The Foundry claims that the defaults, which are balanced, work best for most sequences. You can raise Global Smoothness (all vectors), Local Smoothness (individual vectors), and Smoothing Iterations in order to combat detail noise, but again, in this case it changes artifacting rather than solving it.

  • During the skateboard ollie itself, the 360 flip of the board is a tough one because it changes so much from frame to frame. Build From One Image helps quite a bit in a case like this—instead of trying to blend two nonmatching sets of pixels, Timewarp favors one of them. The downside is that sudden shifts occur at the transition points—the pixels don’t flow.

  • There’s no need in this clip to enable Correct Luminance Changes—it’s for sudden (image flicker) or gradual (moving highlights) shifts in brightness.

  • Error Threshold evaluates each vector before letting it contribute; raise this value and more vectors are eliminated for having too much perceived error.

  • Block Size determines the width and height of the area each vector tracks; as with Smoothing, lower values generate more noise, higher values result in less detail. The Foundry documentation indicates that this value should “rarely need editing.”

    Notes

    Twixtor (RE:Vision Effects) is a third-party alternative to Timewarp; it’s not necessarily better but some artists—not all—do prefer it. A demo can be found on the disc.


  • Weighting lets you control how much a given color channel is factored. The defaults correspond to how the eye perceives color to produce a monochrome image. If one channel is particularly noisy—usually blue—you can lower its setting.

  • Filtering applies to the render, not the analysis; it increases the sharpness of the result. It will cost you render time, so if you do enable it, wait until you’re done with your other changes and are ready to render.

    Tip

    Did you notice back in the Motion Blur section that Timewarp can be used to generate procedural motion blur without retiming footage (Figure 7)?

    Figure 7. Footage that is shot overcranked (at high speed, left) typically lacks sufficient motion blur when retimed. Timewarp can add motion blur to speed up footage; it can even add motion blur to footage with no speed-up at all, in either case using the same optical flow technology that tracks individual pixels. It looks fabulous.


The biggest thing you could do overall to improve results with a clip like sk8rboi is to use Roto Brush  to separate out each moving element—the van, skater, and background.

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