3 RE:Vision ReelSmart Twixtor 1.2
Oct 1, 2001 12:00 PM, BY S. D. KATZ
In the past, if an editor wanted to speed up or slow down existing material, he or she could do one of two things. The options were frame blending, which creates new frames that are little more than crossfades, or simply repeating frames. Neither method is ideal. When more than slight stretching of footage is required, the slow motion stutters.
ReelSmart Twixtor, a plug-in for Adobe After Effects, Discreet Combustion, and Pinnacle Systems Commotion, employs a far superior method of speeding up or slowing down footage. Twixtor synthesizes new frames by calculating a motion vector for each pixel. Essentially this is morphing at the pixel level. The method works in the majority of situations and produces excellent results — a convincing imitation of over-cranking the camera.
Extremely easy to use for slow motion, Twixtor can also remove 3:2 pulldown to provide smoother 24fps motion than that allowed by traditional solutions available on After Effects and the other host compositing programs. Twixtor can also use 24fps footage to create new 29.97 video that includes 59.94 fields. And it can create faster motion from footage with results that are superior to frame blending. However, these frame-rate conversions are only available in After Effects. (The good news is that a new product, ReelSmart Fields kit, offers a set of enhanced interlace/deinterlace tools available for any program that accepts AE filters.)
But there are limits to what Twixtor can do. The first issue is the problem of blurring. Real slow motion is created in the camera by operating at speeds above film's sync-sound speed of 24fps or video's 30fps — e.g. 48fps, 96fps, or 120fps. This requires a faster shutter speed; the slower the slow motion, the shorter the exposure of each frame of film. Shorter exposures produce sharper images. In a motion picture camera at 96fps, the shutter speed would be approximately 1/200 of a second, which tends to freeze motion reasonably well.
Twixtor, however, is going to be used with 24fps or 30fps footage captured with a shutter speed of 1/50 to 1/80 of a second. At these frame rates, fast motion causes blurring, which seems normal when the footage is viewed at normal speed. In fact, it tends to smooth the motion of objects that are moving quickly. When you apply Twixtor to this footage to create synthetic slow motion of 96fps (a 4X speed-up), you get the same amount of blurring as from a 24fps shutter. That's because Twixtor faithfully replicates each pixel. So while the motion may be nicely slowed down, the blurring causes parts of an image to smear.
If you can control the shooting of the footage, one solution is to select a faster shutter speed during acquisition. In a film camera this means closing down the shutter angle. Digital cameras offer even more flexibility, and some support 1/1000 second shutter speeds.
But if the footage is already shot, you have one other option: Real Motion Blur. Another of RE:Vision's plug-ins and available for the same compositing programs as Twixtor, Real Motion Blur adds and subtracts motion blur from existing footage. The subtraction of blur is more limited than adding blur, so this is not a complete solution.
The other limitation of Twixtor is that for certain footage, the algorithms have difficulty interpreting fast motion. For example, with footage of someone throwing a punch, the camera — depending on the frame rate — will capture that action as slices in time. Let's say the camera captures a fast punch: Frame 1 is an image of the arm cocked and ready, Frame 2 is the arm extended, and Frame 3 shows the arm pulled back. Twixtor will morph new frames between original Frames 1 and 2 because the path of the arm is known. The same is true when the arm pulls back. Twixtor knows to morph along a vector that connects the extended arm to the arm returned to its starting point. In this situation Twixtor will work fine.
But the way the camera captures these positions is arbitrary. Sometimes the camera does not capture the necessary extremes of motion. In the example above, what would happen if the camera failed to expose a frame of film at the moment of the arm's full extension? Twixtor would try to morph frames pixel-by-pixel between the arm in mid-thrust and the arm coming back to the cocked position. This would cut short the natural arm motion and could look wrong. As a rule, very fast changes in direction can fool Twixtor.
As an AE plug-in, Twixtor is easy to use and the settings are reasonably simple. The most significant aspect of the operation is that you cannot apply time changes to the footage directly. Either create a sub-comp or apply Twixtor to a Solid and in the Time Warp layer set the footage to be affected.
The most important control is the stretch factor, which allows you to stretch or shrink footage by a factor of zero to 10 in increments of .01. You also have the choice of using Constant Stretch or Keyframes Stretch. The second makes footage change speed dynamically, either accelerating or decelerating. Also, a Motion Blur option helps create consistent blur throughout footage. While Twixtor uses motion vectors for its best interpolation method, it also does traditional frame blending.
I used Twixtor for speeding up several kinds of footage and generally enjoyed terrific results. Using surfing footage with complex motion, I slowed the footage by factors of 3X, 4X, and 5X with very convincing slow motion. The aforementioned blur issue was apparent in some areas, but acceptable. The motion is smooth and “liquid,” as slow motion should be. This is certainly a big improvement over frame blending.
Twixtor is a first-rate product with many uses. For example, animation frames can be interpolated to produce smoother motion — though animators are pretty fussy about what their in-betweens look like. But for non-character animation motion, the results can be very useful. How successfully Twixtor works is largely dependent on the source footage.
Currently, though, it's the best plug-in method for creating speed changes, slow-motion effects, and frame-rate conversions. Since I completed this review newer versions, Twixtor 2.0 and Twixtor Pro, have added 16-bits-per-channel, new blending options, speed enhancements, and other features. I highly recommend this as an essential compositing plug-in.
S. D. Katz is a writer and director at Pitch Studios in New York. He is the author of Shot by Shot and Cinematic Motion.
Company: RE:Vision Software San Francisco
Product: ReelSmart Twixtor 1.2
Features: Creates a motion vector for each pixel to interpolate frames for slow motion; removes 3:2 pulldown for smoother 24fps output; can use 24fps footage to create new 29.97 video that includes 59.94 fields; offers animatable acceleration and deceleration of footage
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