Twixtor

Slowing footage down in an ordinary editor gives you two bad options. Repeat each frame, which stutters, or blend between frames, which produces a soft double image. Twixtor takes a third approach, inventing the frames that were never recorded.

The Twixtor method explains every strength and every failure that follows. The plugin tracks how each part of the picture moves between one frame and the next, then builds intermediate frames showing where everything would have been partway through that movement.

Done well, footage shot at an ordinary frame rate slows to a fraction of its speed while staying sharp and continuous, which is what people are actually paying for.

The idea is not exclusive to this plugin, and a compositor with its own motion-based retiming in the video sequencer implements the same principle less conveniently and without a licence.

The shutter matters more than the software

Here is the practical fact that decides your Twixtor results, and it is about the camera rather than the plugin.

Real slow motion comes from a camera recording many frames per second, each one exposed briefly and therefore sharp. Ordinary footage is exposed for longer, so anything moving carries motion blur, and that blur is recorded into the image permanently.

Slowing blurred frames down and inserting synthesised frames between them produces smooth motion made of blurred pictures, which looks wrong in a way viewers notice without being able to say why. Sharp frames are what makes slow motion read as slow motion.

So the advice is to shoot with a fast shutter whenever you know footage will be slowed later. That single decision improves the result more than any setting in the plugin, and no amount of processing recovers detail the sensor never captured.

Where the analysis fails, and it always does somewhere

Being specific here is the difference between using Twixtor well and being disappointed by it.

Tracking motion works by matching parts of one frame to the next, so it fails wherever that matching becomes ambiguous. Objects crossing in front of each other, anything entering or leaving the edge of the frame, rapid rotation, and low-contrast areas where nothing distinctive can be matched are the reliable trouble spots.

Fine repeating detail is the worst case. A fence, a grille, hair or foliage produces matches that are individually plausible and collectively wrong, and the result smears or tears.

The failures have a characteristic look. Edges warp, parts of a shape stretch towards where the analysis thought they were going, and something briefly melts. Once you have seen it you recognise it immediately in other people’s work.

The professional version exists because of those failures

This is where the two Twixtor versions separate, and understanding why clarifies which one you need.

The basic version analyses and produces a result. The professional one adds the ability to intervene, meaning manual correction of the motion tracking, separating foreground from background with a matte so each is analysed independently, and warping controls for pushing a result into shape.

That intervention is the actual professional workflow. Somebody delivering a shot to a client does not accept whatever the analysis produced, they find the frames where it broke and fix them, which is laborious and is why the capability is worth paying for.

For anybody whose editing application already includes optical flow retiming, DaVinci Resolve has that built in, and comparing its output on your own difficult footage before buying anything is the sensible first step.

Variable speed, not just a factor

Twixtor changes speed over time rather than applying one multiplier to a clip.

That is how a shot ramps from normal speed into slow motion and back out again, which is the effect most people actually want rather than a uniformly slowed clip.

Setting it means drawing the speed over the length of the shot, and the plugin synthesises whatever frame rate each moment requires. The failures described above apply throughout, so a ramp that passes through the difficult part of a shot needs the same attention as a constant slowdown.

It needs a host application

Being clear about the requirement. Twixtor is a plugin rather than a program, so it runs inside an editing or compositing application and does nothing on its own.

That constrains the decision. The version you need depends on what you already use, and somebody without a supported application has nothing to install it into.

Free editors generally do not support this class of plugin, and Shotcut is a capable editor whose own speed controls repeat or blend frames rather than synthesising them.

Conclusion

Twixtor does something properly clever, which is inventing frames that plausibly belong between the ones you have. On suitable footage the result is slow motion from a camera that never shot slow motion, and it has been the standard tool for that for a long time.

Two things determine whether you get that result. The source frames need to be sharp, which is a decision made while shooting rather than while editing, and the analysis will fail somewhere in any difficult shot, which means either accepting the artefacts or doing the manual correction the professional version exists for.

Test it on your own hardest footage before deciding, particularly if your editing application already offers something similar.