Slow motion is where fake physics goes to be caught

Why slow motion exposes generated footage

A generated collision, splash or impact is plausible at speed and wrong on inspection. There is no simulation underneath (no mass, no conservation of anything) only frames that look reasonable in sequence.

At normal speed the viewer gets a fraction of a second and the sound carries the event, for the reasons in the previous module. Slow it down and you have handed them time to check, removed the sound’s power to resolve ambiguity, and directed their attention at exactly the frames the model was least sure about.

So the rule is firm: do not put generated physics in slow motion. If a shot has an impact, a splash, cloth settling, or anything breaking, play it at speed or faster.

What retiming is good for

Three legitimate uses.

Speeding up. Generated footage is often slightly too slow, because models default to gentle drifting motion. A clip at 110 or 120 per cent frequently reads better and no one notices. This is the most common retime in this kind of work.

Finding a moment. You generated ten seconds and the expression you want happens at 6.2 seconds. Slipping the clip (moving the content within a fixed timeline position) puts that moment where the cut needs it. This is not a speed change at all and it is the most useful retiming operation there is.

Hiding drift. A clip that degrades toward the end can be sped up in its final second so the bad frames pass quickly. Crude, and it works.

How the editor makes the frames

Three methods, and choosing wrongly is where retimed shots go wrong:

• Nearest frame. Repeats or drops frames. Stutters on slow-downs. Fine for small speed increases.
• Frame blend. Averages neighbouring frames. Soft ghosting, which is forgiving and reads as motion blur at modest ratios.
• Optical flow. Estimates motion and generates new intermediate frames. Best for smooth slow-downs, and it produces the occlusion halos discussed earlier: most visible exactly where a limb crosses a body, which is where a slowed generated clip is already under scrutiny.

A practical guide: up to 150 per cent speed, any method. Slowing to 50 per cent, use flow and inspect the result. Below 50 per cent on generated footage, do not.

Speed ramps

A ramp changes speed within the shot: fast, then slowing into a moment, then fast again. Done well it is invisible and gives weight to a beat. Done badly it is the single clearest marker of an amateur edit.

Two rules keep it honest:

• Ramp on motion, not on stillness. A speed change during movement is absorbed; on a static frame it looks like buffering.
• Ease the change. An instant jump from 200 per cent to 50 per cent reads as a glitch. Every editor lets you curve the speed change; use it.

And remember what slowing does to generated physics. A ramp that slows into an impact is exactly the shot this lesson says not to make.

Audio and retiming

Speed changes pitch-shift audio unless you tell the editor otherwise. For a small speed change on ambience nobody notices. For anything with speech, detach the audio before retiming and keep it at normal speed, or the voice goes up.

This is another argument for building sound separately: a soundtrack assembled from your own material is unaffected by whatever you do to the picture’s speed.

Freeze frames

A freeze is a retime to zero and it is worth a note. Freezing a generated frame is safe (it is just a still), but the transition into and out of it exposes any drift immediately, because the eye suddenly has a fixed reference to compare the moving frames against.

If you freeze, freeze on a frame you have inspected, and consider adding a small push-in on the frozen image so it does not sit completely dead.

Today: take a generated clip with any physical event in it, slow it to 40 per cent, and watch what you can now see. Then put it back.