1 The idea
Slowing a shot to 50% needs twice as many output frames as the source has, and half of them don’t exist. Nearest repeats the closest frame, so the motion holds, then jumps (stutter). Blend mixes two frames, so nothing holds, but you see a double image (ghosting).
Motion vectors estimate where pixels move and draw a new in-between image. It is the smoothest when the motion is clear, but it can break where objects cross each other, on smoke, or on thin detail. Read the onion skin: evenly spaced dots = smooth, stacked dots = stutter.
2 Hands-on
Task: make a constant 50% slow motion with no repeated frames and no double images
In this demo vectors are perfect because we know the true motion. On real footage, step through frame by frame looking for warped areas
3 Deep dive
Three ways to make frames that don’t exist
At 50%, every second output frame needs an image “half-way” between two source frames. Nearest uses the closest image; blend mixes two images by weight; vectors estimate pixel motion and push pixels half-way.
| Method | At 50% | Downside | Good for |
|---|---|---|---|
| nearest | half the frames repeat | stutter | real frames only, whole-number speed-ups (e.g. 200%) |
| blend | half the frames are mixes | ghosting | smoke, haze, slow-moving content |
| vectors | a new image every frame | warping at crossing edges, thin detail | clear motion, little overlap |
Motion blur is baked into the frames
Footage carries the motion blur of its original shutter. Slow it to 50% and motion per frame halves, but the blur in each image stays the same length, so it looks too long for the motion. Speed it up and the blur looks too short and the motion gets choppy. Some retime tools can add motion blur when speeding up.
If you know a shot will be slowed, shoot it at a high frame rate. Nothing beats real frames.
Speed ramps and checking
A speed ramp changes speed within a shot, for example 100% → 25% → 100%. The easiest control is keying the source frame against the output frame: a flatter curve = slower. The slowest part is where nearest stutters most and vectors work hardest.
A lesson from real work: some AI-generated clips come back with a different frame count from the source. Filling the gaps with repeated frames made visible stutters. Re-matching the frames and filling with optical-flow in-betweens fixed it, then a check confirmed no repeated frames were left.
- 1Set speed or key source frame
- 2Pick the method (frame/blend/motion)
- 3Step through for warping
- 4Watch at real speed
4 In real work
In Nuke
- Retime: change the speed, or map an input frame range to an output range.
- OFlow (and Kronos, which may need NukeX): set Method to Frame, Blend or Motion, matching this lesson’s three methods, and drive timing by speed or by a keyed source frame.
- TimeWarp: a curve sets which source frame each output frame reads; handy for speed ramps.
- FrameBlend: averages several neighbouring frames, for a deliberate blended look.
- VectorBlur: adds motion blur from motion vectors when a speed-up leaves the blur too short.
Common mistakes → the fix
- ✗ Using nearest for slow motion (any slow-down repeats frames; a non-whole factor also makes the rhythm uneven)✓ Use blend or motion and check the result.
- ✗ Trusting motion vectors without stepping through✓ Check crossing edges, smoke and thin detail; patch with roto or switch method just where it breaks.
- ✗ Filling missing frames with repeats✓ Use optical-flow in-betweens, then confirm no repeated frames remain.
- ✗ Forgetting the footage’s blur doesn’t change with the new speed✓ Add motion blur when speeding up; watch for over-long blur when slowing down.
Before you send the shot
- No unintended repeated frames
- No distracting double images
- Stepped through: no warping at edges or in smoke
- Motion-blur length suits the new speed
- Watched at real speed
5 Check yourself
A clip came back a few frames short, and you filled the gaps by repeating frames. Now it stutters. A better fix?
🔒 Answer the question above and this card opens
1. You slow a 24-frame clip to 50%. About how many output frames do you get?
2. Slow, drifting smoke needs 50% slow motion. Which method is usually safest?
Where the numbers come from: A teaching demo: the object moves on a known straight path, so “vectors” can draw the exact position, which real tools can’t. Real optical flow estimates motion from pixels and can be wrong. Blend here is a linear mix of the two nearest frames.