1 The idea
Big things move slowly. The blast here is about 9 m wide, 73 m away; at ×4 it should read as about 35 m, but played at its sim speed it looks like a firecracker blown up. For motion driven by gravity and buoyancy, time stretches with the square root of size, so n times bigger should play about 1/√n as fast.
Pick the size in the shot, then move the speed and compare it with the “speed its size calls for” readout. Slow it with “nearest frame” and watch how long each source frame is held, then switch Retime to frame blend and compare the motion.
speed ≈ 1 / √scale (×2 → 0.71, ×4 → 0.5)2 Hands-on
Task: make the ×4 blast read as huge: slow it to the speed its size calls for, without stutter.
Read the speed its size calls for, then check how many times each source frame is held. Compare ×1 and ×4 at the same speed.
3 Deep dive
Size comes with its own speed
Big or distant smoke and explosions move slowly; an explosion that is too fast reads as small. For motion driven by gravity or buoyancy, time stretches with the square root of size (Froude scaling). That’s why miniatures are shot at √scale times the normal frame rate: a 1:4 model at twice the frame rate, played back at normal speed.
Comp uses the same rule the other way round: scale an element up n times and slow it to about 1/√n. It’s a good starting value, not a fixed one; judge the result by eye.
| Scale | Expected speed | Each source frame held (nearest) |
|---|---|---|
| ×1 | 1 | 1 frame |
| ×4 | 1/2 | 2 frames |
| ×16 | 1/4 | 4 frames |
Where the missing frames come from
Once you slow an element down, you need in-between frames the sim never rendered. The method you pick decides whether the result steps, ghosts or flows.
| Method | How it makes the in-between | What to watch for |
|---|---|---|
| Nearest | Holds the nearest source frame | Visible stepping |
| Frame blend | Mixes neighbouring frames | Ghosting at very slow speeds |
| Optical flow (OFlow, Kronos) | Builds new frames from the motion | Heavy; precomp it right away |
Order: retime first, scale up after
Use TimeOffset to pick a naturally thin section, then retime it at the element’s native resolution before scaling up; that’s cheaper than retiming the enlarged image. Precomp the retime to EXR, then scale it up with a Transform, pivot on the element.
Slowing an element also stretches it: half speed lasts twice as long. If it is still shorter than the shot, offset copies in time and dissolve between them.
- 1TimeOffset: pick a thin section
- 2Retime at native res
- 3Precomp to EXR
- 4Transform: scale up
- 5Place against the plate
Check real scale against the plate
Before you place an element, ask how big it is against the rocks and hills in the desert plate. The sim blast is about 9 m across at its widest, and the readout gives its width at the chosen size. Set the speed from that, not from feel alone.
Speed can go either way. Stock shot at high fps is often already slow, so it is often sped up, for example ×2 with Kronos. But speeding up a big element for energy, with no reason in the story, makes it read small at once.
Play the slowed blast in real time from a cached render; timing only shows at speed.
4 In real work
In Nuke
- TimeOffset to choose the section with thin smoke and a good shape.
- Retime for the speed change, set to blend neighbouring frames rather than hold them when slowing. FrameBlend only averages a range of frames; it doesn’t change speed.
- When it is slow enough to ghost, use OFlow or Kronos to build in-betweens from the motion.
- Precomp the retime to EXR right away; OFlow and Kronos are heavy.
- Transform to scale the element up after the retime, pivot on the element.
Common mistakes → the fix
- ✗ Scaling a small element up without slowing it, so it reads as a model✓ Slow it to about 1/√n, then judge by eye.
- ✗ Slowing it with frame holds (nearest), so it visibly steps✓ Use frame blend or optical flow to make the in-betweens.
- ✗ Using frame blend at very slow speeds, so it ghosts✓ Switch to OFlow or Kronos.
- ✗ Speeding up a big element for energy with no story reason✓ Keep the speed its size calls for; change it only when the story needs it.
- ✗ Scaling up first, then retiming✓ Retime at native resolution first, then scale up.
Before you send the shot
- The element’s size holds up against the rocks and hills in the plate.
- Speed is close to 1/√scale, or differs for a reason you can name.
- At full speed there is no stepping and no ghosting.
- The retime was done at native resolution and precomped before scaling.
- The slowed element covers the whole shot, or offset copies dissolve cleanly.
5 Check yourself
You scale a blast up to read 16 times bigger in the frame. Roughly how fast should it play?
🔒 Answer the question above and this card opens
1. A blast slowed a lot with frame blend shows doubled smoke edges. What should you do?
2. You need to retime and scale up an element. In which order?
Where the numbers come from: The blast is our own Mantaflow sim, 48 frames long and about 9 m across at its widest, measured from the render and its camera. The ×2 and ×4 blasts are the same picture scaled up on screen around its base, so they soften. The 1/√n rule is Froude scaling for buoyant motion (physics, not measured on the pictures) and is approximate: viscosity and turbulence don’t scale perfectly. Frame blend here is a teaching version: a linear mix of the two nearest frames. Every element is our own: Mantaflow sims in Blender 5.2, rendered in Cycles through the plate’s camera (EmberGen could not export headless on this machine). No stock elements.