Time and stability
The real-time clock
Section titled “The real-time clock”Every solver runs on one real-time clock. A Blender frame advances 1/fps seconds of simulated time, multiplied by the solver node’s Time Scale. 1.0 is real time, 0.15 is bullet time and 2.0 is fast-forward. The solver chooses how many substeps that takes.
Auto Substeps
Section titled “Auto Substeps”Auto Substeps (CFL) is on by default. The solver picks the number of substeps per frame from the particle speed and the material’s stiffness.
| Setting | Default | What it does |
|---|---|---|
| CFL | 0.5 | The most grid cells a particle may cross per substep. Lower is safer and slower. |
| Min | 4 | The fewest substeps per frame. Calm frames never go below it. |
| Max | 400 | The ceiling. Stiff materials need hundreds. Above the ceiling the solver runs past its stability limit, and the UI flags frames that hit it. |
With Auto Substeps off, Substeps is a fixed count (default 20), and each substep is 0.25 ms of simulated time multiplied by Time Scale. The result is slow motion. Explicit Atom keeps this fixed-dt slow-motion frame because a fixed substep count over a real-time frame is unstable. Position-Based can divide a real-time frame by a fixed count.
| Mode | Characteristic |
|---|---|
| Explicit | Accurate and crisp. Needs more substeps for stiff materials. |
| Position-Based | 3 to 4 times faster at few substeps, but softer. PB Iterations (default 3) sets stiffness: more is stiffer and costlier. |
Measured on an NVIDIA CUDA GPU with 50,000 sand particles:
| Clock | Time per frame | Simulated per frame | Real-time factor |
|---|---|---|---|
| Fixed 20 substeps of 0.25 ms (Auto Substeps off) | 32 ms | 5 ms | 0.16x |
| 1/24 s frame, Auto Substeps (default) | 64 ms | 41.7 ms | 0.65x |
So Atom is an interactive preview and bake tool, not a real-time engine. A second of motion costs about 1.5 s of compute. Stiff materials need more substeps and cost about 4 times more per frame on a stiff block.
- Raise Max if the UI reports frames that hit the substep ceiling.
- Use Position-Based for fast previews, then Explicit for the final bake.
- Lower the grid resolution or the particle count for faster iterations.
- Soften stiff materials. The Young’s Modulus of Metal and Concrete is deliberately scaled below real values to keep the substep count practical.
- Sleeping. With Sleeping on, particles slower than Sleep Speed (0.02 m/s) for Sleep Time (0.5 s) stop being simulated until the material around them moves faster than Wake Speed (0.1 m/s). This saves time in settled piles.