# Snapshots (2D FDTD)

A **snapshot** captures the field arrays at a chosen simulation step. The simulation API can schedule captures and retain them for later inspection:

```cpp
fdtd::TMz::Simulation sim(256, 256);
sim.add_snapshot(100);
sim.add_snapshot(200);
sim.run(200);

const auto &captures = sim.snapshots();
```

This minimal example illustrates scheduling; a source or initial field is needed to produce a nonzero wave. `snapshot()` instead captures the current state immediately. A handler passed to `run()` can consume scheduled fields without retaining every snapshot.

The driver offers the same idea through `--snapshot-every N`, writing $E_z$ to binary snapshot files. Captured electric and magnetic fields retain their half-step time offset. Frequent snapshots add transfer and storage costs; CUDA's [[fdtd-cuda-asynchronous-transfers|asynchronous path]] can overlap copies with later work.