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document materials, stimuli, measurements, rendering

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Colin
2022-05-04 00:59:35 -07:00
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@@ -176,11 +176,89 @@ although steps 1 and 3 vary heavily based on the user configuration of the simul
# Features
TODO: document Material options, Stimulus options, Measurement options, Rendering options.
this library takes effort to separate the following from the core/math-heavy "simulation" logic:
- Stimuli
- Measurements
- Render targets (video, CSV, etc)
- Materials (conductors, non-linear ferromagnets)
- Float implementation (for CPU simulations only)
the simulation only interacts with these things through a trait interface, such that they're each swappable.
common stimuli type live in [src/stim.rs](src/stim.rs).
common measurements live in [src/meas.rs](src/meas.rs).
common render targets live in [src/render.rs](src/render.rs). these change infrequently enough that [src/driver.rs](src/driver.rs) has some specialized helpers for each render backend.
common materials are spread throughout [src/mat](src/mat/mod.rs).
different float implementations live in [src/real](src/real.rs).
if you're getting NaNs, you can run the entire simulation on a checked `R64` type in order to pinpoint the moment those are introduced.
## Materials
of these, the materials have the most "gotchas".
each cell owns an associated material instance.
in the original CPU implementation of this library, each cell had a `E` and `H` component,
and any additional state was required to be held in the material. so a conductor material
might hold only some immutable `conductivity` parameter, while a ferromagnetic material
might hold similar immutable material parameters _and also a mutable `M` field_.
spirv/rust-gpu requires stronger separation of state, and so this `M` field had to be lifted
completely out of the material. as a result, the material API differs slightly between the CPU
and spirv backends. as you saw in the examples, that difference doesn't have to appear at the user
level, but you will see it if you're adding new materials.
### Spirv Materials
all the materials usable in the spirv backend live in `src/sim/spirv/spirv_backend/src/mat.rs`.
to add a new one, implement the `Material` trait in that file on some new type, which must also
be in that file.
next, add an analog type somewhere in the main library, like `src/mat/mh_ferromagnet.rs`. this will
be the user-facing material.
now implement the `IntoFfi` and `IntoLib` traits for this new material inside `src/sim/spirv/bindings.rs`
so that the spirv backend can translate between its GPU-side material and your CPU-side/user-facing material.
finally, because cpu-side `SpirvSim<M>` is parameterized over a material, but the underlying spirv library
is compiled separately, the spirv library needs specialized dispatch logic for each value of `M` you might want
to use. add this to `src/sim/spirv/spirv_backend/src/lib.rs` (it's about five lines: follow the example of `Iso3R1`).
### CPU Materials
adding a CPU material is "simpler". just implement the `Material` trait in `src/mat/mod.rs`.
either link that material into the `GenericMaterial` type in the same file (if you want to easily
mix materials within the same simulation), or if that material can handle every cell in your
simulation then instantiance a `SimState<M>` object which is directly parameterized over your material.
## What's in the Box
this library ships with the following materials:
- conductors (Isomorphic or Anisomorphic). supports CPU or GPU.
- linear magnets (defined by their relative permeability, mu\_r). supports CPU only.
- a handful of ferromagnet implementations:
- `MHPgram` specifies the `M(H)` function as a parallelogram. supports CPU or GPU.
- `MBPgram` specifies the `M(B)` function as a parallelogram. supports CPU or GPU.
- `MHCurve` specifies the `M(H)` function as an arbitrary polygon. requires a new type for each curve for memory reasons (see `Ferroxcube3R1`). supports CPU only.
measurements include ([src/meas.rs](src/meas.rs)):
- E, B or H field (mean vector over some region)
- energy, power (net over some region)
- current (mean vector over some region)
- mean current magnitude along a closed loop (toroidal loops only)
- mean magnetic polarization magnitude along a closed loop (toroidal loops only)
output targets include ([src/render.rs](src/render.rs)):
- `ColorTermRenderer`: renders 2d-slices in real-time to the terminal.
- `Y4MRenderer`: outputs 2d-slices to an uncompressed `y4m` video file.
- `SerializerRenderer`: dumps the full 3d simulation state to disk. parseable after the fact with [src/bin/viewer.rs](src/bin/viewer.rs).
- `CsvRenderer`: dumps the output of all measurements into a `csv` file.
historically there was also a plotly renderer, but that effort was redirected into developing the `src/bin/viewer.rs` tool better.
# Performance
with my Radeon RX 5700XT, the sr\_latch example takes 125 minutes to complete 150ns of simulation time (3896500 simulation steps). that's on a grid of size 163x126x80 where the cell dimension is 20um.
in a FDTD simulation, as we shrink the cell size the time step has to shrink too (it's an inverse affine relationship). so the scale-invariant performance metric is "grid cell steps per second" (`(163*126*80)*3896500 / (125*60)`): we get 850M.