Water & Fire

An IMU-controlled CardputerZero simulation app built with LVGL 9.5.

The water mode ports the FLIP particle-fluid algorithm from the reference esp32-led-matrix firmware. Fire mode uses a 96 x 52 Eulerian combustion field with shared velocity, temperature, fuel, and emission layers. Buoyancy, pressure projection, vorticity confinement, and semi-Lagrangian transport let neighboring flames merge and pull on one another across a full-width fuel bed. On CardputerZero, gravity comes from the BMI270 Linux IIO interface. The desktop simulator uses the arrow keys as a virtual gravity vector.

Simulation design

Water physics runs on a 48-cell vertical grid (about 97 x 48 cells for the 2:1 tank), with two solver substeps per displayed frame. The pressure projection is the expensive part: its cost scales with grid cells, pressure iterations, and substeps, while the particle separation pass also scales with particle count. A native 320 x 170 FLIP grid would contain more than eleven times as many cells and far more particles. The lower physics resolution is therefore intentional; separable spatial filtering, temporal filtering, and bilinear sampling produce the final 320 x 170 image.

The 96 x 52 fire field is bilinearly sampled at 320 x 170. Physical temperature drives the flow while a separate, shorter-lived emission field controls visible brightness, preventing hot exhaust from becoming a solid glowing wall. Cinders are composited afterward so interpolation does not turn them into soft blobs.

Water uses full-width initial particle packing, wall-aware separation, and a smoothed density constraint to preserve visible volume. It keeps simulating at rest; speed-dependent viscosity removes constraint noise without a sleep timer.

Controls

Key	Action
Tab	Switch between water and fire
1 / 2	Select water / fire
Arrow keys	Simulate tilt on desktop
Space	Change the current mode palette
Enter or R	Reset both simulations
Hold ESC for 3 seconds	Exit

A short ESC press displays the required exit hint without closing the app.

Windows Simulator

$env:VCPKG_ROOT="C:\vcpkg"
cmake --preset win32-mingw64
cmake --build --preset win32-mingw64-dbg
.\build\mingw64\Debug\water_fire_simulator.exe

Optional standalone physics regression test:

cmake --preset win32-mingw64 -DWATER_FIRE_BUILD_TESTS=ON
cmake --build --preset win32-mingw64-dbg
ctest --test-dir build\mingw64 -C Debug --output-on-failure

CardputerZero Package

On a Debian-based host with the AArch64 cross compiler installed:

cmake --workflow --preset cp0-cross-package

The package is written to dist/ with a filename derived from the current project version, for example:

dist/WaterFireSimulator_1.0.9_m5stack1_arm64.deb

Runtime requirements are declared in app-builder.json. The app requires the IMU, does not use the network, and does not run as a background service.

Attribution

The FLIP implementation is adapted from Matthias Mueller's Ten Minute Physics work and the esp32-led-matrix firmware port. The DOOM fire implementation is adapted from the same reference firmware. See source headers and LICENSE for the MIT license notices.