Weather Forecast
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Weather Forecast
Lovebyte 2022
Mecrisp
Matthias Koch
m-atthias@users.sf.net
http://mecrisp.sourceforge.net/
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A particle simulator running in 512 bytes on RISC-V RV32IMC
Weather forecast is designed for GD32VF103 microcontrollers and
draws vector graphics on an analog oscilloscope in XY mode
connected to
GND and the two DAC channels on pins
PA4 (vertical) and
PA5 (horizontal).
At the beginning, a sun is displayed, which bursts into many drops
when rain touches the sun. Waves and spray fill the canvas,
and you can observe intermittent rainfall and forming of clouds.
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Design ideas:
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Weather Forecast implements a "smoothed particle hydrodynamics" simulation
using carefully scaled fixpoint integers only.
https://en.wikipedia.org/wiki/Smoothed-particle_hydrodynamics
Every particle has force, velocity and position vectors,
a temperature and a wall flag and its behaviour is simulated by numerically
integrating newtons laws of motion for all pairs of particles, taking
collisions, viscosity, temperature, and gravity into account.
The particle table is filled with an old, but obscure "Mandala" algorithm:
Rotations of pixels by fractional angles in closed cycles on integer coordinates
sometimes result in long, interesting and artistic curves.
Proof-of-concept Mandala algorithm code for Gforth is included
for further exploration.
Rainfall is implemented by directly setting the y-coordinates of particles
to the top of the image in a pattern derived by shifting from the frame counter,
clearing the wall flag of the choosen particle along with the move.
At the beginning, all particles are flagged as "wall", being immobile.
Collisions with movable particles clear the wall flag, so the sun bursts on
first contact with rain.
A Minsky circle algorithm updates sine and cosine values, which are
- depending on frame counter - fed along with gravity as initial values
into the force integrals of the particle simulator to cause waves and clouds.
All other visible effects result from the physics
simulation running on these inputs.
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How to run:
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If you have connected your
Longan Nano board to
a USB-serial cable with 3.3V logic levels
3V3 or 5V -- VCC (measure the voltage present here)
R0 -- TXD
T0 -- RXD
GND -- GND
you can flash it if you first
* Press and hold BOOT0 button
* Press and release Reset button
* Release BOOT0 button
and then execute
stm32loader -e -w -v -p /dev/ttyUSB0 weather-forecast.bin
on your host machine.
Press and release Reset button to start demo.
PS: There is also a version for Linux with qemu-user-static
that uses a mmap'ed 1920x1080x32bpp framebuffer through syscalls
so that you can at least try the effects of Weather Forecast
on your desktop machine.
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Credits:
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* Particle simulator inspiration for Weather Forecast:
https://www.ioccc.org/2012/endoh1/hint.html
https://github.com/davidedc/Ascii-fluid-simulation-deobfuscated/
* Square root algorithm from the book Hacker's Delight:
http://www.hackersdelight.org/
* Minsky circle algorithm:
https://nbickford.wordpress.com/2011/04/03/the-minsky-circle-algorithm/
* Mandala algorithm based on integer rotations:
https://web.archive.org/web/20061211143039/www.tiac.net/~sw/2005/03/Mandala/index.html
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