PCISPH Lava Simulation on CPU

This is a 3D lava particle simulator based on Smoothed-Particle Hydrodynamics (PCISPH). It is a project developed in the ETH course Physically-based Simulation.

Videos

• https://www.youtube.com/watch?v=K2ffZptTYHk
• https://www.youtube.com/watch?v=teAVuc_flAM

Features

• Predictive-Corrective Incompressible SPH simulation based on 3D grid and neighbor lists (for pure SPH version, see git tag pure_SPH)
• Temperature diffusion inside volume and to the ground and air
• Spring and elastic impact based ground collisions
• Viscosity calculation based on temperature
• Multithreading using OpenMP
• Configurable neighbor lookup distance so that neighbor lists can be used for multiple timesteps
• Multiple rendering modes using a RIB (Renderman) scene output file:
  • Point rendering (with different color modes: density, temperature, surface or shader)
  • Spheres or oriented disks
  • Surface rendering using disk splatting

Compilation

There are no external dependencies for the simulator. The code works under Linux and Mac.

Release build:
$ make

Debug build:
$ make debug

Then to render you need to first compile the shaders in data/shaders. Eg for Pixie:
$ cd data/shaders
$ sdrc *.sl

The simulation output can get considerably large, so there is an option to compile with compressed output (gzip files, requires libz). Pixie can directly read it, however simulation performance suffers quite a bit.
$ make clean && make COMPRESSION=1

Script Dependencies

• render script: Renderman compliant renderer (eg Pixie), ImageMagick for surface rendering (spatial and temporal blurring steps)
• video script: ffmpeg

Usage

Input is an XML scene configuration file under config/ together with a height field (see include/simulation.h for documentation of the config parameters). The output is a RenderMan Scene File (RIB), which can be rendered with a RenderMan compliant renderer. We used Pixie (http://www.renderpixie.com).

Simulator output:
#P: 13656, T: 1.463/10.000, steps: 1.8/s (549ms), avg_nei: 41, nei_upd:50% temp:[617 1000] avg_it:12
With current number of particles, current and total simulation time, simulation steps per second and average time of one timestep, average number of neighbors, percentage of steps where neighbor lists needed to be updated, [min max] temperature and average number of iterations for PCI loop.

Example

$ ./simulator -c config/grid_simple.xml -f 100
$ ./scripts/render.sh config/grid_simple.xml -r <pixie_bin>/rndr
$ ./scripts/video.sh output/rendering/grid_simple --fps 20

Configuration

Configuration files are under ./config using xml files. How to setup & tune configuraton parameters:

• To figure out the particle mass, first use a particles_grid with the desired particle density and calc_mass=true. Running the simulation will output the particle mass.
• Set lookup_dist equal to smoothing_kernel_size, then make sure that the simulation has around 30-40 average neighbors (avg_nei).
• Then slightly increase the lookup_dist to have around 20% neighbor list updates (nei_upd) for optimal performance.
• To get a stable simulation, the time_step needs to be low enough and the viscosity high enough
• Make sure the surface particles are correctly calculated by changing surface_air_threshold. To visualize, set color="surface" in output.
• The rate how fast the temperature diffuses within the fluid and to the air, can be changed with the parameters temperature_diffusion_coeff and temperature_diffusion_coeff_air, temperature_diffusion_coeff_ground. Increasing these will result in faster diffusion.
• Multipass Rendering:
  • Image blur: use pass{i}="blur 0xX file" where file is one of the outputs of the previous rendering pass (eg normal). blurring will be done in-place, so the same file can be used in following rendering passes.
  • Temporal blur: use pass{i}="temporalblur size [name]" where size is the neighborhood size in the past and future (1 means to filter over previous, current and next image). If name is not given, the final output is blurred, otherwise it's the same convention as for the blur. Note: this is not in-place, the new filename is tblur_[name]_out_%06i.tif. So the following passes need to use the prefix tblur_ to use the output in a .rib file. To render the video with final temporal blur use --prefix tblur
• High quality rendering:
  • Increase image resolution (Format x y)
  • Decrease ShadingRate
  • Enable PixelSamples and PixelFilter
  • Change the amount of blur in the config file according to the changed image resolution

Literature

Parts of the implementation are based on these papers:

• Particle-Based Fluid Simulation for Interactive Applications, Matthias Mueller et al., 2003
• Animating Lava Flows, Dan Stora et al., 1999
• Numerical simulation of lava flow using a GPU SPH model, Alexis Herault et al., 2011
• Predictive-Corrective Incompressible SPH, B. Solenthaler et al., 2009

Copyright 2014 Hans Hardmeier hanshardmeier@gmail.com
Copyright 2014 Beat Küng beat-kueng@gmx.net