Physical simulation of sand particles to render photo-realistic images of desert dunes and ripples. This was a course research project for CS 378H Computer Graphics Honors at UT Austin, which I completed in collaboration with my partner Rahul Krishnan.
To create ambitious desert geometries, we simulated the physics of wind-blown sand particles. Additionally, to improve the realism of the images produced, we also implemented Catmull-Clark subdivision and procedurally generated noise.
Height Map:
We represent the desert floor as 2D grid of height values which we call the height map.
At each time step of the simulation, the value at row i and column j of the map is updated with the following equation:
where is the number of particles deposited at that point,
is the number of particles leaving that point, and
is the diameter of any given particle.
is determined by particle collisions with the floor, while
is either 1 or 0 and determined with a pseudo random number generator.
Particles: The particles are stored in a linked list. At each time step their individual velocities and positions are updated with the following equations:
where the is the force of gravity and
is the force of drag from the wind.
A particle's initial velocity is determined by the wind velocity and direction, while a particle's initial position is the physical location of the height map point where it was born.
Collision: When a particle collides with the floor, its velocity is attenuated and then redirected upwards. Then it can either stay alive and bounce if the magnitude of its resulting velocity is greater than some epsilon, or it can be deposited at that point in the floor otherwise.
For more details regarding any aspect of the physical simulation we did please see Wang and Hu's paper referenced below.
The physical simulation above resulted in a mesh which was very polygonal. You could see individual triangles like how models were rendered on old game consoles such as the N64. To fix this, we implemented the classic Catmull-Clark algorithm whereby a surface is recursively subdivided and made incrementally smoother. For example, this image taken from Wikipedia depicts a cube which is made to increasingly approximate a sphere:
For a complete description of how the algorithm works, see the Wikipedia article mentioned above.
To add organic bumps and dune-like geometries to our desert, we implemented a form of noise generation whereby coarse noise is combined with upsampling increasingly finer grained noise. Graphically, the basics of this algorithm can be depicted simply:
We converted the polygonal mesh of the floor which results from the above algorithms into a triangular mesh which we rendered on the GPU using OpenGL.
To modulate the simulation's configuration, you can edit src/config.h. We especially recommend modifying the values for:
- hmap_resolution
- fatten
- ffrac
- num_levels
- subdivs
where hmap_resolution determines the density of height map points in the simulation, fatten is the friction that bouncing particles experience in the normal direction, ffrac is the friction that bouncing particles experience in the tangential direction, num_levels is the number of levels used in the procedural noise generation, and subdivs is the number of subdivision layers used in the Catmull-Clark algorithm.
This demo was primarily intended to run on MacOS. As such, the instructions below are intended for running the demo on MacOS only.
There are a few pre-requisites you need to set up before executing the demo. First, you need to run brew install gcc to install gcc. Then, because MacOS automatically aliases gcc to clang, you need to modify your configuration file (~/.bashrc for example) by adding the following four lines:
export CC=/usr/local/Cellar/gcc/9.2.0_3/bin/gcc-9
export CXX=/usr/local/Cellar/gcc/9.2.0_3/bin/g++-9
alias gcc="/usr/local/Cellar/gcc/9.2.0_3/bin/gcc-9"
alias g++="/usr/local/Cellar/gcc/9.2.0_3/bin/g++-9"
and run source ~/.bashrc (or whatever config file you use). Note that you might have to change the version number from 9.2.0_3 to whatever version of gcc you have installed on your machine. Then, you need to install glew and glfw by:
brew install glewbrew install glfw
Finally, you need to modify one of the cmake files to configure the directory that it will find the glew library. To do so, open the file cmake/gl3.cmake and modify lines 3 & 4 with the directory that the associated files are located. For example, since brew installed version 2.1.0_1 for me, located at "/usr/local/Cellar/glew/2.1.0_1/", I would have these lines read:
set(GLEW_INCLUDE_DIRS "/usr/local/Cellar/glew/2.1.0_1/include")
set(GLEW_LIBRARIES "/usr/local/Cellar/glew/2.1.0_1/lib/libGLEW.a")
And with that, you should be all set for executing the demo! Now just clone the repository and execute it as described below.
The script build.sh builds the project using the cmake settings we've configured for MacOS. Once successfully built, the project can be run with the script run.sh. This should open a new window displaying the sand dunes!
The physical simulation math and physics came from: Real-Time Simulation of Aeolian Sand Movement and Sand Ripple Evolution by Ning Wang and Bao-Gang Hu.