Before building anything, set the EIGEN_INCLUDE_DIR environment variable to where you installed the Eigen library in the Makefile.

SIM_Stokes.C and SIM_Stokes.h are the C++ files defining the Houdini plugin DOP. In order to build them, source a houdini enviroment with

$ pushd /opt/hfs##.#.#/; source houdini_setup; popd

Then build with

$ make install

Note: instructions will differ for Windows

To use the micro-solver,

1. Prepare your viscous fluid scene in Houdini.
2. Locate and dive into the FLIP Solver DOP node.
3. Change the type of the second Gas Project Non Divergent Variational operator named projectnondivergent to the new Stokes node. The parameters should stay mostly the same as in the original node. Note that Min Viscosity is the dynamic viscosity and NOT the kinematic viscosity from the Gas Viscosity node.
4. You may want to increase Error Tolerance by an order of magnitude, and copy the Samples Per Axis parameter from the Gas Viscosity node.
5. Remove the lingering spare parameters.
6. Finally, disconnect the remaining Gas Project Non Divergent Variational and Gas Viscosity nodes named projectnondivergent_viscosity and gasviscosity respectively.

Once disconnected, you can reap the benefits of the Slip On Collision feature on the FLIP Solver when viscosity is enabled. If viscosity is disabled in the FLIP Solver, the resulting simulation will still exhibit viscosity. If you want to bypass the full Stokes solver without modifying the FLIP Solver network further, simply select the Pressure Only Scheme on the Stokes node to disable viscosity.

Note that currently, only the Stokes Scheme is fully optimized with OpenCL, other Schemes will use a more expensive CPU based solver.

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