Graphics Reference
In-Depth Information
4
I
Simple
Rasterization-Based Liquids
Martin Guay
4.1 Overview
Rasterization pipelines are ubiquitous today. They can be found in most of our
personal computers as well as in smaller, hand-held devices—like smart phones—
with lower-end hardware. However, simulating particle-based liquids requires
sorting the particles, which is cumbersome when using a rasterization pipeline.
In this chapter, we describe a method to simulate liquids
without
having to sort
the particles. Our method was specifically designed for these architectures and
low shader model specifications (starting from shader model 3 for 3D liquids).
Instead of sorting the particles, we splat them onto a grid (i.e., a 3D or 2D
texture) and solve the inter-particle dynamics
directly
on the grid. Splatting is
simple to perform in a rasterization pipeline, but can also be costly. Thanks to
the simplified pass on the grid, we only need to splat the particles once.
The grid also provides additional benefits: we can easily add artificial obstacles
for the particles to interact with, we can ray cast the grid directly to render the
liquid surface, and we can even gain a speed up over sort-based liquid solvers—
such as the optimized solver found in the DirectX 11 SDK.
4.2 Introduction
Simulating liquids requires dealing with two phases of
fluid
—the liquid and the
air—which can be tricky to model as special care may be required for the interface
between the two phases depending on the fluid model. In computer graphics,
there are mainly two popular formulations for
fluids
: strongly incompressible
and weakly incompressible.
The strong formulation is usually more complex as it requires a hard constraint
(e.g., solving a Poisson equation), but it is more accurate and therefore more
