Water system simulation
The core of HDRP's water implementation is a simulation of the way environmental factors displace water surfaces.
Simulation
In this context, a simulation is an adjustable mathematical construct.
This particular simulation shields you from some of the complexity inherent in fluid dynamics, because it provides you with a set of simplified inputs—wind and current.
As you modify the simulation inputs the simulation displaces the water mesh.
Wind and wave types
There are two types of wind in the HDRP water implementation: distant wind and local wind. Local wind produces Ripples, small waves that are close together. Distant wind produces Swells (for River, Sea, or Ocean surface types) or Agitation (for River surface types), broader waves that are farther apart.
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Here is an Ocean, Sea, or Lake surface that only has Swells. These Swells significantly displace the water surface.
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Here is an Ocean, Sea, or Lake surface that only has Ripples. These Ripples do not significantly displace the water surface.
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Here is an Ocean, Sea, or Lake surface with both Swells and Ripples.
Simulation bands
A frequency is a measure of the number of vibrations that pass through a specific point in one second. A band is a specific range of frequencies within a broader spectrum. You may have heard of the FM radio broadcast band, for example.
In the context of the HDRP water implementation, a Simulation Band is a specific range of wave frequencies.
Waves that are closer together, like Ripples, are more frequent; they have a higher frequency. Waves that are farther apart, like Swells, are less frequent; they have a lower frequency.
Ocean, Sea, or Lake water surfaces have three simulation bands, two for Swell waves and one for Ripples.
River surfaces have two simulation bands (although they are not labeled as such in the UI), one for Agitation (the equivalent of Swell) and one for Ripples.
Pool surfaces only have one band, for Ripples, also not noted as such in the UI. This because pools are usually much smaller than the bodies of water that the other surface types simulate, and so do not experience the effects that wind can cause from a distance as it blows continuously on a surface tens or even thousands of kilometers across.
Because they separate higher and lower frequency waves, Simulation Bands make it easier to produce realistic wave behavior on both large and small water surfaces.
Current
In the real world, factors other than wind can displace water, like gravity, temperature, salinity, and topography. The Current control accounts for this. Currents can flow in an orientation independent of the wind orientation settings.
On River and Ocean, Sea, or Lake water surfaces, Swell and Agitation band settings for Current override Current values for Ripples by default.
Chaos
To break up overly regular patterns of waves, you can increase the Chaos value, which is the inverse of the wave's directionality.
- At a Chaos value of 0, the waves (ripples and swell) follow the wind direction 100%.
- At a Chaos value of 0.5, the waves (ripples and swell) follow the wind direction 50% and otherwise behave chaotically.
- At a Chaos value of 1, the waves (ripples and swell) disregard wind direction, and behave entirely chaotically.
Patch and Grid
The Volume Override uses the terms Patch and Grid. The Patch is the size of the area on which Unity runs the simulation for a particular Simulation Band. The Grid is the geometry Unity uses to render the water, which is always a rectangle.
Scripting with the water simulation
You can query a water surface for position and current direction which you can use in scripts, for example to create a customized buoyancy effect that makes it possible to float a ball on the waves (for example). See Scripting in the Water System for more information.