Houdini 20.5 Fluids

Oceans

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Overview

The Ocean shelf tab contains tools for creating ocean simulations.

The Small Ocean tool is used for creating an ocean like surface. However, it is not a simulation, it is a deformer that does not use DOPs at all. You can scrub through the timeline and watch the ocean evolve without any simulation. For a simulated ocean, use the Wave Tank tool.

The Large Ocean tool is used for creating a large ocean with no repeating tiles even over very large distances. However, it is not a simulation, it is a deformer that does not use DOPs at all. You can scrub through the timeline and watch the ocean evolve without any simulation. For a simulated ocean, use the Wave Tank tool.

The Guided Ocean Layer tool creates a guided FLIP simulation of a thin particle layer on top of an ocean surface. The particles are initialized with ocean velocities. The ocean surface at a specified depth acts as a collision layer underneath the particles, allowing the particles to closely match the ocean waves. A boundary layer of particles suppresses reflections at the edge of the tank, contributes ocean velocities back to the simulation, and maintains the water volume level to match the ocean. The FLIP Simulation uses a guiding volume approach to maintain ocean velocities through the simulation.

The Wave Tank tool creates a FLIP tank simulation with particles initialized from an ocean surface with velocities. A boundary layer of particles suppresses reflections at the edge of the tank, contributes ocean velocities back to the simulation, and maintains the water volume level to match the ocean.

The Beach Tank tool creates a FLIP tank simulation of waves breaking on a beach or other shallow water terrain. This tool is similar to the Wave Tank, except it enables an option to ramp down ocean velocities in a certain direction across the length of the tank. Reducing the velocities allow the ocean waves to break naturally as the water get shallower.

The Flat Tank tool creates a FLIP tank simulation of a flat tank of particles similar to the FLIP Tank tool, but with the addition of a boundary layer of particles that helps suppress reflections at the edge of the tank and maintain the water level over time. The boundary layer also allows the tank to track a moving object, simulating only the area around the object.

Rendering ocean surfaces in Karma

With the new Karma Ocean LOP you can render oceans inside Karma CPU. Here’s a quick guide for a basic scene.

  1. From the Oceans shelf, create a Small Ocean and dive into the ocean_geometry node. The shelf tool automatically creates a Karma Ocean node in the stage editor.

  2. Save the project.

  3. Click the ROP Geometry Output named save_spectra.

  4. Set Valid Frame Range to Render Frame Range and cache the scene with Save to Disk (in Background).

  5. Repeat step 4 for the File Cache node named bake_foam.

  6. Connect the File Cache node’s output with the Merge node’s (merge_foam) input. When you move the timeline slider, the foam should be visible.

  7. Switch to the stage network and click the Karma Ocean node.

  8. Turn on Foam File. The appropriate path should already be available.

  9. For faster iterations, set Dicing Quality to 0.25 and turn off Interior Geometry.

  10. Add an Environment light node.

  11. Connect the light’s inpout with the Karma Ocean node’s output.

  12. In the viewport, open the Viewport options menu and choose Karma to get a rendered preview.

Masterclass

H16 Masterclass: Ocean tools

Differences between ocean tank types

Tank simulation

Guided Ocean Layer

Creates a layer of FLIP particles on the ocean surface. The layer of FLIP particles is sitting on a collision object that is more or less the shape of the ocean. Simulation only occurs in the top layer of the ocean surface.

Wave Tank

Fills the whole tank with FLIP Particles. Simulation occurs throughout the whole tank.

Beach Tank

Fills the whole tank with FLIP Particles, minus the terrain object. Simulation occurs throughout the whole tank.

Flat Tank

Fills the whole tank with FLIP Particles. Simulation occurs throughout the whole tank.

Ocean Flat Tank

Fills the whole tank with FLIP Particles. Simulation occurs throughout the whole tank.

Waves

Guided Ocean Layer

Pulls in velocities from the ocean spectrum node to create a constant wave simulation.

Wave Tank

Only has the initial wave setup. After the initial frame, the simulation will begin to deviate from underlying ocean spectrum. However, the velocity applied at the simulation boundaries will maintain energy on the surface.

Beach Tank

Pulls in velocities from the ocean spectrum node to create directional waves across the length of the tank.

Flat Tank

Does not start with any waves, but will react to objects that disturb it.

Ocean Flat Tank

Does not start with any waves, but will react to objects that disturb it. The waves are added at render time.

Tracking

Guided Ocean Layer

Can follow moving objects.

Wave Tank

Can follow moving objects.

Beach Tank

Can not follow moving objects.

Flat Tank

Can follow moving objects.

Ocean Flat Tank

Can follow moving objects.

When it is useful

Guided Ocean Layer

Useful for shallow objects moving through the water, like a boat.

Wave Tank

Useful for short ocean simulations where you don’t need to control the shape or movement of the ocean.

Beach Tank

Useful for simulating waves breaking on a beach or other shallow water terrain.

Flat Tank

Useful for any calm water simulations that don’t require waves, or for simulations that are going to displace a lot of water, such as a whale moving around and jumping out of the water.

Ocean Flat Tank

Useful for simulations where the interacting objects are much bigger and/or faster than the wave motion, so the ocean displacement can be added as a post-process at render time.

Understanding the network of nodes

There are three important layers to focus on when creating your ocean. First create the ocean, next add whitewater, and finally add specularity for the whitewater.

Tip

Disable all whitewater nodes at the OBJ level while you work on your ocean, then disable the ocean nodes while you work on your whitewater.

The first set of nodes control the ocean itself.

  1. fluidtank_initial controls the first frame of your simulation. This is where you can shape the initial frame of your tank with Ocean Spectrum and control the outputs with Ocean Source. This includes the size of your tank, the depth of the water etc.

  2. AutoDopNetwork controls the simulation of your tank. This is where you will find the FLIP Object and FLIP Solver. The FLIP simulation will contain Volume Source DOPs to sink and source the boundary layer particles, and a Gas Guiding Volume DOP to add ocean velocities to the simulation. Note, the Beach Tank shelf tool uses POP Advect By Volumes to inject ocean velocities.

  3. fluidtank_fluid is the result of #1 and #2 combined, and is where the results are rendered. After the simulation is done, this node collects the fluid particles, sets up a material, creates some nodes for surfacing to finish the effect.

  4. fluidtank_interior is also used for rendering, and for creating the volumetric effect that one of the shaders applies to the interior of the fluid. It controls the volume beneath the surface, such as how cloudy or murky the water will appear.

The next set of nodes are to control the whitewater.

Note

To add whitewater to your simulation, use the Whitewater tool on the Oceans shelf.

  1. whitewater_source is where the spray and foam is coming from.

  2. whitewater_sim is where the whitewater simulated. This is where you can modify the animation.

  3. import_whitewater is the result of #1 and #2 combined.

For more information see How to animate a wave tank with whitewater.

Note

If you build a completely procedural ocean (using the the Large Ocean or Small Ocean shelf tools for example), the Ocean Foam SOP is used.

Changing the look of your ocean

To...Do this

Set the height of the wave

Navigate to the Ocean Spectrum node and adjust the Wave Scale parameter on the Wave Amplitude tab.

This value is multiplied by the Speed parameter on the Wind tab.

Set the direction of the waves

Navigate to the Ocean Spectrum node and adjust the Directional Bias parameter on the Wind tab.

This controls how many frequencies are moving in the same direction as the wind. Increasing this value will cause more frequencies to travel in the same direction, which is useful for creating shoreline effects.

You can also try increasing the Directional Movement parameter. This will dampen the waves moving in the opposite direction of the wind, leaving only the ones moving in the same direction.

Control the height of the peak

Navigate to the Ocean Spectrum node and adjust the Chop parameter on the Wind tab.

Increasing this parameter creates sharp peaks on waves. However, if this value is too high waves, may invert on themselves.

Add more detail to your ocean

Increase the Resolution Exponent parameter on the Ocean Spectrum node.

Note

The Resolution Exponent parameter will not only determine the quality of your ocean, but also the size of the texture maps that you will eventually write out.

Create a large ocean

Use the Large Ocean shelf tool.

Understanding the extended ocean surface nodes

The Guided Ocean Layer and Ocean Flat Tank tools also create a network for rendering an “extended” fluid surface that can stretch to the horizon and be rendered with ocean displacement to provide a seamless integration between a FLIP simulation and the surrounding source ocean.

  1. The Particle Fluid Surface node (called particlefluidsurface1) creates the extended FLIP mesh. The settings on the Flattening tab of this node will flatten the fluid simulation near the edges of the simulation area, and extrude the edges of the polygonal mesh outwards to form an extended flat surface. This flat area surrounding the simulation is then rendered with ocean displacement.

  2. The two Particle Fluid Mask nodes (called particlefluidmask1 and hifrequency_mask) will create masks on the input ocean spectra that limit the render-time ocean displacement to specified areas of the simulation, usually where there is little velocity, vorticity, or splash height in the fluid. These nodes also blend in ocean displacement around the edges of the simulation bounding box to match the flattening applied while surfacing. The hifrequency_mask node additionally filters the incoming ocean spectra to remove all but the highest-frequency waves, usually so these can be applied to the simulated FLIP mesh and allow it to better integrate with the surrounding ocean.

  3. The surface_preview node is a fast preview of the FLIP simulation that then samples the created mask to allow visualization of the ocean contribution using a viewport visualizer on the mask attribute. The oceanevaluate1 node applies ocean displacements to that preview based on the calculated masks.

  4. The split_spectra_masks node will separate out the static ocean spectra from the time-dependent masks created by the masking nodes. They are cached to two different sets of files to take advantage of the Ocean Surface material’s ability to have separate spectra and mask inputs.

Fluids

For beginners

SOP WORKFLOW

Particle Fluids (SOP)

Viscous fluids (SOP)

Optimization (SOP)

FLIP Configure tools (SOP)

DOP WORKFLOW

Particle Fluids (DOP)

Viscous Fluids (DOP)

Oceans (DOP)

Optimization