Velocity is turned on by default and you can directly map the different velocities to the simulation: From the Visualization dropdown, choose Color Water by Layer. If you already went through the introduction to the Shallow Water Solver, you know that this option adds a turquoise-blue gradient that represents the water’s velocities. The Visualization parameters let you control the gradient’s color distribution.

For more information, open the Geometry Spreadsheet and click the Primitives button. There you’ll see that there’s no combined velocity layer, but you have individual vel.x, vel.y and vel.z layers instead. It’s also important to know that vel.y is always zero, because heightfields are 2D grids without a physical height.

The vorticity layer stores the local rotational motion at each point, with larger absolute values indicating faster spinning. Positive vorticity values signal that rotation is in the counter-clockwise direction when viewed from above; negative values indicate a clockwise rotation.

The Shallow Water Solver does not provide any built-in features to visualize vorticity and you need a custom solution.

Acceleration is the rate at which an object’s velocity changes over time. It can mean speeding up, slowing down, or changing direction. Like velcoity, the acceleration layer consists of three components: accel.x, accel.y and accel.z, but again, only the X and Z components are relevant. There’s no out-of-the-box feature for displaying this layer and you need a custom solution.

The solver’s Binding tab provides an empty Force Mask Layer. You can create a custom, animated mask and connect it to the slot. The solver will use the mask values to push and displace the water.

Houdini’s heightfields provide a HeightField Visualize SOP that lets you define custom colors for up to nine layers. A common application with this node is texturing.

The visualize node is typically the last node of your network, because only then you can be sure that all layers will be available. The node’s usage is straightforward:

• On the Material section you can see a Height Ramp. The gradient tints the terrain according to the values of the height layer. You can click Compute Range to map the entire range of gradient colors to the terrain. When you click the Presets button, you choose from various predefined color ramps.

• Each of the nine Layer parameters has an associated dropdown menu that list the entire range of existing layers. Choose a layer or type its name to an empty parameter field. Then, define a Color. The layer will be displayed with the chosen color.

You can visualize the information from the solver’s layers to create trails with mesmerizing swirling and curling effects. As the name indicates, the node was made to show volumes, but a heightfield is a 2D grid. The Y coordinate is always zero and this is the reason why you will also only see a flat representation.

1. Add a Volume Trail SOP to the network and connect its second input with the output of the solver.

2. Lay down a Grid SOP and connect its output with fist input of the solver.

3. Adjust the grid’s Size parameters so that they match the Size values of the HeightField SOP. For example, if the heightfield is 100 m by 100 m, the Size values for both nodes are 100 as well.

4. The grid’s Rows and Columns parameters define the resolution of the trails. It’s not possible to give exact values here, because they depend on several factors.

5. On the trail node, go to Velocity Volumes and enter the name of the field you want to visualize. You can also choose an entry from the parameter’s associated dropdown menu.

6. Then, adjust Trail Length. Longer trails take longer to be drawn and will also create a denser “map”.

If the shallow water simulation is cached, you can move the Playbar head or press the Play button to see the trails in motion.

Houdini offers a wide range of methods to create custom fields. However, the various techniques have one thing in common if you want to use them with the Shallow Water Solver: The force must be converted into a mask. The following example illustrates how to draw an animated radial mask that pushes the water away from the mask’s center.

// Parameter section
vector center = chv("center");       // Custom parameter: Wave center
float speed = chf("speed");          // Custom parameter: Wave speed
float frequency = chf("frequency");  // Custom parameter: Wave frequency
float amplitude = chf("amplitude");  // Custom parameter: Mask strength (= force strength)
float falloff = chf("falloff");      // Custom parameter: Strength attenuation factor
float time = @Time;                  // Current simulation time

// Current heightfield position
vector pos = @P;

// Distance of the current position to the wave center
float dist = distance(pos, center);

// Wave creation
float wave = sin(dist * frequency - time * speed);

// Remap wave to a [0,1] range and scale the result with the waves' strength
float mask = fit(wave, -1, 1, 0, 1) * amplitude;

// Optional: Wave strength decreases with increasing distance from the center
mask *= exp(-dist * falloff);

// Write the result to the heightfield's @mask layer
@mask = mask;