Houdini provides a wide range of nodes and tools for generating realistic terrains through heightfields. A heightfield doesn’t consist of traditional geometry like vertices and polygons. You can think of a heightfield as 2-dimensional grid with cells. The intersecting points of that grid contain the terrain’s height information.

This approach lets you combine and stack different types of terrain, e.g. a bedrock with a layer of soil on top. These elements are called height layers, while the base layer - here it’s the bedrock - is always named height. Layers play an important role with heightfields in general and it’s all about stacking and manipulating layers to achieve the results you're looking for. The advantage with this technique is that you keep full artistic control, because you can turn on and off layers temporarily, work on each layer individually or add new layers at any point of your network.

Most terrain nodes take a second input that can contain a mask layer to control, which parts of the terrain the node will modify. The default mask layer is named mask. As with height layers, it’s possible to have multiple, differently named mask layers in a geometry stream. Then you can specify which mask layer you want to use, e.g. for applying erosion or terraces.

The Houdini viewport is capable of visualizing the 2D height field as a 3D surface, and the mask field as a red tint on the surface. Heightfields aren’t limited to huge terrains, but you can also create smaller landscapes, riverbeds, seabeds, deserts, and many other geological structures. It’s also possible to paint your own map and use it as a basis for a complex and highly-detailed terrain. The perhaps most realistic approach is to load a displacement map from a real landscape and turn it into a heightfield.

As mentioned before, heightfields are flat 2D grids and all height information is stored in the grid’s points. For this reason it’s not possible to work on a terrain’s vertical areas. A good example are the vertical parts you get from a HeightField Project SOP. This nodes takes regular geometry and projects it onto the 2D grid. Masks can also create very similar effects. The image below shows a sphere that is projected onto an empty heightfield. The vertical parts are just stretched along the Y axis.

If you want to add detail to the vertical parts, you can erode the terrain or convert the heightfield into actual geometry. Another possibility is to scatter objects over those areas to create structures.

Of course, Houdini’s Karma CPU and Karma XPU render engines support heightfields. Anyway, before you can render a heightfield you should convert it into polygons. This step is not absolutely necessary, because Houdini will convert the terrain automatically when you import it to Solaris. However, you have more control over the resulting geometry with manual conversion, for example if you want to you bake colors and layers to the terrain.

Different noise types are one of the centerpieces in conjunction with heightfields and you can find them everywhere: terrain creation, masking, distortion, patterns, etc. Especially the various noise types on the HeightField Noise SOP are perfectly suited to quickly change the characteristics of a terrain. You can create landscapes with smooth hills or mountains with sharp ridges and steep slopes.

On the first look, some noise types like Chebyshev Cellular F2-F1 might create strange patterns. With appropriate distortion and stacked noise types you can create highly-detailed terrains. Then you apply erosion to the mountains to get a realistic look.

The noise settings of different nodes also share several common parameters. The most important are probably Amplitude and Element Size. Amplitude defines the maximum height of the terrain. Element Size, however, refers to the noise pattern. In terms of heightfields you add more spikes and irregularities with small values, while higher settings create a smoother surface.

The ratio between Amplitude and Element Size is also relevant.

• When Element Size is small, you normally decrease Amplitude.

• With large Element Size, the terrain becomes flatter and you’ll most probably increase Amplitude.

The image below shows that you can achieve different types of terrain just by varying Element Size and Amplitude.

To create a basic heightfield, you need only a couple of nodes. The nodes' parameters let you precisely shape the landscape and you can create distant hills with a reduced level of detail or high-resolution ground object for scattering. You normally only have to alter very few parameters to get a completely different look.

Heightfields are created through geometry SOP nodes, so everything starts on Houdini’s obj level, where you press ⇥ Tab to open the tab menu. From there, create a Geometry OBJ that will serve as container for your landscape. Double-click the node to dive inside.

A mask restricts an effect to a certain area. In conjunction with heightfields you can define, where the terrain should appear, while areas outside the mask will remain flat. When you press ⇥ Tab to open the tab menu and enter hfmask, you’ll see a long list of nodes for the creation and manipulation of masks. To illustrate the basic concept, lets start with a noise mask.

If you've already created a basic heightfield, you can proceed with the existing network. Scale and look of your heightfield don’t matter. Here’s a preview of the network you're going to create.

1. From the tab menu, put down a HeightField Mask Noise SOP.

2. You must place the new node between the already existing HeightField SOP and HeightField Noise SOP nodes to see an effect. Connect the mask node’s

   • first input with the output of the heightfield

   • output with the second input of the heightfield noise.

3. When you look at the mask node’s parameters, you’ll recognize the same parameters as with the HeightField Noise SOP: Amplitude, Element Size, and the Noise Settings section. Play with the parameters to achieve different looks. Amplitude, by the way, acts like a like a multiplier for the height values.

To control the mask’s actual look, turn on the mask node’s blue Display/Render flag. Now you’ll see a red and white pattern, and mountains are only created in the red areas.

• The Terrain desktop has a specialized shelf set, a radial menu, and its own pane layout designed for your work with terrains.

• You can take advantage of the ⇥ Tab menu’s smart completion when type height or just hf instead of height field when searching for appropriate nodes. For example, you can type hfnoise to get a list of all available heightfield noise nodes.

• Heightfields and masks are regular Houdini volumes and you can use any geometry (SOP) node that works with volumes to edit heightfields and masks.