Unity requires the heightmap resolution to be a power of 2 + 1. In order to achieve the best results given this constraint, you’ll need to understand how the height field node parms marked in red boxes above work together and should be used.

For Sampling use Corner type to match Unity’s terrain generation, as well as the power of 2 size requirement. Houdini uses Center sampling by default but this results in floating point calculation errors when converting to Unity terrain. Therefore Corner sampling is recommended.

For Grid Spacing, use 1 or use a power of 2 (e.g. 2, 4, 8, etc.) to control height map resolution.

For Size use power of 2 starting at 64 (e.g. 64, 128, 256, 512, 1024, 2048, 4096, etc.).

Example settings using Corner sampling:

In all cases, Unity’s heightmap resolution must be power of 2 + 1. These would be:

• 8193 x 8193

• 4097 x 4097

• 2049 x 2049

• 1025 x 1025

• 513 x 513

• 257 x 257

• 129 x 129

• 65 x 65

When using other heightmap resolution sizes, Unity will automatically change to the next lowest power of 2 size, which means the height values will not be properly applied. Therefore stick to power of 2 values for the height field size and grid spacing.

The plugin ships with an example HDA (Assets/Plugins/HoudiniEngineUnity/HDAs/TerrainGenerator.hda) which shows the various recommended configurations of sizes and grid spacing values.

For using Unity terrain as input to generate Houdini height fields, it is just a matter of reversing the above calculation to figure out the height field size and grid spacing. Note that both the terrain width and length must be the same value (i.e. square sized terrain). Use the following steps:

1. decide on the grid spacing, either 1 for simplicity, or a power of 2 (e.g. grid spacing = 2) 2. for terrain width and length, choose a power of 2 size starting at 64, and add the grid spacing (e.g. terrain width, length = 512 + 2 = 514) 3. for heightmap resolution, it should now be the terrain width - grid spacing divided by the grid spacing plus 1 (e.g. heightmap resolution = (514 - 2) / 2 + 1 = 257)

The simplest method is to first choose a power of 2 value for terrain width and height, then add 1 for both the final terrain width and height, as well as the height map resolution (e.g. 512 + 1 = 513).

A Houdini height field volume consists of height and mask layers, and potentially could have a number of other layers. A Unity terrain consists of a Terrain component, and a TerrainData with height values, any number of TerrainLayers (or SplatPrototypes for Unity versions prior to 2018.3), and corresponding number of splatmaps. A TerrainLayer specifies the material to paint on the terrain. The splatmap specifies the masking or weights of each pixel from the TerrainLayer material at each point on the terrain.

The following describes how Houdini’s height field layers are treated by the plugin:

The height layer will be used for the terrain’s heightmap values. The range of the heightmap values form the height range for the terrain. The unity_hf_terraindata_file attribute can be specified on this layer (and only on this layer) to reference an existing TerrainData asset file to use (Houdini Heightfields to Unity Terrain). There will NOT be a corresponding TerrainLayer created for this layer.

The mask layer is NOT used.

For each of the other layers, a corresponding TerrainLayer and a splatmap will be created or loaded, and set on the TerrainData. Therefore a height field volume with, for example, 4 layers (height, mask, ground, cliffs), will have 2 TerrainLayers and 2 splatmaps; a TerrainLayer and splatmap for the ground and the other 2 for the cliffs. The height field layer’s name will be used for the TerrainLayer’s name. An existing TerrainLayer asset can be specified via the unity_hf_terrainlayer_file attribute. On Recook, existing TerrainLayers will be reused if their name matches with the height field layers' name. If not specified or not found, the TerrainLayer will be created. In order to maintain the reference in the Unity project, the TerrainLayer will be saved to an asset file (see Generation for the saved file path).

The splatmaps are generated from the height field layer’s height values. The values should be between 0 and 1 inclusive (i.e. normalized). If there are values outside this range, the plugin will normalize them back to within this range, using the minimum and maximum values (each value is subtracted by the minimum and divided by the range). Also, it is recommended that the splatmap values of each layer at any coordinate sum up to a total of 1. For example, if there are 2 splatmap layers, then for the splat values at (0,0), if the 1st layer has value of 0.7, then 2nd layer should have a value of 0.3. Note that ultimately these values are used by the material and shader used by the Unity terrain, so the recommended range is for the default Unity terrain material.

To create values between 0 and 1, a HeightField Remap SOP node could be employed. The following screenshot shows an example of this for height field layer named base:

TerrainLayers UI ¶

The plugin provides a Strength control for each splatmap layer, exposed on the HDA’s Inspector UI. The Strength value is a multiplier applied to each splatmap value, with a default value of 1. It allows to control the splatmap values generated from Houdini with a single parameter, and is useful for troubleshooting and when additional TerrainLayers have been manually added by the user.

TerrainLayers Example with Tiles ¶

The following HDA contains a height field network that outputs 2 merged terrain tiles, with 3 height field layers (height, mask, and base). The base layer is a copy of the height layer, with values remapped to 0 and 1. An string attribute is created on the base layer to specify the TerrainLayer file to set in Unity for the base layer.

The following image shows the final merge node’s info. There are 6 layers in total (3 layers (height, mask, base) for each terrain tile):

Finally, the HDA is loaded in Unity and generates 2 terrains, each having a single TerrainLayer (from the base height field layer):

During generation of a Unity terrain, the plugin does several things depending on the asset settings, existing terrain state, and cooking type:

Materials ¶

The material used for generated Terrains can be specified through the Plugin Settings window, in the GEOMETRY section:

For Unity HDRP projects, it is recommended to set this to the DefaultHDTerrainMaterial that is shipped with the HDRP package. For other render pipelines, use the default Unity terrain material shipped for that pipeline.

Similarly, the default splat texture can be changed in the Plugin Settings to point to another texture file.