Unity terrains can be furnished with trees. Patches of trees can be painted onto a terrain in much the same way that heightmaps and textures are painted but the trees are solid 3D objects that grow from the surface. Unity uses optimisations (eg, billboarding for distant trees) to maintain good rendering performance, so you can have dense forests with thousands of trees and still keep an acceptable framerate.
The tree button on the toolbar enables tree painting.
Initially, the terrain will have no trees available but if you click the Edit Trees button and select Add Tree you will see a window to select a tree asset from your project.
Unity comes with a tree object in the standard assets for prototyping purposes (menu:) but you can also create suitable objects with a 3D modelling app or by using Unity’s Tree Creator tool.
With a tree selected, you can paint onto the landscape in the same way you paint textures or heightmaps. You can remove trees from an area by holding the shift key while you paint and remove just the currently selected tree type by holding down the control key. The familiar Brush Size option is available for tree painting but the Opacity property is replaced by Tree Density, which controls the average number of trees painted into a given unit of area. There is also a control for Color Variation and the trees’ heights and widths can be specified, again with variation. The variation options help to create the impression of a random, natural-looking forest rather than an artificial plantation of identical trees.
The Mass Place Trees button is a very useful way to create an overall covering of trees without painting over the whole landscape. Following a mass placement, you can still use painting to add or remove trees to create denser or sparser areas.
Unity has its own Tree creator that you can use to produce new tree assets but you can also use a standard 3D modelling app for the task. The tree mesh should have fewer than 2000 triangles (for performance reasons) and the anchor point should be right at the base of the tree where it emerges from the ground. The mesh should always have exactly two materials, one for the tree body and the other for the leaves.
Trees must use the Nature/Soft Occlusion Leaves and Nature/Soft Occlusion Bark shader. In order to use those shaders you also have to place the tree in a special folder that contains the name “Ambient-Occlusion”. When you place a model in such a folder and reimport it, Unity will calculate soft ambient occlusion in a way that is specifically designed for trees. The “Nature/Soft Occlusion” shaders rely on the folder naming convention and the tree won’t render correctly if you don’t follow it.
When you save a tree asset from the modelling app, you will need to click the Refresh button (shown in the inspector when the tree painting tool is selected) in order to see the updated trees on your terrain.
You can add a Capsule Collider to a new tree asset by instantiating it in the scene, adding the collider (menu:) and creating a new prefab for the modified tree object. When you add the tree to the terrain for painting, make sure you select the prefab with the collider rather than the original object. You should also enable Create Tree Colliders in the terrain’s Terrain Collider component inspector to allow.
Note that the physics engine has a limit of 65536 colliders in a single scene. This is normally not an issue but when trees are used with colliders, you should make sure you don’t exceed this limit (also bear in mind the colliders other than those on trees that you are using in the scene).