Types of Tools
The Open World Tools in Unreal Engine 4 allow you to place Static Meshes inside your levels using a computer algorithm to determine placement instead having to do this work manually. This enables you to quickly generate a natural looking forest comprised of many different species of trees, plants and bushes.
When talking about the Open World tools we are actually referring to the two different tool sets inside of UE4. The tool set that you use depends on what you want to accomplish. For example if you want to make an expansive pine and birch tree forest then you're going to want to use the Procedural Foliage Tool. If you want to densely populate the Landscape Terrain used in that forest with grass, flowers, and small rocks, then you would use the Grass Tool.
Procedural Foliage Tool
The Procedural Foliage Tool is used for simulating huge forests that are filled with lots of different kinds of trees and bushes. This tool works by simulating how a forest grows over the years using Steps to define years of growth. At each Step in the simulation new virtual seeds are cast into the level to act as spawn locations for other Foliage Type Actors. By tweaking the various Foliage Type options like Spread, Size, and Priority Settings, you will affect how those Foliage Types spread and compete for spawn locations through the level the Procedural Foliage Spawner in placed in. To learn more about how to use the Procedural Foliage Tool check out the Procedural Foliage Quick Start.
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Grass Tool
Currently the Grass Tool will only work in conjunction with the Landscape Terrain system.
The Grass Tool is used to densely cover the Landscape Terrain with things like grass, flowers, and small rocks. You can use the Grass Tool to define a Static Mesh that will spawn when a certain Landscape Layer is painted on the Landscape Terrain. This tool does not use any kind of simulation like the Procedural Foliage Spawner does. Instead, it simply fills the specified area at the specified density with the specified Static Mesh. To learn more about how to use the Grass Tool, check out the Grass Tool Quick Start that is linked below.