Painting Terrain Tool
The Painting Terrain Tool is available from the list of tools whenever you are in Terrain Mode. Essentially, the Painting Tool does exactly as advertised, it "paints" on whatever texture you have selected in the Terrain Editing window. This can be used to mask/unmask texture layers on the terrain, as well as modify the heightmap for you terrain (producing increases or decreases in the height of your terrain).
Using the Painting Terrain Tool
In general, holding down Ctrl+LMB and moving the mouse over your terrain will paint more white onto the selected texture, and holding down Ctrl+RMB and doing the same will paint more black. This has different effects depending on what you are painting (see below).
This tool is mostly useful in a 3D UnrealEd Viewport, though it will work in an Orthogonal viewpoint, which can be handy for working with texture layers (you'll need to enable terrain view in that viewport first).
Painting your Heightmap
By far the most common use of the Painting Tool will be with the heightmap for you terrain. Ensure that it is selected in the Terrain Editing window (the Terrains tab about half-way down the window should display all the heightmaps in your map) and then begin painting. The heightmap is based on having white represent the highest point in your map and black the lowest, so if you use the LMB to paint the Painting Tool adds more white to your heightmap, and the height of the terrain will increase. You can use this to make moutains, valleys, whatever you want.
Painting Texture Layers
The other common use of the Painting Tool is to mask or unmask terrain texture layers on your terrain. You do this much as you would a heightmap, just select your texture layer in the Layers tab of the Terrain Editing window and being painting. Here, adding more white (LMB) causes the texture to become more opaque, and more black (RMB) causes it to become more transparent.
If you remove every layer from your terrain it will become invisible, so if your terrain suddenly vanishes while working with this tool, just switch to the wireframe UnrealEd viewport and you should be able to see your terrain again.
Painting Decoration Layers
The Painting Tool can also be used to mask or unmask the alpha map for a terrain decoration layer. See the page on this for more information on the specifics. The general principles will still apply.
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Discussion
Birelli: I've made a start on this, but it could probably use some re-writing and clarification, if nothing else.
Category To Do – This could probably use some re-writing and clarification
