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This is a paid asset, but now you can download Erosion Brush Free.
Erosion Brush v1.52
A brush tool for creating erosion. It generates an effect of realistic hydraulic action, weathering and sediment under your mouse cursor, setting both heights and terrain textures.
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This script is targeted not to make the terrain of your dreams in one click, but to provide the artist with a handy and useful tool. It gives an artist the ability to modify terrain to make it realistically eroded in the places where it is really needed to, leaving all the others untouched. Terrain is edited in Unity without using third-party standalone toolkit; not before creating a location, but while working on it – all this gives the level artist a chance to create amazingly looking scene in a very efficient way.