Eneroth Fractal Terrain Eroder
Splits every selected face to smaller ones and randomly moves around endpoints to make it look more like a natural terrain.
Eneroth Fractal Terrain Eroder

How to use it

First draw your terrain or other geometry you wish to erode. Then select the faces and click Plugins > Erode.

What it does

The plugin starts by converting selected faces to triangles. These are split into smaller triangles of which the endpoints are moved randomly. This is repeated a number of times to get a natural looking surface. Points binding other faces than those selected are not altered, meaning other faces doesn't get changed.

The original geometry is backed up to a group on a hidden layer in case you want to go back to work with the less complex geometry.

Known bugs and issues

If the selection makes up an inner loop of another face its sides will be disconnected to that face causing unexpected results. An easy workaround is just to draw a temporary edge between the selection and an outer endpoint  of the face.