How to "Erode" Terrain on an Elevation Map?
My normal map-making process involves a greyscale layer for elevation: Black- sea level, White- Everest, Greys- everything in-between. I have no problem painting in most of the elevation in Photoshop, using custom brushes, etc. The detail that bogs me down is the erosion. If you look at an elevation map of the world you can see the river valleys carved out of the mountains, and the various tributaries. That is very time-consuming to paint in detail (so usually i don't). I've spent some time browsing the forums for this answer.
Does anyone have a technique, plugin or software add erosion detailing to an existing hight-map?
I use OsX, so Windows only software won't help.