How to add Grass Area dynamically ?

I want to add grass dynamic, but the Grass only can added by density map,
so ,if i want to add a new area in real time, i should generate a new density map in real time,


Is there any way ?

Take a look at PropertyMaps.h, and specifically the documentation for DensityMap::getPixelBox(). All you need to know should be described in detail there (also in the manual).