*
You would use the Distribution Shader setting/input of a Population for this. You can click the little ... button to the right of this setting and simply create a new shader there for the purpose, or link it to an existing one. You can use the Distribution Shader to control height/slope constraints or an Image Map shader (set to PlanY projection and with appropriate scaling) to do image-based distribution control.
- Oshyan
*
Oshyan,
[edit] After finding the explanation by Frank about how to use the Density Shader and Oshyan's explanation here about using the Displacement Shader, I made this work. Took some tweaking.