A mask rendered using Terragen 2. It might make someone a nice terrain. It did not work as a mask, maybe I do not know how to do this yet.
http://www.flickr.com/photos/njeneb/4993451047/
Interesting
I don´t know if I can do better, but how did you try to use the mask?
A new camera pointing down at the population center and camera projection. I tried making the black transparent (zero opacity). I tried mapping to x,z.
Maybe it has to be png format with alpha channel turned on?
For a geographic mask the best bet is to use it in an image map shader - Play Y Projection. Set it to be the exact same size as your heightfield.
See here for far too much info on the subject: http://www.archer-designs.com/tutorials/terragen2/working-with-gis-data/index.html (http://www.archer-designs.com/tutorials/terragen2/working-with-gis-data/index.html)
That may be another issue. This is a fractal terrain, not a height field. I kept the camera exactly where the image was rendered from. I used Walli's method of using the preview color and rendering.
http://forums.planetside.co.uk/index.php?topic=10456.0
Anyway, I went back to using distribution shaders with altitude and slope constraints.
Although it may be obsolete now:
the only problem I found using this image map as blending shader for surface layers or populations was that it made the populations very thin, being perhaps simply to dark.
After adjusting in Photshop (70% brighter or more) it made a population that looks quite nice.
Best regards,
Jan