Hi,
My guess would be you need to find some way to limit the population using the density shader param of the populator. An example might be to use a distance shader to limit how far from the camera instances appear. You might also use a painted shader to paint out parts of your terrain which never get seen during the animation ( back sides of mountains etc. ). You'll need to invert it because normally the areas you painted would be the ones populated. You can use a distribution shader to avoid places grass might not grow. Depending on your scene you may need a more or less sophisticated approach, but I'd say the density shader is where you need to start.
Depending on your scene and animation if may be quicker just to do the populating up front and not worry about limiting it. If you use any of the stuff I've mentioned above, aside from painting out where the camera never sees, you'll need to turn on "Repopulate every frame" and the time to repopulate each frame might add up to be more than if you just did it once. On the other hand :-), doing the population every frame could save you some memory because less instances would be created.
In the next release there will probably be a "Clip to camera" param to limit population to the area inside the camera frustum, but that's not going to be much help to you now.
Regards,
Jo
