World opacity is very important for my workflow, and I thought I'd share a word about it for you guys, in case you didn't know. It's been made possible again after a long absence, but still has its bugs (like the leaf thing).
Benefit is that you can now use a color mask to really get all grass (e.g.) off a certain area, especially handy when using wider patches of grass.
I found that using an altitude restraint didn't work as expected; with a final position minimum altitude (e.g.) the iterations come in the right place allright, but when also using world opacity the distribution shader also needs Y to be checked, or they won't show up at all, at least not in RTP.
It is cool you got this working, though it's a shame it apparently broke most other opacity. Not really a good trade off currently. I use opacity far more for other stuff that is OBJ based. For example help topic I just shared and a planet I wanted to transform into a half planet base with a core sticking out like a image I saw. Or my old snow globe landscape. A lot of my objects with holes for certain things use opacity maps too, like swords and such.
By the way, the result looks good for the paths. I'd usually just use smaller patches and mix with larger patches at a distance. Probably more work.
I think Matt should look into this. I can't miss the world opacity anymore, but the fact that you need Y checked also is a bug, actually. You can't have accurate altitudes a bit from 0/0/0 that way. And of course the stuff you mention.
Indeed, for a 6k render and up, so getting enough acurate detail in the distance as well, with small one-man paths and such, having to fill a minimum of a square kilometer with 10cm patches of grass is quite undoable.