opacity issue

[Dune]

OK, here goes; I'm trying to designate part of a forest to be burnt, without using different populations. So trying to re-texture the bark in a certain area (which works), but also getting rid of the leaves within that area. Should work through the opacity tab, but doesn't. The strange thing is that only the distribution shader works! But that's only a Y limitation, and I need an XZ limitation as well. Here's a couple of tests I did this morning, so you know what doesn't work.

There are also some strange conclusions to be drawn if you check the (absence of) leaves and their shadows in the different scenarios.

If anybody can get this to work I'd be a happy guy again 

[Tangled-Universe]

17 examples which somehow show your issue, but that's for us to figure out

There's an issue here, but I can't wrap my head around it yet...hmmm.

So given your bug report on the alpha forum it seems it is related to the fact that you're using a transform shader in world space.
Distribution shaders do not work in world space, but in texture space and is the reason why it works on restricting for Y.

So there seems to be a "space" issue here where you are trying to do things in the wrong type of "space", but I'm not sure how to get around it and whether that's even possible if it's truly a bug.

[Dune]

Yes, I thought I'd give you a nice challenge and all the info I got. I particularly find it strange that you see the white on the terrain in the right area (XYZ), but can't translate that to the trees' opacity or merge shader control. Only the height through the distribution shader.

[zaxxon]

At the risk of being overly simple: why not create a couple of 'burnt trees' objects and then just populate as needed? 

[Dune]

Yes, that's the easy way, and I will probably do that. This just doen't work, I'm afraid. But it would be handy if you could change burnt areas by PF-etc, even for animation purposes.

[Tangled-Universe]

Yeah I feel with you Ulco, being a 'purist' myself

[Dune]

Regarding animation of a burning forest; I can envision the line of burnt wood moving (by animated transform shader), where ever more trees get darkened, leaves crumpled (partial opacity... even temporarily illuminated/lit), smaller branches also 'opacitated out', while a line of smoke and fire (cloud also) moves with it. 
Can't very well be done by replacing pops.