Compositing fog destroys PNG alpha in Images as Planes when using Cycles I'm following Andrew Price's spaceship tutorial and taking advantage of the inbuilt addon called Import Images as Planes.



Here you can see 2 Images as Planes used for the steam columns, close to the door.
If you ask why?, it's because I already have enough polygons, for my laptop to render...
And I thought that a smoke simulation would simply be an overkill.

Issue:

I wanted to try compositing, even if I'm still not ready with the definitive 
geometries and textures.
And... All is fine except for mist.

You can see I can use some Blur and Glare withuot big issues (apart the orange LED 
wall panels which became white - so I thought to assign them a different Material 
Pass and a separate ID Mask - now they are pinkish, but not so bad at all).



But! If I enable the Map Value node, which creates the mist... what happens it 
that the steam columns contours are MANGLED (well, not only the contours)!!



Now, is there a way to fix this issue (is it a bug or am I doing something wrong?) or do I have to give up and sacrifice that wornderful oniric mist?

[EDIT]

To whom can be interested in: the original blend is here (much better han mine).

Here's the Steam column I used for both Images as Planes:



You can't see it because it's white on a transparent background, resulting in white on white.
The problem is Z Channel in Cycles - it does not take into account material transparency:



I recommend not using the Z channel pass but to render nice custom Z pass like this:

Assign this material to scene objects (this material has value based of ray length - distance to camera):





You will get nice anti-aliased Z-depth render that you can use for the mist.  For alpha-transparent planes mix in Transparent shader like in the image and render them on separate render layer so there's nothing behind them (then composite them back in).

To create group node group them with Ctrl+G. Go inside and outside group node with Tab.