Video games, especially the early 3D games, use a "distance fog" to improve performance, like this:

 

How could I achieve the same in Poser? Here's what I've tried so far:

1. Atmosphere: This works, but render time goes over the roof, so it's not an option for me. 

2. Simple (or several) planes with cloud nodes: Got close, but I have to be moving the planes around for every shot, and I can't get the depth effect right, things look IN or OUT of the fog, but not comming in or out of it (like the trees of the first picture)

One thing I haven't tried is post working it in (not doing animation so that's an option). I suppose I would need to generate a distance gradient map or something similar. Any help is appreciated.

I think the two-planes solution will work.  If you watch a real fog-bank closely, you'll see that everything appears to be either in or out of the layers.  For objects on the scale of cars or trees, there isn't a visible 'in-between'.  I suppose the in-between-ness *might* show on something like a very long warehouse that runs parallel to the road.

If you want the fog to be 'lifting' so it's stronger above, you can use the V variable to control the strength of the plane's transparency.

The power function makes it possible to shape the up-down fade better.

What about a curved prop? Flip it around so the flat side is closer, and the curve curves into the distance towards the bottom?

Quote - Atmosphere: This works, but render time goes over the roof, so it's not an option for me.

You're using the wrong kind. The distance fog effect is almost free.

It's the checkbox "DepthCue_On" - turn that on but do NOT enable "Volume_On".

Adjust the DepthCue_Color to a light gray as seen in the shots you posted.

The DepthCue start defines where the fog begins (distance from camera) and the end defines where the fog becomes 100%.

Bill's suggestion is marvellous if you want/need to render all in one go.  As I'd never thought of that solution, I've just tried it - the attached render took less than three minutes just now on a 5 year old Quad Core system, so I can vouch for Bill's assertion that it's cost-free; not an ideal scene to demo fog with but I happened to have it loaded as I was browsing.

For me, I usually go down the ZDepth render/postwork route as the amount of fog or haze is then easily adjusted, plus the ZDepth render is essential for masking and other 3D effects like depth of field.

Thanks for the hint - it's a nice and easy way. 

Henning 

@ockham, You are right, it was a mild case of "I expect reality to be different than it actually is", probably having to do with "learning how to see" artist talk about, I lack that lol. Thank you.

@bagginsbill, thank you! I played with the options but for whatever reason it didn't occur to me to untick volume, that did the trick and like you say, it's almost free.

Thank you everyone.

Fascinating!  Thanks.

Quote - Bill's suggestion is marvellous if you want/need to render all in one go.  As I'd never thought of that solution, I've just tried it - the attached render took less than three minutes just now on a 5 year old Quad Core system, so I can vouch for Bill's assertion that it's cost-free; not an ideal scene to demo fog with but I happened to have it loaded as I was browsing.

For me, I usually go down the ZDepth render/postwork route as the amount of fog or haze is then easily adjusted, plus the ZDepth render is essential for masking and other 3D effects like depth of field.

Beautiful render.

May I ask how do you create the ZDepth mask in Poser?

Update: Found the answer in this post