I am trying to make this image, with the ship emerging from a gas cloud.  I used a v-node on the volume density, but it seems to be filling from the back of the image, although I want it to stop halfway up the ship.  Also, the higher I make it stop, it tends to arc and stop being a straight line.  I have tried a lot of different things, but they all seem to work the same as the settings in the screenshot.

 Also, I can't seem to get any decent shadows in the atmosphere.  The fog looks ok with real high atmospheric strength on the light and a real low (.008) density or vice versa, but when I bring the numbers closer together the fog gets real bright and starts to wash out the image. 

Sorry, here's the original image.

You want to modulate based on height above ground? But you're using the V coordinate of the texture mapping. I don't know that the "fog" of atmosphere even has a V coordinate.

You want to use the P node, not the V node. P is position. The X, Y, Z coordinate returned is first multiplied with the values in the parameters of the node. So if you set them up as 0, 1, 0, it will discard the X and Z coordinates. You can then just use the number coming out.

Technically the P node is a coordinate, not a color, but it will be handled like a color. This means if you plug it into a Math node where a number goes, the "color" will be collapsed to a gray level by averaging the R, G, and B. If you use the 0, 1, 0 values, then the resulting numerical value is (0 + Y + 0) / 3 or Y/3. Note also that Y is in tenths of an inch - an odd unit.

So I usually set it up as 0, .3, 0 and then the averaged output is exactly in inches. Of course you could use .3/12 instead of .3 and you'd get feet. If you're into metric, .3*2.54 would be centimeters, and .3 * .0254 would be meters.

Attached Link: Making low lying fog/mist

See the linked thread. There are screen shots of height-limited atmosphere.

 

I have to say, I was secretly hoping you would see this :)  Thanks for the tip, that was all I needed to get the height working.

 The positioning of the fog worked out rather well, and i'm happy with it, but I just can't seem to get the shadows right. I decreased the atmospheric density to.06 so as not to obscure any shadows, and increased the atmospheric strength on the single overhead light to 2, while giving the volume color a dark gray so as to offset any glow from the increased light strenght, but still nothing.  For now I'm going to revisit the old tutorial for rays through blinds and see if I can get any new insight from a step-by-step walkthrough.