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How do I make a smog layer in the sky with volume?

Minyassa

Enthusiast
I've successfully used 3dcheapskate's height-dependent fog shader to make a ground-hugging fog in Superfly (I just switched depth cue to volume), but now I want to do the reverse and have a fog layer that's almost clear at ground level and thickens as it goes up. It doesn't translate directly just by inverting things, which I'm guessing is because trying to layer something onto infinite sky is much different than layering it onto a ground plane? So I've put in an atmosphere box, but still not having much luck. I just don't know the math that's involved in that sort of blending. I'd appreciate any help I can get.
 

Glitterati3D

Dances with Bees
Afrodite-Ohki's AO Level Up Superfly lights include a very effective atmosphere for this type of image. I used it here.
 

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  • BrunoLondonAtmo.jpg
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Sounds like you want a sort of cyberpunk/noir city, relatively clear at ground level but quickly getting foggier in the towers further up. I did make the 3dcheapskate node layout and tinker, but the best I could do was raise the fog slightly so that the sidewalk became a little more visible.

Then my thoughts were:

Make a 180-degree rotation on all the models? Render and flip it back in Photoshop?

Load the Poser scene to Vue, and there use one of Vue's highly controllable atmospheres?
 

KageRyu

Lost Mad Soul
Contributing Artist
I did something like this on a magma pit that sank below ground level, but can't even load that up to show the node setup. May not even have those scenes anymore (HD corruption, not sure how bad it is currently). There might be a way to invert it using blend nodes and the subtract node... unfortunately I can't even experiment right now as I have no working graphics machine.
Try some simple color experiments on a primitive to get the right combo of nodes, then you will know how to set it up on the atmosphere shader.
 

3dcheapskate

Engaged
I've successfully used 3dcheapskate's height-dependent fog shader to make a ground-hugging fog in Superfly (I just switched depth cue to volume)...
I know that a year and a half is a long time for a reply, but I've only just seen this. When you say "I just switched depth cue to volume" do you mean that you simply unticked the Atmosphere root node DepthCue_On and ticked the Volume_On instead ? Because that shouldn't have worked at all !
 
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3dcheapskate

Engaged
(ran out of time editing the above post)

Assuming you're talking about the shader below from post #5 of Upping the ante with the Poser atmosphere's Depth Cue (I'm still using Poser 11 with Firefly) | HiveWire 3D Community, if you did that then the rest of the nodes are doing nothing at all - DepthCue_EndDist only has any effect when DepthCue_On is ticked, and all the node network's doing is making the DepthCue_EndDist height dependent.

Basic Height Fog.png



If you want to use Volume then you need to plug height dependent networks into one or more of Volume_Density, Volume_StepSize, and/or Volume_Noise.
 

3dcheapskate

Engaged
A few test renders (Poser 11) to double check - I've used three 1 PNU wide x 10 PNU high flat rectangles at -10, -20 and -30 PNU on the Z axis to give an indication of the depthcue/atmmosphere effect, the 100PNU groundplane from my ground shaders freebie for a flat surface at zero height, and my worldball freebie scaled up to 1000% (so that the three rectangles fit inside it) for the backdrop

Left to right below: 1) Basic setup with no atmosphere; 2) With height dependent fog shader on default settings but pink DepthCue_Color; 3) Switching off DepthCue and using Volume instead.

Render 3.jpgRender 2.jpgRender 4.jpg

And if I make the worldball backdrop invisible it's clear that neitherDepthCue (below left) nor Volume have any effect if there's nothing behind.
Render 1.jpg Render 5.jpg

(I accidentally made the ground invisible too for the one on the left)
 

3dcheapskate

Engaged
Whoops! Sorry, I didn't use the default settings for the height dependent fog shader in the previous post - without thinking I automatically set Value_1 of MaxVisHeight_Feet to something like 10, and Value_1 of ZeroVisHeight_Feet to 50 or 80 or 100, which should give me something along the lines of what Minyassa was after.

Just started again from scratch (I didn't save the scene from the previous post) and with a bit of trial and error got this render

Render 1.jpg

Here's the height dependent fog shader with the values I changed.
Shader.jpg

I also changed the WorldBall scale to 400% just to be able to see some sky on the backdrop. This looks unrealistic, but by simply increasing the scale, or decreasing the DepthCue_EndDist in the Atmosphere node, or by a combination of both, I should be able to get the fog lower as it intersects the backdrop, making it look less bad.

And here's a quick sketch showing what the shader is really doing - hopefully it makes some sense ?

pic.jpg
 

3dcheapskate

Engaged
(Note: for the material room screenshots in this post my units are set to FEET)

Here's an easy test - use a Math_Function>Step node to make the input to Atmosphere>DepthCue_endDist go from 0 to 1 (or 1 to 0) at a specific height. The value 25 in the Step node is arbitrary and was selected to get a nice-looking render.

2 DepthCue.JPG
1 DepthCue.JPG

Now try the same, but plugging it into Atmosphere>Volume_Density instead:
2 volume.JPG
1 volume.JPG
So the basic idea does seem to work with volume, but there are a couple of observations:
1) Plugging 0 into DepthCue_EndDist makes the 'fog' thick, and 1 makes it thin, whereas the opposite applies if you plug it into Volume_Density.
2) If you look at the first of the two renders with green fog, the high fog, you'll notice that where the backdrop is white you don't see the green of the fog. So you may need darker colours on the backdrop and distant objects for the fog to work.
 

3dcheapskate

Engaged
Same thing but with the basic height dependent shader. I've using 24 and 26 in the two ###Height_Feet nodes and swapped the values to make the fog high or low.
Here it is with DepthCue
3 DepthCue.JPG
4 DepthCue.JPG
And with Volume:
3 Volume.JPG
4 Volume.JPG
So it does work.

And of course using a white fog will get rid of that background problem. Play with the values in the red boxes to adjust things to your liking.
0.JPG
 

3dcheapskate

Engaged
But watching that last volume render reminded me of why I was playing with DepthCue. The render on the left below uses volume, settings as in previous post, and took 2 minutes:
0 Volume (2min).jpg 0 DepthCue (7sec).jpg
The render on the right, using DepthCue (settings in the screenshot below), took just 7 seconds. Seventeen times faster.

0 DepthCue settings.jpg
 

3dcheapskate

Engaged
Playing around with some Dystopia city blocks, scaled up to 4000%. Left, basic render (one infinite, one IBL); right same thing but with the latest DepthCue fog shader (not yet uploaded anywhere, but the previous post points to the details.

Capture.JPG fog.JPG
 
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