I'm trying to flesh out a way to do a simple atmospheric fog in Houdini to show distance. I've seen examples of light fogs on a spotlight and similar effects, but I'm looking for something a little more simple, can even be a composite effect.

Can anyone point me in the right direction?

I would render out a z-map with an extra image plane in the mantra node. That used as a mask in composite does a good job for atmospheric perspective. Often you need to tweak the gamma to get enough contrast in the Pz-plane.

Yes you need to render out a Pz extra image plane. For the Pz image plane probably best to set the Pixel Filter to “Closest Sample Filtering” or minmax min (no antialiasing or feathering of edges which will return nonsense depth information).

There is a Fog COP that manipuates image pixels to simulate fog and expects a Pz channel to be present on the input image.

Thanks that's exactly what I was looking for.

EDIT: Nevermind, used a Channel Copy node to just combine/swap what I needed before applying the Fog node.



New question about the Fog compositing node: Is there a way to use the color from one image/sequence and the Pz information from another image/sequence?

I've got a live shot of some foreground buildings and I'm adding 3D buildings in the background, and want to add some fog over both.

So I've got a color render of the live shot + bg buildings composited together which I want to add the fog too, and another render of the bg buildings + fake versions of the fg buildings with a Pz layer for the z-depth information.

Use the Channel Copy COP to copy the Pz from one image sequence to the Pz of another image sequence.

jeff
Use the Channel Copy COP to copy the Pz from one image sequence to the Pz of another image sequence.

Thanks, got it! Just had to add a fake Pz node to my color sequence so I could do the swap. I guess that's the only way?

Yes. You have to initialize the target plane with the correct name.

If you do this a lot, roll your own asset.