Atmospheric Fog

To those who joined the Wed session,
check out the Thursday session since the "simulated Zdepth" issue was covered.
(Watch from 1h16m38s)

Download Wed Session

Download Thu Session  <---Check this one out!

Download (Nuke file, images)

<Atmospheric Effect>
1. Why is the sky blue?
The sun ray scatters in the atmosphere.
The blue scatters first and becomes dominant in the sky.

2.
-Rayleigh scattering:
When the particle is extremely small as air molecule.
Scattering in all directions (omni)
The scattering color is blue (the sky)

-Mie scattering:
When the particle is larger than air molecule.
The scattering direction is more directional(forward).
Depending up on the size of the particle, the color is different. . (The larger the particle, the whiter the scattering )
The fog / the cloud

3. When you render something really large or something really far, you need to take "Distance" into account.
You can just use zdepth information.

4. To add atmospheric effect,
-Add grade to scale the brightness toward black.
(The fog can block the background pixel)
-Add another grade node to add Fog color
-Use the Zdepth image as a mask to manipulate the above grade nodes.

5. Make sure to use unpremult and premult to avoid
edge artifacts and unwanted extra color to the BG.

6. Pick the fog color from the background.
Any color from the background is gonna work.
But the color near the horizon is recommended.

7. Using  regular Zdepth may create some edge artifacts. Since it is not anti-aliased.
I recommend using  Atmospheric fog ( environmental fog ) in Maya.