All CGI have circular bands of colors without post processing

Why this bands of different colors appear on any CGI by default?

If you can't see them from this image here is the same with higher exposure to highlight them:

I see these bands always without any vignette or other post processings

 

What you see is called Colour banding and is caused by color space limitations. On that page below is a pretty good explanation of it:

• https://simonschreibt.de/gat/colorbanding/

Dithering

can mitigate it.
.

 

You can also set the Display Buffer to 32bit in the player settings (may not be available for all platforms however)

 

That's only an option for Android because other platforms already default to 32 bit. The above example image is also already 32 bit. The problem is 32 bit isn't enough, especially when you have wide gradients of darker colors.

See tha article @Mauri linked to.

The TLDR version is if you look at the actual color values in the original example image posted above in Photoshop or Gimp you'll see they're "1 value" apart. They're greyscale values of RGB(29,29,29), RGB(30,30,30), RGB(31,31,31). That's hitting the limit of 32 bit color representation. And when you focus on a dark gradient between two similar values, the human eye can tell the values apart.

We could render at a higher bit depth, say a 16 bits per color channel RGBAHalf (the default "32 bit" RGBA32 is 8 bits per channel), but basically all consumer displays still only display at 8 bits per color (some only do 6 bits per color!) so you'll get banding again even then.

The answer to why you don't see (as much) banding in profession media is two fold. One is they make sure to include brighter areas in the image so the contrast between the brights and darks make it harder to see the banding, and they use dithering to hide the banding.

 

That article is interesting, I'm reading it right now.