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i tried to add the volume scatter to my scene which looks like this
when i render it without the volume scatter i get this view
but as soon as i activate the world node and volume scatter i get this result, an extremely over exposed image
and those are my world settings
I reduced the Density to 0.001 and it got way better, but i still don't know why ?
Think of Volume scatter like smoke or dust (or murky water). If it gets too dense then you can't see through it.
Here's the same scene with different density values.
The effects of volume scattering will be more pronounced in objects that are further away, so the distance from the camera to the objects is also a factor to be considered.
Lights might not be seem bright when having to travel through volume scattering, so be prepared to use very large values for their intensity.
All lights on your scene will bounce on the volume. If you have any lights close to the camera those will tend to wash away anything else in the image (as when you drive through fog where the car lights will bounce right back):
the same scene, but with a light behind the camera:
There is no "right" or "magic" number that will work for each and evry scene, you'll have to find the one that gets you closer to what you are after...
Other considerations:
Rendering scenes with volume scattering will always result in noisier images. You'll have to increase the number of samples, which will have an impact on rendering time as well. You'll see more fireflies, created by the indirect bounces, to control use clamp indirect (a value of 3.00 is a good start point). On the light paths bring up the bounces for volume. A smaller step size for Heterogenous volume sample helps a lot (but will increase rendering time as well).
