Noise threshold vs samples.

[philip_nel]

What is the difference between the two - does it produce the same result: more speed but with additional noise.
Lowering the noise threshold to say 0,3 will it have the same result as keeping it on 0,1 but lowering the render samples?

What is the best way to get more render speed and quality for animations: lowering noise threshold or lowering samples?
Not sure how these 2 settings are interlinked?

[npg]

The two settings are not directly comparable, but both affect how long the frame would get sampled and therefore how long it would render.

From our docs https://docs.chaos.com/display/LAV/Render+Tab :
Noise threshold - Specifies the noise threshold that determines when to stop refining a pixel. Higher values allow more noise in the image, but the render is faster. Lower values help reduce the noise, but take more time to render. A value of 0.0 traces the entire image unconditionally.

When a non-zero threshold is set an active pixel mask is continuously computed to determine which pixels are still noisy and which ones can be skipped. You can visualize it with the "Active pixels overlay" checkbox found at the bottom of the advanced render settings panel.
There is an undocumented detail that the mask is only computed after the first 16 sampling passes are done. In other words even if there is no noise (e.g. empty sky) each pixel will have at least 16 samples taken.

The Samples setting is pretty simple - how many sampling passes to take before the frame is considered complete. If your noise threshold is 0 this setting determines render time pretty much linearly, meaning that a 200 sample render would take roughly twice as long as a 100 samples render. However when the noise threshold is non-zero, pixels may start to turn off at some point, lets say after the 100th sample in our example. So the second half of the 200 sampling passes could be computed a lot faster and the 200 sample render may be only 40% slower than the 100 sample render (arbitrary example numbers, of course).

So with the help of the active pixels overlay you could determine what noise level you're happy with - see which pixels turn off and whether quality there is ok (start with a high threshold value and move to lower values) and this could speed up your renders. Of course if it takes 500 samples to reach the noise threshold and you're only rendering 200 passes in your render job, the threshold won't be reached.

Or, to put things differently: the offline sampling passes setting is what you set to get the desired quality and the noise threshold is an optimization which skips oversampling parts of the image while doing those sampling passes (when the threshold is chosen correctly, otherwise if the threshold is too high the image may remain undersampled even when the number of sampling passes is large).

Finally a note about V-Ray: these settings are similar to those of the progressive sampler in V-Ray, but instead of "Max. subdivs" we have the simpler "Samples".

[philip_nel]

Great explanation, thanks Nikola.
Will turn on the active pixels overlay to see what threshold will work for me - 14 000 frames so every second makes a difference!