Getting the Primary Samples right is the first step. The primary samples affect the quality of geometry and all other sampling Karma performs during the render. Because secondary samples are multiplied by pixel samples, you can reduce time spent making later adjustments by setting Min/Max Secondary Samples to 1. This lets all of the post-primary sample controls build on the primary sampling that Karma is already doing.
Tip
Setting primary samples to values that are easily squared (i.e. 4, 9, 16, 25, etc…) may help to resolve noise.
Looking at the beauty pass is usually enough to tell where more primary samples are needed.
While the beauty pass alpha channel is helpful, adding Cryptomatte AOVs can be even more useful to identify under-sampled primitive edges in complex scenes. If any of the following features are grainy or noisy (in the beauty, alpha, or Cryptomatte planes), start increasing the primary samples until the quality is where you want it:
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Noisy hair or fur (BasisCurves), or aliased edges of geometry.
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Strands of curves look pixelated or noisy.
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Noisy motion blur, or aliased depth of field.
The primarysamples
AOV can be useful to inspect, if increasing primary samples doesn’t seem to improve quality. This can happen for renders dominated by depth of field (DOF) or motion blur quality. If the actual number of pixel samples aren’t reaching the value set in the Primary Samples parameter, the pixel oracle is probably preventing sufficient pixel sampling. Try lowering the oracle’s Variance Threshold and see if that sends more pixel samples. If it doesn’t, you can continue to increase primary samples, or switch over to the uniform pixel oracle.
Tip
Sometimes what appears to be motion blur noise in heavily motion blurred areas is actually secondary samples. You should check other secondary sample AOVs once in a while, even while you are finding the correct pixel samples.