How can I tell whether noise comes from direct or indirect light?


The easiest way to find out whether the noise visible in your rendering comes from direct or indirect (GI) light is to add a Direct and Indirect pass (found in the Basic pass type list - see What multi-pass channels are available?).


After doing this, you will be able to see the selected render elements on the roll-out menu in Corona's VFB during and after rendering.


If you can see that there is more noise visible in the Direct pass, this might suggest that your scene needs more direct light samples. To add more direct light samples, go to Render Settings > Corona > Performance Settings > Sampling balance and slightly increase "Light samples multiplier".


If you can see that there is more noise visible in the Indirect pass, this might suggest that your scene needs more GI samples. To add more GI samples, go to Render Settings > Corona > Performance Settings > Sampling balance and slightly increase "GI vs. AA balance".


Note: The total amount of samples is dependent on the "Light samples multiplier" and "GI vs. AA balance" values. If "Light samples multiplier" is set to 1 and "GI vs. AA balance" is set to 16, this means that 16 GI samples and 16 direct light samples will be used for each pixel per pass. If "Light samples multiplier" is set to 2 and "GI vs. AA balance" is set to 16, this means that 16 GI samples and 32 direct light samples will be used for each pixel per pass. In some situations it may pay off to reduce one of these values and increase the second one, or even to reduce both. Typically, there is no need to increase "GI vs. AA balance" above 64 and "Light samples multiplier" above 4.

See: Light samples multiplier

See: GI vs. AA balance

Read more about getting rid of excessive noise here.



Examples:

A standard beauty render after 16 passes with no denoising.


Noise visible in direct light (the Direct pass)

After 16 passes there is still visible noise in direct light. If it persists, this may suggest that more direct light samples are required to efficiently render the image.



Noise visible in indirect light (the Indirect pass)

After 16 passes there is visible noise in indirect light. If it persists, this may suggest that more GI samples are required to efficiently render the image.