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

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 the following two render elements (passes):

• CESSENTIAL_Direct
• CESSENTIAL_Indirect

How to add those elements (passes)?

3ds Max:

Go to Render Setup > Render Elements > Render Elements rollout and press the "Add..." button. Pick CESSENTIAL_Direct and press ok, then repeat the same process with the CESSENTIAL_Indirect element:

Cinema 4D:

Go to Corona > Multi-Pass..., enable the Multi-Pass, then add a Basic > Direct and Basic > Indirect passes:

Once this is done, you will be able to see the selected render elements on the roll-out in the Corona VFB during rendering.

What do the elements tell me?

• If you can see that there is visibly more noise in the CESSENTIAL_Direct pass:
  This might suggest that your scene needs more direct light samples. To have more direct light samples rendered per each render pass, go to:
  
  
  • 3ds Max:
    Render Setup > Performance > Performance Settings > Sampling balance and slightly increase the Light samples multiplier. The default value is 2, and it is not advised to increase this value above 4.
    
    
    
  • Cinema 4D:
    Render Settings > Corona > Performance Settings > Sampling balance and slightly increase the Light samples multiplier. The default value is 2, and it is not advised to increase this value above 4.
    
    
    
• If you can see that there is visibly more noise in the CESSENTIAL_Indirect pass:
  This might suggest that your scene needs more GI samples. To add more GI samples per each render pass, go to:
  
  
  • 3ds Max:
    Render Setup > Performance > Performance Settings > Sampling balance and slightly increase the GI vs. AA balance. The default value is 16, and it is not advised to increase this value above 64.
    
    
    
  • Cinema 4D:
    Render Settings > Corona > Performance Settings > Sampling balance and slightly increase the GI vs. AA balance. The default value is 16, and it is not advised to increase this value above 64. 
    
    
    
    

Additional notes

• The total amount of samples is dependent on the Light samples multiplier and GI vs. AA balance values. 
• If the Light samples multiplier is set to 1 and the 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 the Light samples multiplier is set to 2 and the 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 makes sense 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.
• GI vs. AA balance should never be lowered below 2. 

The default value of 16 works well in most cases, but sometimes it should be increased if there is more noise in indirect light areas, or decreased if there is strong DoF (3ds Max | C4D) or motion blur (3ds Max | C4D) in the scene. 

Note: GI vs. AA balance works in correlation with Light Samples Multiplier.

Read more about getting rid of excessive noise.

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.