Very often, when we render with Corona, we are bothered by the large amount of fine noise in the rendered image. This problem is common to all renderers that use ray tracing, and it is especially severe during the test-render stage. Sometimes even the final render at high settings cannot get rid of it, which may mean that something is wrong with your render or your scene setup. Corona does come with its own denoising option, which removes that fine noise through very slight blurring, but beyond that you also need to make sure your scene and your settings are correct.

To remove this noise more effectively, you need to know which bad habits in your workflow produce it:

Wrong Material Settings

This kind of noise tends to be concentrated on one particular object, or on objects that use a certain material, so it is localized. The main causes are:

1.Unrealistic albedo

You can use the CShading_Albedo render pass to check whether your scene contains objects with an unrealistic albedo. Red means an unrealistic albedo, while gray represents a normal albedo.

2.Incorrect glass material

Corona has two kinds of glass. When you make things like glass windows or light bulbs, choose Thin (no refraction); when you make materials with light-absorbing properties, such as liquids and glassware, do not check that option.

3.Overusing self-illumination materials

Light produced by this kind of material carries a lot of noise

Wrong Light Settings

This kind of noise usually appears concentrated at the boundary where light and shadow meet, or inside the shadows. The main causes are:

• 1.Windows that are too small

  In this case you need to add a portal light. This raises the light level indoors and reduces noise. In the image below, the left side has no portal and the right side is after adding one.

  

  When adding portals, note that you should add them even for windows that are not visible in the render, and try to make them fit the window opening as closely as possible.

• 2.Not enough light subdivs

  You can add the CESSENTIAL_Direct and CESSENTIAL_Indirect render passes to check whether it is the direct light or the indirect light that produces the noise, and whether the subdivs need to be increased. Below is the CESSENTIAL_Direct pass, where you can see noise.

  

  Then you need to increase the setting shown below (the default is 2; raising it above 4 is not recommended):

  

  Below is the CESSENTIAL_Indirect pass, where you can see noise.

  

  Then you need to increase the setting shown below (the default is 16; raising it above 64 is not recommended):

  

• 3.Defocus

  You can lower the strength of this effect to ease the noise

• 4.Too many lights

  Try to use efficient light types to simulate the lights in your scene

• 5.Overusing self-illumination materials

  Light produced by this kind of material carries a lot of noise

Wrong Render Settings

This kind of noise generally appears across the whole render, or is concentrated in one direct-lighting area or one indirect-lighting area. The main causes are:

• 1.Max sample intensity set too high

  

• 2.GI vs. AA Balance or Light Samples Multiplier set too high or too low (the defaults are recommended)

  

White bright spots in the render

These bright spots may be caused by model objects that have no thickness; simply giving them thickness solves the problem.

Noise caused by anti-aliasing and filtering

If you see a lot of jaggies or hard edges, especially near lights or windows, then you need to increase the values of the following parameters:

Highlight compression

The default is 1; raising it to 10 solves the hard-edge problem

You can also enable Sharpening/Blurring to remove jaggies and noise

Switching to the Box filter type solves severe aliasing

Noise caused by the camera: both DOF and motion blur produce noise. Besides all of the above, some real-world physical phenomena also produce noise, such as:

• 1. Reflections on small objects such as grass and leaves
• 2. Refraction inside small objects
• 3. Complex texture types, especially bump and displacement maps
• 4. Moiré patterns

Finally, you can also raise the highlight clamping value