How to get rid of excessive noise?
Subtle high-frequency noise is normal for Corona and other path-tracing renderers, especially at the beginning of the rendering process. But strong noise that won't go away after many passes, or after long rendering time, may suggest that the rendering engine encountered some problems in your scene setup. Starting from Corona 1.4 you can use the denoising feature (3ds Max | C4D) to get rid of the noise by smartly blurring it. It is however recommended to make sure that there are no mistakes in the render setup, or the scene itself.
To efficiently get rid of problematic noise, you need to find out where it comes from. Most common causes of persistent noise include:
Wrong material setup:
This is usually manifested by noise visible only in specific parts of the scene (some objects or some materials).
- How to create basic materials? (3ds Max | C4D)
- Unrealistic material albedo - see: What is Albedo? (3ds Max | C4D)
- Wrong glass type - see: What type of glass should I use? (3ds Max | C4D)
- Too many mirror-like materials in scene - see: How to set up realistic glass / metal materials? (3ds Max | C4D)
- Misuse of Corona Light material or self-illumination- see: Should I use Corona Light material or self illumination? (3ds Max | C4D)
- Small window holes. Always use portals in such scenes - see: How to use light portals? (3ds Max | C4D)
- Area lights or global illumination - see: How can I tell whether noise comes from direct or indirect light? (3ds Max | C4D)
- Caustics - you can usually reduce them using max sample intensity parameter - see: MSI (3ds Max | C4D) or by using Corona Ray Switch material. (3ds Max | C4D)
- Using many light sources in scene - try to reduce number of lights in your scene or use other lighting technique (for example emitter objects or texture maps).
Wrong render settings:
- Too high max sample intensity value - see: MSI (3ds Max | C4D)
- Too high or too low values of GI vs. AA Balance (3ds Max | C4D) or Light Samples Multiplier (3ds Max | C4D)
Fireflies in corners:
Single bright pixels visible in corners of the room or on geometry without thickness.
- See: I can see noise in the corners of my scene! (3ds Max | C4D)
Noise caused by anti-aliasing or image filtering:
- If you can see sharp or "jagged" edges, especially around lights, windows or other bright parts of your scene, see: I can see jagged edges! (3ds Max | C4D)
Noise in camera effects:
Noise is visible in areas affected by depth of field or motion blur effects.
- Depth of field - see: How to enable and control DoF? (3ds Max | C4D)
- Motion blur - see: How to enable and control motion blur? (3ds Max | C4D)
Flickering/splotches in animation when using legacy HD cache GI solver:
Apart from user errors, there are some physically-correct phenomena that can also cause "noise" effect in real life (for example in photography). This includes:
- Sunlight or other strong light reflected by grass/leaves/small objects, see: I can see bright pixels in grass! (3ds Max | C4D)
- Refraction, especially visible inside small objects
- High-frequency textures, especially as bump maps or displacement
- Moiré pattern
- To clamp sample intensity and reduce noise visible in highlights (fireflies), you can use MSI (3ds Max | C4D) and highlight clamping. (3ds Max | C4D)
To determine the cause of noise, it may be sometimes useful to enable material overrides. See: How to create clay renders? (3ds Max | C4D). If the noise is still visible after applying the diffuse grey material to all objects, it indicates that most probably it is not caused by scene materials.