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 a long rendering time, may suggest that the rendering engine encountered some problems in your scene setup. You can always use denoising (3ds Max | C4D) to get rid of the noise by smartly blurring it, but it is much better 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. The 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?
- Wrong glass type - see: What type of glass should I use?
- Too many mirror-like materials in the 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?
- Area lights or global illumination - see: How can I tell whether noise comes from direct or indirect light?
- 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 the scene - try to reduce the number of lights in your scene or use other lighting technique (for example emitter objects or texture maps).
Misuse of Corona Light material or self-illumination - see: Should I use Corona Light material or self-illumination?
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 or Light Samples Multiplier.
- The Adaptive light solver is not active. It can greatly reduce the overall noise in an image: What is the Adaptive light solver option?
Fireflies in corners:
Single bright pixels are visible in corners of the room or on geometry without thickness.
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 the camera effects:
Noise is visible in areas affected by the 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 the UHD Cache GI solver:
- See: How to set up animation flicker-free? (3ds Max | C4D)
- See: How to fix flickering in animations?
Apart from user errors, there are some physically correct phenomena that can also cause a "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 the 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 the 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.
Too much noise may also indicate that the rendering process is abnormally slow. To find out about common causes of performance problems, see: Rays per second too low.