Ray types can be divided into four categories:
Camera: the ray comes straight from the camera.
Reflection: the ray is generated by a reflection off a surface.
Transmission: the ray is generated by a transmission through a surface.
Shadow: the ray is used for (transparent) shadows.
Reflection and transmission rays can further have these properties:
Diffuse: the ray is generated by a diffuse reflection or transmission (translucency).
Glossy: the ray is generated by a glossy specular reflection or transmission.
Singular: the ray is generated by a perfectly sharp reflection or transmission.
The Light Path node can be used to find out the type of ray the shading is being computed for.
The object ray visibility settings.
The maximum number of light bounces can be controlled manually. While ideally this should be infinite, in practice a smaller number of bounces may be sufficient, or some light interactions may be intentionally left out for faster convergence. The number of diffuse reflection, glossy reflection and transmission bounces can also be controlled individually.
Light paths are terminated probabilistically when specifying a minimum number of light bounces lower than the maximum. In that case, paths longer than minimum will be randomly stopped when they are expected to contribute less light to the image. This will still converge to the same image, but renders faster while possibly being noisier.
The transparent BSDF shader is given special treatment. When a ray passes through it, light passes straight on, as if there was no geometry there. The ray type does not change when passing through a transparent BSDF.
Alpha pass output is also different for the transparent BSDF. Other transmission BSDFs are considered opaque, because they change the light direction. As such they cannot be used for alpha-over compositing, while this is possible with the transparent BSDF.
Note that, while semantically the ray passes through as if no geometry was hit, rendering performance is affected as each transparency step requires executing the shader and tracing a ray.
Maximum number of light bounces. For best quality, this should be set to the maximum. However, in practice, it may be good to set it to lower values for faster rendering. A value of 0 bounces results in direct lighting only.
Maximum number of diffuse bounces.
Maximum number of glossy bounces.
Maximum number of transparency bounces.
The maximum number of transparent bounces is controlled separately from other bounces. It is also possible to use probabilistic termination of transparent bounces, which might help rendering many layers of transparency.
Maximum number of transmission bounces.
Maximum number of volume scattering bounces.
- Clamp Direct
This option limits the maximum intensity a sample from rays which have not yet bounced can contribute to a pixel. It reduces noise at the cost of accuracy. Setting this option to 0.0 disables clamping altogether. Lower have a greater affect (dimmer samples) on the resulting image than higher values.
This option provides a way to limit Fireflies. However, note that as you clamp out such values, other bright lights/reflections will be dimmed as well.
Care must be taken when using this setting to find a balance between mitigating fireflies and losing intentionally bright parts. It is often useful to clamp indirect bounces separately, as they tend to cause more fireflies than direct bounces. See the Clamp Indirect setting.
- Clamp Indirect
The same as Clamp Direct, but for rays which have bounced multiple times.
A common source of noise is Caustics.
See Reducing Noise for examples of the clamp settings in use.
- Filter Glossy
When using a value higher than 0.0, this will blur glossy reflections after blurry bounces, to reduce noise at the cost of accuracy. 1.0 is a good starting value to tweak.
Some light paths have a low probability of being found while contributing much light to the pixel. As a result these light paths will be found in some pixels and not in others, causing Fireflies. An example of such a difficult path might be a small light that is causing a small specular highlight on a sharp glossy material, which is observed through a rough glossy material. In fact in such a case there practically occurs a caustic.
With path tracing it is difficult to find the specular highlight, but if you increase the roughness on the material, the highlight gets bigger and softer, and so easier to find. Often this blurring will hardly be noticeable, because it is blurred by the material anyway, but there are also cases where this will lead to a loss of detail in lighting.
While in principle path tracing supports rendering of caustics with a sufficient number of samples, in practice it may be inefficient to the point that there is just too much noise. This option can be unchecked, to disable reflective caustics.
The same as above, but for refractive caustics.