The Adaptive SamplingA method of sampling that determines if areas of a rendering require more sampling than other areas instead of sampling the entire rendering equally. parameter is a rendering option that disables sampling for pixels that reach a specified noise threshold. This lets kernels focus its processing on areas that still need refinement. You can find the Adaptive Sampling options in the attributes for the Direct Light, Path Tracing, and Photon Tracing kernels.
Figure 1: The Adaptive Sampling parameter found in the Direct Light, Path Tracing, and Photon Mapping kernels
Adaptive Sampling - Stops sampling pixels that reach a specified noise threshold, which allows the kernel to focus its processing on areas that still need refinement.
Noise Threshold - When Adaptive Sampling is enabled, Noise Threshold specifies the smallest relative noise level. When the noise estimate of a pixel becomes less than this value, sampling switches off for this pixel. Good values are in the range of 0.01 - 0.03. The default is 0.02, which is pretty clean.
Min. Adaptive Samples - Specifies the minimum samples to calculate before Adaptive Sampling kicks in. Pixel noise estimates are just estimates with a large initial error. The higher you set the noise threshold, the higher you should also set this parameter to avoid artifacts.
Pixel Grouping - When Adaptive Sampling is enabled, Pixel Grouing specifies the number of pixels that are handled together. When all pixels in a group reach the noise level, sampling stops for all of these pixels.
Expected Exposure - This parameter should be about the same value as the image exposure, or 0 (the default value) to ignore these settings. This parameter is used by Adaptive Sampling to determine the pixels that are bright and those that are dark - which depends on the exposure setting in the Octane Imager. If the value is not 0, Adaptive Sampling reduces the noise estimate of very dark areas of the image. It also will also increase the Min. Adaptive Samples limit for very dark areas, because very dark areas tend to find paths to light sources irregularly, resulting to an otherwise over-optimistic noise estimate.