Arnold has the capability of adapting the sampling rate of each pixel when the
enable_adaptive_sampling render option is enabled, allowing it to dedicate a greater number of camera samples (and thus also a greater amount of render time) to the pixels that show a greater variation in their sample values. When used, all pixels will receive a sampling rate of at least
AA_samples, but no more than
AA_samples_max. The adaptive sampler's sensitivity to noise may be controlled through the
adaptive_threshold render option, where lower threshold values will apply higher sampling rates to a greater number of pixels.
Adaptive sampling is good for situations where small areas of the image have high levels of noise that require an impractical number of samples to clear up. For example scenes with bright, motion-blurred speculars, DOF, buzzing rim lights, or scenes with the hair shader.
Adaptive/Progressive and IPR
In Arnold plugins that perform a series of render passes at lower AA settings for the purpose of previewing the output before rendering at the final sampling rates, enable_adaptive_sampling should be DISABLED for all but the final render pass to enhance performance.
Likewise, when the progressive_render render mode is used, for best performance plugins may skip performing preview renders with AA settings of 1 or higher since the progressive rendering mode will already display all of the intermediate AA sampling steps from 1 to the user-selected setting to the render view drivers.
Adaptive Sampler and Custom Filters
Adaptive sampling consists in placing samples in a non-uniform fashion over the rendered frame. Because of this, some pixels will have a higher density of samples than others, and this sample density must be taken into account by weighted-average filters otherwise the sample values of high-density regions will become over-represented and those from low-density regions will become under-represented in the filtered result.
Each sample's sampling density can be obtained through an AiAOVSampleIteratorGetInvDensity() function call in each iteration of a sample filtering loop. The scalar return value can be used to properly adjust each sample's value in the weighted average filter through a multiplication.
The AA_inv_density AOV can help to visualize the sample density with Adaptive Sampling. Use it with a Heatmap filter.
AA_inv_density AOV with Heatmap filter
Max. Camera (AA)
Sets the maximum amount of supersampling. It controls the per-pixel maximum sampling rate and is equivalent to the units used by AA_samples. Adaptive sampling is enabled when AA_samples_max > Camera (AA) samples and Camera (AA) samples >= 2. Scenes with a large amount of depth of field or motion blur may require higher Max. Camera (AA) values. This parameter can also help with 'buzzing' speculars and hair shading as well.
Setting AA_samples to 1 or lower does not yet allow Arnold to calculate the measures needed for adaptive thresholding, and setting AA_samples_max to AA_samples or lower does not leave a margin for any adaptive AA samples.
Variance filter showing effect of Max. Camera (AA)
The threshold which triggers/terminates adaptive-AA. This value controls how sensitive to noise the adaptive sampling algorithm gets. Lower numbers will detect more noise. The default value (0.05) should work well for most scenes.
Variance filter showing effect of AA Adaptive Threshold