Adaptive Sampling

This feature allows the user to set a desired overall noise level instead of explicitly setting the number of samples per pixel (SPP). Instead, the renderer will adaptively choose the number of samples per pixel to reach the desired error threshold over the entire image. Since we want the noise at each pixel to fall below this specified error threshold, it also has the beneficial side effect of removing fireflies from the output. The error is measured on the beauty image only.

SceneVariables attributes

To enable adaptive sampling, the sampling_mode attribute must be changed from the default 0 (uniform) to 2 (adaptive)

Attribute Name Description
sampling_mode uniform (0): Render with a fixed number of samples per pixel: the square of the pixel_samples attribute.

adaptive (2): Render with adaptive sampling. The pixel_samples attribute is ignored, and the attributes below are used.
min_adaptive_samples The minimum number of samples taken per pixel, regardless of error level. A value too small may result in areas that are prematurely deemed converged. A value too large will waste time rendering where it is not necessary. Unlike pixel_samples, this value is linear.
max_adaptive_samples The maximum number of samples taken per pixel, regardless of error level. A value too small may result in unconverged areas and pixels. A value too large will waste time rendering where it is not necessary. Unlike pixel_samples, this value is linear.
target_adaptive_error When adaptive sampling is turned on, this represents the desired quality of the output images. Lower values will give higher quality but take longer to render. Higher values will give lower quality but render quicker.

To tune the above adaptive sampling parameters, check the value shown for “Pixels at max adaptive samples” in the output log (when rendering use the -info flag for verbose output). If the value is significant and your image is noisy, consider increasing max_adaptive_samples. On the other hand, if the value is small and your image is converged, you may be able to increase your target_adaptive_error.

Example RDLA Syntax

SceneVariables
{
    ["sampling_mode"] = 2,              -- adaptive mode
    ["min_adaptive_samples"] = 16,      -- use at least 16 samples per pixel
    ["max_adaptive_samples"] = 4096,    -- use at most 4096 samples per pixel
    ["target_adaptive_error"] = 1.5,    -- stop sampling once this error is reached
}

In moonray_gui, hotkey “8” will toggle to a grayscale view showing the number of samples rendered per pixel. The brighter the pixel, the higher the sample count. In addition, a weight AOV is available that also records the absolute number of pixel samples. To be human-viewable, the weight AOV may need to be normalized to [0, 1] in compositing software.

Questions and Answers

What happens if I set min_adaptive_samples too low?

Setting min_adaptive_samples too low can cause the algorithm to classify tiles prematurely as converged, leading to noisy areas or image discontinuities.

What happens if I set min_adaptive_samples too high?

Setting min_adaptive_samples too high can cause excessively long render times. Higher values give the adaptive sampler more information about the scene before limiting samples, but it also results in a longer render time.

What happens if I set max_adaptive_samples too low?

Setting max_adaptive_samples too low may result in noisy images or unresolved fireflies.

What happens if I set max_adaptive_samples too high?

Setting max_adaptive_samples too high can result in excessive render times if the renderer has difficulty getting an area of the image to converge.

What happens if I set target_adaptive_error too low?

Render times will be longer than desired.

What happens if I set target_adaptive_error too high?

The final image will be noisier than desired.