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Adaptive Sampler: Make use of the improve sampling algorithms instead of the default. You may need to slightly increase your Pixel Variance setting to avoid longer render times (but better resulting quality on complex images) Your min samples should be set to 0 which has special meaning that the square root of the max samples is used. If you manually input a low value, it will terminate early.

Bucket Order: RenderMan subdivides the output image into small rectangular regions called buckets, and renders a single bucket at a time. This allows a small portion of the image to be in memory at any one time. The drop-down list allows users to choose the order in which buckets are displayed in "it" or the Render View. The default is Spiral. Note that only some bucket orders are supported for checkpointing and recovery.

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