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Checkpoint Interval

This is a periodic checkpoint. This can be specified with an interval measured either in a number of increments (i.e., passes over the image), or by the elapsed wall clock time. For example, you could ask the renderer to write checkpoints with an interval of 100i, meaning every 100 increments and it will update the images on disk with the state of the render on the 100th, 200th, 300th increment, and so forth.

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