Although no communication is necessary to compute all the points, the nodes ultimately have to communicate in order to output the complete image. When designing the output algorithm, we have to keep in mind our requirement that no node should ever have to store the whole image - rather, every node should be responsible for its own part of the output. At this time, we face the problem that no node has a continuous block, but only non-consecutive lines of the image. If each node was to output its lines as soon as they are completed, the resulting overall output would be completely out-of-order since the nodes will take different amounts of time to compute different lines. Thus, we need a synchronization mechanism to output the distributed lines sequentially and in order.

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