Just out of curiosity, what exactly is going on behind the scenes when that "Cancelling" is shown on the top header?

Example: In the compositor, add a viewer node and a couple of nodes. Preview the final result. When Blender is computing the nodes there is a bar on the top header with the completion percentage. But if I decide that I want to stop the preview for some reason and disconnect the viewer node then the bar gets lighter and a "Cancelling" message appears next to it.

This "Cancelling" can take a looong time when working with heavy scenes. What exactly does it mean? Thank you!

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    $\begingroup$ Not sure but I think some tile or piece of information have probably been sent to the active computing device for processing, until that chunk is processed and the processor returns the digested information Blender can only wait. It probably has no way of telling it to stop mid process. Until information is returned and a "stop" signal can be sent and acknowledged it is "cancelling" $\endgroup$ Commented Mar 20, 2017 at 1:42


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