Primacy of the B Feed
A Nuke script will typically consist of many Merge nodes arranged one after the other. When troubleshooting a script (and scripts will always need troubleshooting) it is necessary to be able to switch off the Merge nodes one by one (keystroke: select node then D) yet still preview the current state of the composite. To do this it is highly recommended to always preserve the B steam running down the script. In other words: preview is only possible if, by deactivating a Merge node, the main trunk of the script above that node is still active and not just a side branch.
In the image below the B stream is preserved. The highlighted yellow branches indicated the nodes that are being processed, showing us that nodes above the deactivated node are still being processed.
In the next script the B stream has not been preserved. This shuts off processing of all nodes above it.
- Tip: sometimes to preserve the B feed, an 'Under' merge operation should be used, instead of an 'Over'.