Jonathan de Blok
If I take the hip file you posted and add this line the the "set_new_focal_lenght" node it works fine and prints updates.
#dummy comment to trigger update `optransform( chs("../camera_path") )`
But again, this is solution is not going to win any prizes and as Graham suggest doing this though attributes is better way.
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And btw.. a much overlooked feature in the wrangle/expression nodes is in the wrangle's src edit window context menu->reference->scene data' where you can generate vex to get you started on retrieving all sorts of scene data:
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Hey, I love this info, this is super helpful!
I'm experiencing two issues: if I have the camera reference the new focal length in a detail attribute, this creates infinite recursion (I guess when the detail attribute changes, the camera changes, which causes the Python node to cook again which recooks the node the computes the new focal length...)
Also, thanks for the tip on using the scene reference data! Looking at the code that makes it up it appears that it still references the entire camera SOP and would thus recook the node even if only the focal length changes. Any tips on how to avoid this?
Thanks!
Anson