Thumbnails in 5.7 - how to force generation of a new thumbnail for existing files ?

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Hi.
I added a new thumbnail declaration programmatically for my site. I then use that thumbnail through

$file->getThumbnailURL($type->getBaseVersion());


This works great for new files. However, existing files that were uploaded before I made that change do not have that thumbnail and I get the original much bigger image instead of the thumbnail.
One workaround I found was to edit and then immediately save a file - then the thumbnail is generated. But that's not very acceptable with hundreds of existing files.
It there a way to force the generation of all the missing thumbnails either through the backoffice or programmatically ?

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studio108 replied on at Permalink Reply 1 Attachment
studio108
I believe you have to click 'Rescan' in the image properties window in File Manager but unfortunately that is on an individual file basis.

UPDATE-----
Actually it appears you can rescan a batch by clicking the tick box in the top blue panel in File Manager and then select 'Rescan' under the 'Items Selected' dropdown.
Example image attached.
dangrth replied on at Permalink Reply
No, I just tried. Rescan works (at least of the images that do not trigger an out of memory, some of my contributors just upload insanely huge images, that was the purpose of using thumbnail generation - I have to dig some to find how to increase memory), but the thumbnail is not generated.
What is the purpose of the 'required' flag for the thumbnail generation ? I set it to true, but I'm not sure of it's exact purpose.