I think “Photos” does not allow to create a “New Smart Album” with Square edited photos.
At least I did not find “square” as a criteria in the list provided.
Do I need to flag the square edited photos one by one or is there a means of choosing a selection of photos whose width is strictly equal to height, eventually outside of “Photos” ?
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Emile NICAISE
You are right, there's no way to search or create a Smart Album using "square" as the criteria. You can't even use dimensions as a criteria. It would be nice if they would add this. I can't find any better way to do this other than to simply keyword tag your square photos manually and search for that keyword. If there are 100 photos, no problem. If 10,000 then problem.
It is potentially possible to do this with AppleScript/JXA in Automator. You could loop through your photos, and add a keyword if the width and height are equal. Then create a Smart Album for that keyword. But it would be difficult. So if this is a critical work task, then investigate it further. If this is just something that would be convenient, then it is probably not worth it.
Thanks a lot for your prompt response and suggestions.