Manual:$wgUploadStashScalerBaseUrl

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Server URLs and file paths: $wgUploadStashScalerBaseUrl
To enable remote on-demand scaling, set this to the thumbnail base URL.
Introduced in version:1.17.0 (r86169)
Deprecated in version:1.36.0 (Gerrit change 664086; git #d08c5ba3)
Removed in version:still in use
Allowed values:(string) or false
Default value:false

Details[edit]

To enable remote on-demand scaling, set this to the thumbnail base URL. Full thumbnail URL will be like $wgUploadStashScalerBaseUrl/e/e6/Foo.jpg/123px-Foo.jpg where 'e6' are the first two characters of the MD5 hash of the file name. If this is set to false, thumbs are rendered locally as needed.