How to Use the Google Nik Collection With Photos for Mac
The famous Google Nik collection with photo editing plug-ins is free since March last year (https://plus.google.com/+NikCollection/posts/AFGsG2Di7EK) - download it here:
https://www.google.com/nikcollection/
If you are using Photos 3.0 on High Sierra you can call the stand-alone applications of the Nik collection simply using "Image > Edit with", while you are viewing a photo enlarged in Photos.
In earlier versions of Photos you need a photo editing extension, that will call an external editor, for example the "External Editors extension". This extension is no longer available from the AppStore, after High Sierra has been released. I hope, everyone purchased this extension in time.
After downloading the NIK Collection from Google's site https://www.google.com/nikcollection/, run the installer. You will find the stand-alone programs in the Applications folder in the subfolder "Nik Collection". Run each of them to test them (but not the Uninstaller 😁).
After you restart the Mac, the External Editor Extension will be able to see them and show them in Photos.
To use them, open the photo in Photos in Edit mode by selecting the thumbnail and hitting the Return key.
Now click "Extensions" in the Edit panel and select "External Editors".
In the External editors select the NIK plug-in you want to use:
Now you will see the editor window of the selected plug-in and can edit your photo. When done, use the "Save" button in the plug-in and then the Save Changes in "External Editors".
Now the edited photo will appear in Photos.
Using photo editing extensions will usually create an additional original master image file. It increases the storage required for your photos. Don't use external editors for tasks, that can be done in Photos. By doing simple things in Photos you are able to revert all adjustments individually and to save storage.
On a stand-alone basis Dfine2 and Viveza2 are working well. I am using tis work-around to use them in Photos 2.0. I export the original from Photos to the Desktop, then ctrl-click to to use "Open with Dfine (or Viveza2). Then I am using the External editor extensions "Replace with Image from Disk" to pair the Viveza edited photo with the Version in Photos. This is giving the same result as directly editing from Photos.