Smart Previews are a Lightroom 5 feature that allows you to edit your image with Lightroom adjustments while the original file is not present. This is possible because Lightroom creates a version about 5% the size of the original image and saves the instructions for your edit until you the original files are available again.
The Nik Collection plug-ins use all of the image information to accurately edit the image. For this reason, the plug-ins do not have the ability to edit the image when the original is not available.
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