Updating preview database

[TomTurnschuh]

Question

Is there a way to update preview database to old files or new settings? What is the best method for this task?

One example

1. Previews are generated for widthSizes "128".
2. widthSizes is changed to "128 256".
3. Missing preview files should be generated.

Another example

1. previewgenerator is missing in the installation.
2. A few images are uploaded.
3. previewgenerator is installed and enabled.
4. Missing preview files should be generated.

Background

The momentary application is the inclusion of the previewgenerator for NextCloudPi, cf. nextcloud/NextCloudPi#662.