exporting images with lua

So far we have learned to use lua to adapt darktable to our particular workflow. Let’s look now at how to use lua to easily export images to an online service. If you are able to upload an image to a service via the command line then you can use lua to integrate this into darktable’s user interface.

In this next example we will use lua to export via scp. A new storage type will appear in darktable’s UI that will export images to a remote target via the copy mechanism in ssh.

darktable = require "darktable"

darktable.preferences.register("scp_export","export_path",
  "string","target SCP path",
  "Complete path to copy to. Can include user and hostname","")

darktable.register_storage("scp_export","Export via scp",
  function( storage, image, format, filename,
     number, total, high_quality, extra_data)
    if not darktable.control.execute("scp "..filename.." "..
      darktable.preferences.read("scp_export",
         "export_path","string")) then
      darktable.print_error("scp failed for "..tostring(image))
    end
end)

darktable.preferences.register will add a new preference to darktable’s preferences menu, scp_export and export_path allow us to uniquely identify our preference. These fields are reused when we read the value of the preference. The string field tells the lua engine that the preference is a string. It could also be an integer, a filename or any of the types detailed in the API manual relating to types_lua_pref_type . We then have the label for the preference in the preference menu, the tooltip when hovering over the value and a default value.

darktable.register_storage is the call that actually registers a new storage type. The first argument is a name for the storage type, the second is the label that will be displayed in the UI and last is a function to call on each image. This function has a lot of parameters, but filename is the only one we use in this example. It contains the name of a temporary file where the image was exported by darktable’s engine.

This code will work but it has a couple of limitations. This is just a simple example after all:

  • We use preferences to configure the target path. It would be nicer to add an element to the export UI in darktable. We will detail how to do that in the next section.

  • We do not check the returned value of scp. That command might fail, in particular if the user has not correctly set the preference.

  • This script can’t read input from the user. The remote scp must use password-less copy – scp can’t be provided with a password easily, so we will leave it that way.

  • There is no message displayed once the example is done, only the progress bar on the lower left side tells the user that the job is complete.

  • We use coroutine.yield to call an external program. The normal os.execute would block other lua codes from happening.

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