What edits the metadata inside a stored file for the Storehouse.Analyzer.EditMetadata step: Storehouse's own (Storehouse.Image.MetadataEditor.Builtin, in Elixir, for JPEG, PNG and WebP), or one of the app's, around ExifTool, a NIF, a service.

The changes are Storehouse's, whichever editor makes them:

• strip: :gps - where the photo was taken.
• strip: {:keep, tags} - all metadata but the tags named (Storehouse.Image.JPEG lists them by name), and what draws the image.
• put: %{artist: _, copyright: _, image_description: _} - tags written, or removed with nil, after :strip.

An editor reads the file at from, writes the edited file to to, and never changes from. It answers :unsupported for a type it does not edit, and the step is skipped; :unchanged when there is nothing to change (the step also sees an edit that changed no byte).

Another editor

For every step, in config:

config :storehouse, :metadata_editor, MyApp.ExifTool

or for one, as the step's :editor. One around ExifTool, which edits the formats Storehouse's does not:

defmodule MyApp.ExifTool do
  @behaviour Storehouse.Image.MetadataEditor

  @tags %{artist: "Artist", copyright: "Copyright", image_description: "ImageDescription"}

  @impl true
  def edit(from, to, _content_type, changes) do
    args =
      Enum.flat_map(changes, fn
        {:strip, :gps} -> ["-gps:all=", "-xmp-exif:gps*="]
        {:put, tags} -> for {tag, value} <- tags, do: "-#{@tags[tag]}=#{value}"
      end)

    case System.cmd("exiftool", args ++ ["-o", to, from], stderr_to_stdout: true) do
      {_output, 0} -> :ok
      {output, _status} -> {:error, output}
    end
  end
end