Lightroom Export Filter Plugin, Work in progress

exportFilter.lua

-- Lightroom Lua Plugin for export filter 
-- Page 49 to 51 of the SDK guide
-- http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/devnet/photoshoplightroom/pdfs/lr5/lightroom-sdk-guide.pdf
function SimpleExternalToolFilterProvider.postProcessRenderedPhotos(
  functionContext, filterContext )
  -- Optional: If you want to change the render settings for each photo
  -- before Lightroom renders it
  local renditionOptions = {
    filterSettings = function( renditionToSatisfy, exportSettings )
      exportSettings.LR_size_maxHeight = 400
      exportSettings.LR_size_maxWidth  = 400
      exportSettings.LR_size_doConstrain = true
      return os.tmpname()
    end,
  }

  for sourceRendition, renditionToSatisfy in filterContext:renditions( 
    renditionOptions ) do
    -- Wait for the upstream task to finish its work on this photo. 
    local success, pathOrMessage = sourceRendition:waitForRender()
    if success then 
      -- Now that the photo is completed and available to this filter,
      -- you can do your work on the photo here.
      -- It would look somethinglike this: 
      -- local status = LrTasks.execute( 'mytool "' .. pathOrMessage .. '"' )
      local status = 1
      -- use something like this to signal a failure for this rendition only: 
      -- (Replace "error message" with a user-readable string explaining why
      -- the photo failed to render.)
      if status ~= (desired status) then
        renditionToSatisfy:renditionIsDone( false, "error message" )
      end
      
    end
  end