Why is MiniMagick's crop method returning an empty string?

Question

My screenshot model has a shot image attached via CarrierWave:

class Screenshot < ActiveRecord::Base
  attr_accessor :crop_x, :crop_y, :crop_w, :crop_h 
  mount_uploader :shot, ShotUploader
  process_in_background :shot unless Rails.env.test?
end

I have a form that gathers cropping information for the @screenshot#process_crop action:

  def process_crop
    @survey = Survey.find(params[:survey_id])
    @review = @survey.review
    @screenshot = @survey.screenshots.find(params[:id])
    @competitor = @survey.competitor
    screenshot = MiniMagick::Image.open(@screenshot.shot.url)
    if screenshot_params["crop_x"].present?
      x = screenshot_params["crop_x"]
      y = screenshot_params["crop_y"]
      w = screenshot_params["crop_w"]
      h = screenshot_params["crop_h"]
      cropped_image = screenshot.crop("#{w}x#{h}+#{x}+#{y}")
      uploader = ShotUploader.new
      uploader.store!(cropped_image)
    end
    redirect_to survey_screenshot_path(@survey, @screenshot)
  end

I have confirmed that x, y etc are sensible values given the dimensions of the image.

MiniMagick::Image.open(@screenshot.shot.url) returns a MiniMagick image object as you'd expect.

But why does screenshot.crop("#{w}x#{h}+#{x}+#{y}") return an empty string?

Clearly I need it to return a cropped image in a format that will make sense to uploader.store!.

Answer 1

Minimagick tends to operate inplace, so

screenshot.crop("#{w}x#{h}+#{x}+#{y}")

modifies screenshot rather than returning a new cropped image