def default_mask(mask_arg)
      mask = mask_arg.dup
      mask[:width] = image_file_width if mask[:width].zero?
      mask[:height] = image_file_height if mask[:height].zero?

      crop_size = size_when_fitting({width: image_file_width, height: image_file_height}, mask)
      top_left = get_top_left_crop_corner(crop_size)

      point_and_mask_to_points(top_left, crop_size)
    end
def default_mask(mask_arg)
      mask = mask_arg.dup
      mask[:width] = image_file_width if mask[:width].zero?
      mask[:height] = image_file_height if mask[:height].zero?

      crop_size = size_when_fitting({width: image_file_width, height: image_file_height}, mask)
      top_left = get_top_left_crop_corner(crop_size)

      point_and_mask_to_points(top_left, crop_size)
    end
Returns the default centered image mask for a given size.
 If the mask is bigger than the image, the mask is scaled down
 so the largest possible part of the image is visible.