Extracting image dimensions

Gem

See https://github.com/teeparham/paperclip-meta

DIY

Image dimensions can be calculated and stored upon creating a file. In this case, we serialize image width and height into one field in the table: dimensions.

Asset.rb

# Table name: assets
#
#  id                  :integer(4)      not null, primary key
#  upload_file_name    :string(255)     not null
#  upload_content_type :string(255)
#  upload_file_size    :integer(4)      not null
#  upload_updated_at   :datetime
#  dimensions          :string(30)

has_attached_file :upload
before_save :extract_dimensions
serialize :dimensions

# Helper method to determine whether or not an attachment is an image.
# @note Use only if you have a generic asset-type model that can handle different file types.
def image?
  upload_content_type =~ %r{^(image|(x-)?application)/(bmp|gif|jpeg|jpg|pjpeg|png|x-png)$}
end

private

# Retrieves dimensions for image assets
# @note Do this after resize operations to account for auto-orientation.
def extract_dimensions
  return unless image?
  tempfile = upload.queued_for_write[:original]
  unless tempfile.nil?
    geometry = Paperclip::Geometry.from_file(tempfile)
    self.dimensions = [geometry.width.to_i, geometry.height.to_i]
  end
end

AssetsController.rb

@asset = Asset.find(params[:id])
width, height = @asset.dimensions