class Asciidoctor::AbstractNode

def generate_data_uri_from_uri image_uri, cache_uri = false

and the mime type specified in the Content Type header.
Returns A data URI string built from Base64 encoded data read from the URI

is used to cache the image for subsequent reads. (default: false)
cache_uri - A Boolean to control caching. When true, the open-uri-cached library
image_uri - The URI from which to read the image data. Can be http://, https:// or ftp://

which can then be used in an image tag.
constructed from the content_type header and Base64 data and returned,
The image data is read from the URI and converted to Base64. A data URI is

Public: Read the image data from the specified URI and generate a data URI 
def generate_data_uri_from_uri image_uri, cache_uri = false
  if cache_uri
    # caching requires the open-uri-cached gem to be installed
    # processing will be automatically aborted if these libraries can't be opened
    Helpers.require_library 'open-uri/cached', 'open-uri-cached'
  elsif !RUBY_ENGINE_OPAL
    # autoload open-uri
    ::OpenURI
  end
  begin
    mimetype, bindata = ::OpenURI.open_uri(image_uri, URI_READ_MODE) {|f| [f.content_type, f.read] }
    # NOTE pack 'm0' is equivalent to Base64.strict_encode64
    %(data:#{mimetype};base64,#{[bindata].pack 'm0'})
  rescue
    logger.warn %(could not retrieve image data from URI: #{image_uri})
    image_uri
    # uncomment to return empty data (however, mimetype needs to be resolved)
    #%(data:#{mimetype}:base64,)
    # uncomment to return 1 pixel white dot instead
    #'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=='
  end
end