code: def init_app(self, app):
          
          self.enabled = app.config.get("CAPTCHA_ENABLE", True)
          self.digits = app.config.get("CAPTCHA_LENGTH", 4)
          self.max = 10**self.digits
          self.image_generator = ImageCaptcha()
          self.rand = SystemRandom()
  
          def _generate():
              if not self.enabled:
                  return ""
              base64_captcha = self.generate()
              return Markup("<img src='{}'>".format("data:image/png;base64, {}".format(base64_captcha)))
  
          app.jinja_env.globals['captcha'] = _generate
          
          # Check for sessions that do not persist on the server. Issue a warning because they are most likely open to replay attacks.
          # This addon is built upon flask-session.
          session_type = app.config.get('SESSION_TYPE', None)
          if session_type is None or session_type == "null":
              raise RuntimeWarning("Flask-Sessionstore is not set to use a server persistent storage type. This likely means that captchas are vulnerable to replay attacks.")
          elif session_type == "sqlalchemy":
              # I have to do this as of version 0.3.1 of flask-session if using sqlalchemy as the session type in order to create the initial database.
              # Flask-sessionstore seems to have the same problem. 
              app.session_interface.db.create_all()
docs: Initialize the captcha extension to the given app object.