The London of “The Golden Compass” owes a debt to the unrealized vision of 17th-century architect Christopher Wren. His St. Paul’s Cathedral is the only part of his plan for rebuilding the city after the Great Fire of London in 1666 to have been built.

“He never got to do it in reality,” says production designer Dennis Gassner. “That’s what we get to do in movies, we get to do an homage to that.”

The city, seen from a zeppelin through the eyes of young Lyra, combines Wren’s vision with futuristic and magical elements to give it the right combination of reality and fantasy, Gassner says.

The sequence was designed down to the zeppelin’s flight path and brought to life with CGI tools far beyond anything Wren could have imagined. “Computers are great pencils,” says Gassner, now at work on the next Bond picture. “They are magnificent tools to create imagery, and this movie could never have been made without that facility.”