By DAVID GERMAIN, AP Movie WriterLAS VEGAS - This time out, there's no vat of chemicals to explain how Batman's greatest enemy came to be the twisted sociopath known as the Joker.Heath Ledger's Joker springs full-blown in this summer's "The Dark Knight," the sequel to 2005's "Batman Begins" that was previewed for theater owners Thursday with a clip showing the new movie's opening sequence.Unlike 1989's "Batman," in which the deranged, disfigured clown appearance of Jack Nicholson's Joker resulted from a dip in chemical goo, "The Dark Knight" starts right in with the bad guy in all his psychopathic glory."I believe whatever doesn't kill you simply makes you stranger," Ledger's depraved Joker cryptically tells an accomplice in the opening scenes, in which he pulls off a daring bank robbery.In an interview at ShoWest, a theater-owners convention where distributor Warner Bros. showed off footage of "The Dark Knight" and the rest of its summer lineup, director Christopher Nolan said it was almost inevitable that the sequel would pit Christian Bale's Batman against the Joker."The psychopathic clown, that's an icon to stand with the guy with the ears and cape," Nolan said. "It's just a wonderful visual relationship, and it's a terrifying image."Long before Ledger's death of an accidental prescription drug overdose in January, the marketing of the movie had focused on the villain's rise to power and his creepy appearance.There had been speculation among critics and fans that the studio and filmmakers might take a different approach to selling the film in light of Ledger's death, but the marketing has gone on as originally planned."I think he'd be very pleased to see we're just moving ahead as is," Nolan said. "If you try to honor somebody, you honor them by respecting their work and putting it out there for as many people to see. He was immensely proud of the work he did on the film. I feel a great burden to present that in an undistorted form.""The Dark Knight" is due i...