深伪视频，躺枪的为什么总是汤姆·克鲁斯？

A trio of viral videos allegedly depicting the actor Tom Cruise performing a magic trick, telling a not-so-funny joke, and practicing his golf swing are some of the most sophisticated examples yet seen of deepfakes, highly convincing fake videos created using A.I. technology, according to experts in the forensic analysis of digital images.

The three videos, which were posted last week on the social media platform TikTok from an account called @deeptomcruise, have collectively been viewed about 11 million times. The account has garnered more than 342,000 followers and 1 million likes from other users of the social media platform.

The person or people behind @deeptomcruise have not yet been definitely identified, but Cruise impersonator Evan Ferrante told website Mic over the weekend that he believed the videos were the work of an actor named Miles Fisher, who resembles Cruise and has done impressions of him in the past. Several people on social media sites also said they believed Fisher is depicting Cruise in the videos, with his face modified using deepfake technology.

Hany Farid, a professor at the University of California at Berkeley who specializes in the analysis of digital images, says he is convinced that the videos are deepfakes but that they are “incredibly well done.”

According to an analysis by Farid and one of his graduate students, Shruti Agarwal, there are a few tiny pieces of evidence that give away the fact that the videos are A.I.-generated fakes. In one video, in which Cruise seems to perform a magic trick with a coin, Cruise’s eye color and eye shape change slightly at the end of the video. There are also two unusual small white dots seen in Cruise’s iris—ostensibly reflected light—that Farid says change more than would be expected in an authentic video.

The A.I. methods used to create deepfakes often leave subtle visual oddities in imagery and videos they create—inconsistencies in eye color or shape, or strange ear contours or anomalies around the hairline.

Deepfakes are most often used to swap one person’s head or face for another’s as opposed to generating the entire body, and Farid notes that the hands performing the coin trick don’t look like the real Cruise’s hands. Presumably they belong to an actor, who was filmed performing the coin trick and then had Cruise’s face substituted for his.

Farid also says that while a true deepfake often involves a full-face swap, a more convincing result can sometimes be obtained by using the A.I. technique to generate only a portion of the face. He and Agarwal suspect that this is the case with the three Cruise videos. They think that the mouth is probably real, but that the eye region has been created with deepfake technology.

“This would make sense if the actual person in the video resembles Cruise, did some good work with makeup perhaps, and the swapping of the distinct eyes is enough to finalize a compelling likeness,” Farid says. “It is also possible that there was some postproduction video editing.”

Deepfakes are created using a machine-learning technique called a GAN (generative adversarial network), in which two deep neural networks—a type of machine learning loosely based on the way the human brain works—are trained in tandem. One network is trained from pictures or videos of the real Cruise to generate new images of Cruise in different settings or poses that are realistic enough to fool the other network, which is trained to pick out images of Tom Cruise from those of other people.

As with most A.I. methods, the amount and quality of the data help determine how good the system is. That goes to explain why Cruise has been a frequent target for deepfakes: He is among one of the most photographed celebrities on the planet. All that data makes it easier to train a very good Tom Cruise image generator.

Farid says it also doesn’t hurt that Cruise has a distinctive voice and mannerisms that add to the entertainment value and social media virality of deepfakes involving him.

Prior to the current trio of Cruise videos, one of the most wildly circulated and uncanny examples of a deepfake also involved Cruise. Released last year by a person who goes by the Internet handle Ctrl Shift Face, who has created a number of highly realistic deepfakes, it involves a video of the comedian Bill Hader doing an impersonation of Cruise on the David Letterman show in 2008. Deepfake technology is used to modify the video so that Hader’s face seamlessly morphs into Cruise’s as he does the impression. Deepfakes first surfaced in 2017, about three years after GANs were invented. Some of the earliest examples were videos in which the head of a celebrity was swapped with the body of an actress in a pornographic film. But since then they have been used to create fake videos of a lot of different celebrities in different settings. There is now off-the-shelf software that enables users to create fairly convincing deepfakes, and security researchers have become increasingly alarmed that deepfakes could be used for sophisticated political disinformation campaigns. But so far, despite a couple of possible examples that are still being debated by experts, deepfakes have not become a major factor in disinformation efforts.

While today’s deepfakes are usually identifiable with careful digital forensic analysis, this process is time-consuming and requires a certain amount of expertise. Researchers are working to create A.I. systems that would be able to automatically identify deepfakes, and Facebook in 2019 launched an annual competition to find the best of these. But in its inaugural running, the top performing system was accurate only 65% of the time.

Agarwal says it is possible to create deepfakes of the quality seen in the three Cruise videos using commercial software for deepfake generation. But doing so requires some skill, as well as a significant amount of data and training time for the A.I. system involved—and that training time can be expensive. So whether it would have been worth that sort of effort and cost for a viral TikTok video remains uncertain.