SAN FRANCISCO — The speculation started popping up on social media almost as soon as President Trump posted a video Thursday afternoon to his Twitter feed showing him on the South Lawn of the White House.

“Trump faked this video released today," one Twitter user posted. “It’s shot in front of a green screen, the background is fake & on 3-second loop.”

“This is in front of a green screen,” actor George Takei tweeted, in a message that has more than 3,000 retweets.

Video experts say that’s unlikely. Instead, the odd elements people are pointing out — the focus of the grass, the background that looks like it’s on a loop, the shadows — are more likely a result of standard compression that gets applied to videos when they are posted on Twitter.

“I don’t see clear visual evidence that the video is shot with a green screen, the crisp shadows appear to be consistent with the sun as the light source, and the reverberation in the audio does not sound like it is indoors,” said Hany Farid, a professor at the University of California at Berkeley who researches digital forensics.

White House spokesman Judd Deere said the president was filmed on the South Lawn and did not use a green screen.

Earlier, a White House official said that both videos posted to Trump’s Twitter feed Thursday were filmed on the South Lawn on Wednesday.

Since the president announced he tested positive for the coronavirus last week, unsubstantiated rumors have flown online that he might not be as healthy as he and his doctors proclaim. In a video posted earlier this week, people speculated online that a coughing fit might have been edited out. Thursday’s video was the latest presidential message to face such scrutiny.

Adding to the swirl of speculation is Trump’s choice to broadcast his videos across social media. It’s a favorite way of the president to communicate to his 87 million followers on Twitter, something that has taken on added significance after his diagnosis.

People love to play “arm chair detective,” Farid said, but oftentimes, they miss the mark.

Some experts think it’s good that people view the president’s posts with a spoonful of skepticism. Jennifer Grygiel, a professor at Syracuse University and social media expert, pointed out that messages coming directly from the president — or other government officials accounts — have not been fact-checked by the press. People should push for information to be vetted, Grygiel said.

“And without that step, there is risk," Grygiel said.

As for green screen videos, it can be hard for an untrained eye to tell for sure whether a video had special effects applied, said Dylan Reeve, a television editor and documentary filmmaker in Auckland, New Zealand.

“My advice would be before looking for why something might be fake when you suspect it could be, start by trying to find out why it might not be,” he said. “Is there any other angles, corroborating details, hints in the image that it’s all one thing? Then does it being faked make sense?”

Sometimes there might be “color spill,” or green/blue light reflecting onto a subject in the foreground. But that doesn’t necessarily mean a green screen was used — it could also be a reflection from something natural outside, he said.

Reeve carefully listed the reasons he doesn’t believe the president’s video used a green screen in a public Twitter thread.

The shadows of the sun on the White House change throughout the video as the light changes, he said in an email. Natural outdoor lighting is very hard to re-create indoors. And the looping people think they saw in the background? That can be explained by the way Twitter compresses videos, Reeve said.

Even the way Trump’s hands seem to blur are a sign the video was probably shot without a green screen, Reeve said.

Reeve pointed out that Trump’s messaging has made people look for “little lies everywhere,” but that doesn’t mean the visual effects of the video were messed with.

“In this case it’s a two-and-a-half minute video. It tells us nothing of what went on immediately before or after, or how many takes it took, or what was necessary to make it possible,” he said.