Hills of San Francisco transform into optical illusion in tilted video

In YouTuber Karen Cheng's latest video, she plays around on the hills of San Francisco. The results will play tricks on your brain.

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Cheng and friends created the video by tilting the camera to be level with the hills of San Francisco. They then play around with the strange effect by throwing a ball, running up and down the hills and jumping rope.

If you recognize Cheng, it's probably because she's the viral video star who learned to dance in a year and created the "donut selfie."

Filming this project proved to be a bit of an uphill battle, Cheng explained to ABC.

"There were a bunch of challenges when we filmed -- cars kept going by, interrupting our shot, so we had to reshoot some scenes many times," she said.

Some careful planning helped to maximize the effects, she explained.

"I spent the week before we shot practicing on the hills, trying different body positions to figure out which ones work the best," she said. "The reason we're all walking so funny (like penguins) is because walking that way shows the leaning illusion effect the best -- walking normally doesn't do so well!"

Of all the effects, Cheng said the "pendulum" at 1:06 was the hardest to capture.

"I had to keep my body straight while starting to fall over," she said.

Director Ross Ching was inspired when he saw photos using this effect, Cheng explained in the YouTube description. Ching thought it would work well as a video filmed around San Francisco.

The video has struck a chord with optical illusion enthusiasts, garnering hundreds of thousands of views in less than a week.

Images used with permission. To see more projects visit Cheng's Facebook page and Ching's website.