Heijnen, 53, describes the cats that live in shops as "Zen-like".

"They are quiet and relaxed, especially the ones living in the stores. They are used to so much noise around them, including the sound of boxes being moved in and out, and a busy street nearby."

When Heijnen was working as a graphic designer earlier, he switched cities every three or five years, he says.

And when he decided to move to Hong Kong in 2015, he left the cat he was raising in Singapore with a friend. In Hong Kong, it was for the first time in about 40 years that he was without a cat.

At first, he just took a few photos with his smartphone and posted them on social media. A big response drove him to show more. He published a picture book titled Hong Kong Shop Cats toward the end of 2016. A thousand copies or so sold out in three weeks in Hong Kong, he says. In June, it was listed as one of the 11 best books set in Hong Kong by Time Out magazine.

Heijnen owes his success to a combination of interesting images and an understanding of traditional customs in Hong Kong.

"Young people care about these things because it's the reflection of their identity. And I think they like it when somebody presents their culture in a kind of nostalgic way."