A Microsoft Surface advertisement plastered to a bus stop shelter gave Giovanni Donelli his next business idea. At the time, the device was making waves as a long-awaited solution for digital illustrators. Rather than drawing on a tablet and looking up at a monitor to see their progress, artists could work directly on the device.

Donelli, who illustrates as a hobby, knew that most artists prefer Apple products. So Donelli and his business partner, Matt Ronge, set out to fill an apparent gap. Astropad, a Minnesota-based app launched in 2015, lets artists illustrate directly on an iPad without hooking it up to a computer. Now a team of five people, the fledgling start-up rolled out its second version, Astropad Studio, in January. About 320,000 people have downloaded either version, Ronge said.

Donelli and Ronge had first met as Apple interns. When they first got the idea for Astropad, they were working as a consulting team, dubbed Astro HQ, for businesses that wanted to make iPad apps.

Donelli and Ronge spent a year and a half working on the app on top of their consulting gig. After about six months of setting aside weeknights and one day per week for the app, some friends at Pixar showed serious interest. They let go one of their clients and began racing to beat the rumored launch of the Apple Watch, which they feared would leave little media attention for anything else.

"We were hitting burnout at that point," Donelli said. "I was taking a lot of naps."

The duo launched the product in February 2015. Months later, they won the Minnesota Cup, a statewide entrepreneur contest. Serious interest from consumers convinced them to make the app a full-time commitment.

"There was just such reception from artists and designers, this was something they'd been looking for," Ronge said. "We were like, 'OK, we need to give this a shot.' "

Geared toward professional graphic designers or illustrators, Astropad Studio offers users customization and mobility. Artists can use the app anywhere that has Wi-Fi without hooking the iPad up to a Mac. Illustrators can adjust almost every aspect of their experience, from brush size to the app's pressure sensitivity to the stylus.

"That new version is unlike anything else out there," said Brian Rood, who illustrates for Star Wars merchandise.

Rood said he first downloaded the app so he could meet deadlines from his couch while recovering from surgery.

"Time is money. Anything that speeds it up a little bit, any time you can just use your fingers to do a gesture or do a shortcut … time savings is a big deal," he said.

The app is an inexpensive alternative to products from its largest competitor, Wacom, Rood said. The Japan-based company offers a wireless tablet, but Rood likes that he doesn't have to buy a whole different machine for work.

While Wacom tablets sell for upward of $500, the Astropad standard is $30 and Astropad Studio has an annual subscription fee of $65.

"You get all the perks of having that big expensive machine but you get to use it on your skinny iPad pro, so that's pretty awesome," Rood said.

So far, competition has been limited. In an industry where competitors can move in on their market any minute, the Astro HQ team's ability to deftly adjust their product based on customer feedback is one of their biggest strengths, said Brian Olsem, who advises Astro HQ through Minnesota Emerging Software Advisory, an organization that pairs start-ups with mentors.

"They pay very close attention to any social noise," he said. "They start hearing what the reaction is and they adjust in days. Worst case, weeks."

To stay ahead of the curve, Astro HQ will need to invest to branch out in more markets.

The team's frugality has helped sustain the business, Olsem said. "They ran it very carefully as far as expenses go," he said. "We had to encourage them, saying you might want to pay yourselves a little bit."

In the meantime, Ronge says their biggest challenge has been getting noticed. The app has garnered word-of-mouth and social-media buzz among artists, and the company has cross-promoted with other companies, like the comic drawing software Clip Studio.

It doesn't hurt that the Apple Store has featured their app.

"Being the cheap people they were, they had a real hard time saying they were going to give Apple 30 percent of their revenue to be in the Apple Store," Olsem said. "But as soon as they launched in the store, this thing really took off."

Ronge credits the Twin Cities start-up community — with organizations like MESA and Minnesota Cup — as a big factor in their success.

"There's also more of a community here to the start-up ecosystem, as opposed to Silicon Valley," he said. "There's a lot of tech out there, it's easy to get lost in the noise. Out here, resources have been more accessible."

Jackie Renzetti is a University of Minnesota student on assignment for the Star Tribune.