This weekend, America's oldest outdoor show of original art will take place in Philadelphia. The Rittenhouse Square Fine Art Show is celebrating its 90th year.

For the first time, the competitive exhibition will feature art by formerly homeless people.

As one of the most prestigious open-air art markets in the country, the Rittenhouse Square Fine Art Show maintains high standards. All the artists are vetted and there is a long list of rules they must abide.

For example, nothing can be sold for less than $35, and most pieces are priced significantly higher. Compare that to its downmarket cousin, Art for the Cash Poor, operating simultaneously at the Crane Arts Building in Kensington this weekend. Art for the Cash Poor enforces a mandatory price cap, where nothing can be sold for more than $199.

Former board member Richard Rossello floated the idea of inviting residents of Project HOME - an organization offering housing and services to homeless people in Philadelphia - to participate in the Rittenhouse Square show. There was some push-back. Some on the board were protective of the show's reputation for high-quality work.

"We wanted to make sure that bringing in another organization was going to stay on the same level as the rest of the exhibitors," said board member Sandra Sedmak Engel.

Two years ago, Project HOME struck up a partnership with Studio Incamminati, a small art school specializing in classical realist art, founded by the late Nelson Shanks. Rossello is also a board member there.

The Project HOME students receive the same level of instruction as Studio Incamminati's full-time students, just less of it: regular students are expected to put in 30 hours behind the easel every week, for the 20-week semester. The Project HOME residents get four hours of instruction a week, for 10 weeks.

"The progress is slower, but the standards are the same," said instructor Rachel Pierson.

Casundra Pierson, formerly homeless and now a Project HOME resident, tentatively signed up for a class a year ago. She came in with one foot pointed toward the exit.

"I had already planned an escape plan to get out of the classes," she said, skittish of the 10-week commitment. "The first thing we had to draw was a vase. I liked it. It jumped out at me. It's something I wanted to stay involved in."

The studio in the Callowhill neighborhood is set up with three object arrangements, in order of difficulty: cups and vases, rag dolls on a chair, and a lamp and reading glasses.

The task is simple, draw what you see.

Lesson #1: it's not so simple.

"Trying to capture what's really there, sometimes my mind gets in the way," said student Wesley Mitchell. "I'll have a preconceived thing about what a doll is, and I'll try to draw that instead of what's there."

Basic lessons of art sometimes sound like basic lessons of life. Mitchell is a recovering alcoholic; he has a string of DUIs and spent time in state prison. He is now living on disability benefits. Mitchell signed up for art classes because too much idle time is dangerous for an alcoholic.

"I'm finding out that my favorite tool in the art box is my erasure. It took me three weeks to understand that," said Mitchell. "I was married to every stroke, I couldn't let it go. Just like the big thing in my life: I can't drink successfully. I took me a lifetime to understand that. I just had to let it go."

While these art classes have been personally therapeutic for the residents of Project HOME, it is the quality of the results that got them invited to show in Rittenhouse Square. After a studio visit to meet the students and watch them work, the board members had little hesitation to accept them, said board member Engel.

The Rittenhouse Square Fine Art Show normally accepts only professional artists and full-time students. Because the residents of Project HOME fit in neither category, the board had to take a vote to make special exception for the group. The artists will show as a group in the center of Rittenhouse Square, near other art students.

The exhibition is rain or shine, meaning all participants - from professionals to students to the formerly homeless - are required by the rules governing the show to stick it out regardless of weather.