The building once used for boat repairs at the former Aspinwall Marina obstructed views of the Allegheny River. Now it is the open-air welcome center in Aspinwall Riverfront Park.

From the parking lot, you can look through its two open entrance ways — one the size of a large garage door — and see the river framed by a row of openings along the opposite wall.

Architect Eric Fisher’s design deconstructed the obstruction without removing it.

This is a transformative piece of the continually developing park. It is full of air and light, revealing its possibilities for any kind of use.

The park’s opening festival will take place from 5 to 8 p.m. Friday, featuring food trucks, free kayaking, a climbing wall, music and face painting. (See a more complete schedule of programs at the bottom of this column.)

When the park opened in 2015, the boat repair building was a closed-up hangar. It sat against a marina clubhouse that faced the river. A hodgepodge of smaller buildings against them held offices, a marina store and a gasoline station.

It was a mishmash of ugly — the industrial side of boating — but the renovation has retained shortened pieces of the front walls of the demolished buildings, outlines that Mr. Fisher called “the memory of those buildings.”

Park board member Trish Klatt said the park, a private nonprofit, could not afford to tear down the rest. What that lack of money afforded was Mr. Fisher’s creative subtraction, a whittling down to the essence. The welcome center’s large openings suggest something missing without suggesting loss.

The concept reminded me of a description I read somewhere about how Michelangelo sculpted, or rather how he approached sculpting. It may be how all sculptors approach sculpting, but it was enlightening to me as a kid and has stuck with me for more universal consideration: He chipped away at a block of marble to “find” David.

Even as a kid, I knew that didn’t mean just anyone could chip away marble and find Michelangelo’s David. But what I mulled over, the magic of the idea, was that, for him, David was already in there. He just had to know when to stop chipping.

Mr. Fisher sent me a link to his recent talk about subtractive architecture for the Pittsburgh History and Landmarks Foundation, and as I scrolled down, I found his comparison of the practice, in which “portions of a volume are removed to reveal their essence, like a Michelangelo sculpture.”

In his speech to the landmarks foundation, he noted that subtraction is “associated with disruption, degradation, and diminishment: I worry if my 20-pound kitty loses weight. Zombies are decayed humans. Potholes appear in diminished streets. Overly reductive political ideas discourage deep thinking about complicated issues.”

“Yet subtraction is also associated with improvement,” he continued in his speech. “When you subtract the leaves from an artichoke you reveal its heart. When a river courses through a plane for six million years, the resulting canyon reveals fossils and minerals. When you subtract the clouds from the sky, the sun shines. In each of these circumstances, subtraction clarifies an entity, laying bare a more natural pre-existing condition.”

In the same talk, he noted that when a caterpillar sheds part of itself to become a butterfly, it is certainly no less than it was before — to the contrary.

The welcome center is a perfect example of less being more.

By their nature, river industry and private marinas obstruct views of the water for everyone who isn’t using the marina or working in the industry. But the river is the essence. It was the reason the marina and industry were there and it outlasts them.

After leaving the site, where I met Mr. Fisher, I continued to think about subtraction as a means to find meaning, to get at essence, to get at more of something — a view of the river or a bigger perspective.

Diana Nelson Jones: djones@post-gazette.com or 412-263-1626. Park programming after Memorial Day: Venture Outdoors kayak rentals 4 to 8 p.m. Thursdays and Fridays and 10 a.m. to 8 p.m. Saturdays and Sundays.. Kindermusik classes are 10 a.m. Wednesdays. On Sundays, pure barre is at 10 a.m., yoga flow at 11 a.m. and mindful movement class for children at 1 p.m. For a schedule of events, visit aspinwallriverfrontpark.org/​events.

Correction (made May 23 at 11:17 a.m.): An earlier version of this story contained a caption that contained incorrect information and a name misspelling. Susan Crookston is the founder of Aspinwall Riverfront Park.