Picture this: You are walking along the Strand in Galveston, the beautifully restored main drag of a time-warp port city in Texas, and across the street you see a couple of especially pretty Victorian commercial buildings that have just been given a fresh coat of paint. You cross the street to get a closer look, and . . . surprise!

That deep Italianate cornice, those keystone window moldings and the course of sharply angled bricks separating the first and second floors are not more than a few millimeters thick, the thickness of a coat of paint. The whole thing, shadows and all, is nothing more, or less, than a carefully concocted deception.

Or place yourself in Boston, strolling along Newbury Street. You turn around to get another look at the outsized concrete modern building you have just passed, and . . . tilt!

The enormous flat plane of the back side of the building opens up astoundingly to reveal vaulted passageways and an Ionic colonnade supporting a huge coffered dome. This time, there's no mistaking the deception for what it is: a coat of paint, a phantasmagorical cross-section, probably the largest architectural "drawing" ever made.

What is going on in both places, and at appropriate sites in Munich, New York, Milwaukee and Chicago, is that the artist Richard Haas has been there, and left his distinguishing mark. Haas' unusual brand of wall-painting, which occupies a curious border zone between architecture and art, building and decoration, is the subject of an exhibition sponsored by the American Institute of Architects Foundation that will open Wednesday at the Octagon House.

Haas, 45, accomplished his first building mural in the mid-'70s, replicating the front facade of a six-story cast-iron building in the SoHo district of Manhattan, where he lives, on the building's exposed, blank side wall. Like the other artists in the city-scale mural movement that developed from different sources, and on opposite sides of the continent, in the 1960s, Haas has helped to enliven many of the unsightly loose ends of contemporary cities--leftover building walls, unbecoming rear facades, even whole buildings that are just ugly, ordinary or somehow out of sync.

Unlike most of the others, however, Haas from the beginning has emphasized architecture itself in his work. This clearly separates him from the political muralists of Chicago, Los Angeles, Washington and elsewhere, and also from the esthetic muralists who simply transform paintings into urban-scale abstract signs. Mame Cohalan, who painted the imaginary, brightly colored perspectival spaces on the vacant walls at Market Square on Pennsylvania Avenue, works in a parallel direction, but by contrast Haas' work has a peculiar, particular resonance. His huge murals invariably reflect and mimic specific places; his illusions are as much mental as they are optical.

Reviewing his projects at the Octagon show (and in a recent Rizzoli book, "Richard Haas: An Architecture of Illusion," with an introduction by New York Times architecture critic Paul Goldberger) suggests the main reason for this is that, in addition to adopting architecture as the subject matter of his murals, Haas actually conceives them in architectural terms. That is, he doesn't simply paint buildings on buildings. He also thinks of the paintings themselves as buildings or, at the very least, as essential ingredients of the architecture.

Although Haas is deeply indebted to the example of the other muralists, and his work is related in more distant ways to the urban-scale projects of artists such as Rockne Krebs, his theoretical interests in surface decoration, in architectural history, in stylistic context and historic preservation put him much closer to the camp of Post-Modernist architects than to other artists. And his work is more securely placed in the historical context of architectural illusionism than in the history of painting.

Haas' illusionistic paintings don't really fool the eye--at least, not after a first encounter--but they tease it insistently. In Munich, for instance, Haas was commissioned to cover a wall at the end of a block left exposed by bombings in World War II. Responding to cues from the immediate architectural environment, Haas created an entirely new design, an elegantly imagined 19th-century townhouse, that nonetheless fits right in.

At the top of this mural, illusion rivals reality, as the new, painted roof line is every bit as persuasive to the senses as the actual roof line. At the bottom, illusion takes over entirely where the painter cuts through the solidity of the wall with a perspective rendering of an improbable courtyard, "a reminder," Haas wrote, "of typical earlier courtyards."

This kind of space- and time-warp is typical of Haas' newer projects. The Boston mural, for instance, unlike the straightforward Galveston painting, is a space-defying architectural impossibility. We know right away that the painting is a painting, but we also accept its happy intrusion. The cutaway painting of a huge neo-classical interior (based in part, Haas says, on his respect for the architectural drawings of 18th-century visionary architects Boullee and Ledoux) fits Boston in a way that it wouldn't fit, say, San Francisco. The conceit of the thing is also benign: the building whose backside it adorns houses the Boston Architectural Center.

With similar intentions Haas adopted Art Deco motifs for his mural on a blank wall at the entrance of the Lincoln Tunnel in Manhattan; he concocted an impressively sculptural Deco facade for the Centre Theater in Milwaukee (complete with a "reflection" of the Pabst building, a Victorian skyscraper that was torn down, in a stepped-back mirror-glass wall); and he composed a giant tribute to Louis Sullivan for the LaSalle Towers in Chicago, his largest commission so far, where he completely redesigned all four facades of a new apartment building. In the LaSalle project, differences between architecture and decoration become completely obscured. On the outside Haas' design is the architecture.

In additon to scale models and paintings Haas made for various projects, the Octagon show also includes etchings, lithographs and watercolors Haas has made of buildings and cityscapes. This more conventional, and less interesting, aspect of his art preceded the urban-scale paintings (and continues today). These pictures clearly establish Haas as a man in love with buildings, with the way light both defines and alters their facades, and it is interesting to learn that Haas' first love was architecture.

He was born in Spring Green, Wis., and he vividly recalls visits to the town from the master, Frank Lloyd Wright, who lived and worked a mile away at Taliesin. When he was 19 Haas even spent a period of time soaking up the atmosphere at Taliesin, but shortly thereafter he opted for art school. It took Haas more than a decade to combine his learned trade with his real love. The result of that delayed union is a set of immensely likable, vigorous, intriguing urban transformations.

The exhibition opens Wednesday and continues through July 18. The Octagon, at 18th Street and New York Avenue NW, is open Tuesday through Friday from 10 a.m. to 4 p.m. and Saturday and Sunday from 1 to 4 p.m. (It will be closed Sunday, May 30.)