He began his career as an abstract painter. Between 1968 and 1974, he traveled extensively in Europe where he was influenced by the illusionistic painting of the Italians Tiepolo and Veronese, and the German baroque artists Balthazar Newmann and the Zimmerman brothers.
His first trompe l'oeil mural was 112 Prince Street Facade (1974-1975), with illusory windows and architectural elements.
Later murals were less literal. In Boston Architectural Center (1977), for example, Haas projected a cross section of an imaginary domed structure onto the rear facade of a severe 1960's concrete building.