On the streets of Brooklyn, stray bursts of color have been dotting the urban landscape, fleeting works of art that are impossible not to notice.

The chalk outlines - often of street fixtures such as fire hydrants, lamps or even buildings - are mysteriously signed "Ellis G. 2006. "

"My art has always been in the street," said Ellis Gallagher, a former graffiti artist, as he surveyed a just-finished hot pink tracing of a lamppost with traffic box and street signage on the corner of Smith and Dean Sts.

He followed the arc of the street lamp into the street, darting into traffic and squatting barely a foot away from slow-moving cars.

"It's temporal. And temporary," the artist said, as cars tread over the pink lines. Within minutes of completing a piece, the sun has moved the shadows.

People can't tell how much time has passed since the work was finished. Gallagher's artistic inspiration, however, sprang from a traumatic event.

Coming home late one night last month, Gallagher spotted a man lurking a block from his house. The man turned up his hood to cover his face, a sign Gallagher knew meant trouble. As Gallagher fumbled for his keys, he saw the man's shadow flicker across his front door. The man, wielding a 2-foot machete, demanded money.

Police caught the mugger that same night, but Gallagher was still rattled. He became nervous and jumpy, often reliving the moment when the thief's shadow merged with his own. "I became fixated with shadows," he said.

Now he spends several hours each day and night on his shadow art, stopping only for work or sleep. He prefers late afternoon shadows that extend far down the street. He even does night drawings, which use the effects cast by stationary ambient light. Because he uses chalk, police do not consider his work graffiti.

Gallagher has turned his newfound popularity into commissions and gallery work. He created 10 shadow-themed paintings for a show, on display until Sept. 20, at restaurant Apartment 138, also on Smith St.

As he traced another lamppost on Smith St., passersby approached with encouragement. "It's very graphic," said Karl Lucas, a fellow Cobble Hill artist who paused to admire the art. "When the shadow's gone, it turns into abstract art."

By early evening, a passing summer shower had washed away that morning's designs. "I like when it rains because it's a whole new canvas," Gallagher said.