The missing condition is the arresting of time, without which the sidewalk is merely a poor man’s camera obscura. To create a photograph I needed a way to record time’s passing. Conventional light-sensitive photo paper (the kind used in a darkroom) is too sensitive and expensive to handle in an uncontrollable environment like the city street, so I opted for blueprint paper, which is more forgiving. Years ago, the blueprint — or diazotype — was the tool of the architect, but its use has steadily declined with the advent of CAD and digital plotters. Still, the medium exists — due mostly to demand from emerging economies — so I was able to procure a steady supply of paper and my own processor.