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Stare at a blank wall in any room, and you are unlikely to learn much more than the paint color. But a new technology can unnoticeably scan the same surface for shadows and reflections. 1． But they can be got by one tool and analyzed to determine details including how many people are in the room — and what they are doing. This tool could gather information from a partial view of a space. It can spy on activity from around a corner. 2．

As people move around a room, their bodies block a portion of any available light to create subtle and indistinct “soft shadows” on walls. 3． But these faint signals are usually drowned out by the main source of surrounding light. Sharma and other M.I.T. researchers separated that main source of surrounding light by filming a wall in a room as its occupants moved around and averaging the frames over time. Then the researchers removed shadows from furniture or other still objects from the video in real time, only revealing humans’ moving shadows on the wall.

4． They crouched, jumped or moved around, alone or in pairs, outside the camera’s view. Then the team fed the videos into a machine-learning model to teach it which soft shadow patterns indicated which behavior. The resulting system can automatically analyze footage of a blank wall in any room in real time, determining the number of people and their actions.

5． It can register only group sizes and activities for which it has been trained, and it requires a high-resolution (清晰度) camera; a standard digital camera creates too much background noise, and smartphone camera results are even worse.