Pool Watch

Swimmers can drownin busy swimming pools when lifeguards fail to notice that they are introuble.The

Royal Society forthe Prevention of Accidents says that on average 15 people drown in Britishpoois each

year,but many moresuffer major injury after getting into difficulties.Now a French company hasdeveloped

an artificialintelligence system called Poseidon that sounds the alarm when it sees someonein danger of

drowning.

When a swimmer sinkstowards the bottom of the pool,the new system sends an alarm signal to a pooi-

side monitoringstation and a lifeguard's pager. In trials at a pool in Ancenis,near Nantes,it saved a life

within just a fewmonths,says Alistair McQuade,a spokesman for its maker,Poseidon Technologies.

Poseidon keeps watchthrough a network of underwater and overhead video cameras.Al software

analyses the imagesto work out swimmers'trajectories.To do this reliably,it has to tell thedifference be-

tween a swimmer andthe shadow of someone being cast onto the bottom or side of the pool."Theunderwater

environment is avery dynamic one,with many shadows and reflections dancing around,"saysMcQuade.

The software doesthis by"projecting"a shape in its field of view onto an image of thefar wall of the

pool.It does thesame with an image from another camera viewing the shape from a differentangle.If the

two projections arein the same position,the shape is identified as a shadow and is ignored.But ifthey are

different,the shapeis a swimmer and so the system follows its trajectory.

To pick outpotential drowning victims,anyone in the water who starts to descend slowly isadded to the

software's"pre-alert" list,says McQuade.Swimmers who then stay immobile on thepool bottom for 5

seconds or more areconsidered in danger of drowning. Poseidon double-checks that the image reallyis of a

swimmer,not ashadow, by seeing whether it obscures the pool's floor texture when viewed fromoverhead.

If so,it alerts thelifeguard,showing the swimmer's location on a poolside screen.

The first full-scalePoseidon system will be officially opened next week at a pool in High Wycombe,

Buckinghamshire.Oneman who is impressed with the idea is Travor Baylis,inventor of the clockwork

radio.Baylis runs acompany that installs swimming pools一and he was once an underwater escapologist with

a circus." Isay full marks to them if this works and can save lives,"he says.But headds that any local

authority spending ￡30,000-plus on a Poseidon system oughtto be investing similar amounts in teaching

children to swim.How does Poseidonsave a life?

A.He plunges intothe pooi.

B.It alerts thelifeguard.

C.He cries for help.

D.It rushes to thepool.