It depends whether the traveler is passing through the Singapore Immigration or is staying within the terminal only. If the traveler stays in the area of terminals 1, 2 and 3 of the Changi airport and do not pass through the immigration, then he does not require a visa. In cases where the airlines do not have proper transfer facilities, the traveler needs to pass through the Singapore Immigration. In such a scenario, the traveler needs to obtain a Singapore visa prior to the trip.