should partially overlap the blocks e g the blocks 12 defined in the previous round when an object touches an edge of a specific one of the first round blocks 12 the same object is very likely to touch a central portion or near central portion of a corresponding one of the second round blocks 22 therefore the probability of erroneous determination can be reduced to avoid missing any point in a third embodiment of the present invention touched points 105 which are determined in the point scanning stage of the first round will be sensed again in the point scanning stage of the second round before point scanning each point 101 of an involved block 15 can be marked in the third embodiment of the present invention each point 101 of an involved block 15 is initially designated with a flag when a specific point 101 is scanned and sensed this point 101 is de flagged unless this point is determined as a touched point 105 that is after a point is checked it is maintained to be flagged if it is a touched point 105 otherwise it is de flagged if it is an un touched point the points remaining flagged after the previous round in finished are also checked in addition to the points of the involved blocks 15 in the current round the touched points 105 identified in a round can be re checked in more following rounds in a fourth embodiment of the.