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I think the approximate time synchronizer is waiting until the next image on either topic arrives before it determines that the current pair should be published: http://wiki.ros.org/message_filters/ApproximateTime

Note the description of Inter message lower bound

Inter message lower bound: if messages of a particular topic cannot be closer together than a known interval, providing this lower bound will not change the output but will allow the algorithm to conclude earlier that a given set is optimal, reducing delays. With the default value of 0, for messages spaced on average by a duration T, the algorithm can introduce a delay of about T. With good bounds provided a set can often be published as soon as the last message of the set is received. An incorrect bound will result in suboptimal sets being selected. A typical bound is, say, 1/2 the frame rate of a camera.