Exchange data between subscribers. How to perform efficiently?

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Thank you. My concern is decoding image in waiting time. (No wasting of time). Since image comes 10 ms before depth. I want to perform image decoding in that 10ms, In the same node.

I'm not sure what you're asking.

By using message_filters, you'll get a single callback for both the depth image and the rgb image. No other callbacks will be called, until you have processed the current pair.

So unless I misunderstand you, that would seem to do exactly what you're after: synced availability of depth and rgb at the same point in time.

The time synchroniser approach will create a single callback that only gets called when both messages have been received. So the two subscribers will be synchronised automatically by message_filters. You will not have two callbacks, but only one. In that single callback, you'll decode both the depth and the rgb image pair that have the same timestamp.

Do you want to make sure that you're using two timestamp-matched messages (ie: depth and rgb), or are you more concerned with "optimal" processor utilisation?

If I use 'message_filters', in its callback it will lets say first decode 'rgb' and then it will decode 'depth'. In this case processing time would be the sum of both decoding. And I wont be able to utilise the time gap between 'rgb' msg & 'depth' msg. i.e., the 10-4 = 6ms. I hope you got what I mean, and please correct me if I am wrong..

If you really want to decode rgb in the time that you're waiting for the depth image then message_filters will not allow you to do that.

You'd have to implement the synchronisation yourself.