What is the best way to send transform (with scale)?

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You cannot send non-normalized transforms (rotations or translations) via tf. It's designed to keep track of coordinate frames in space and apply transforms to data. A scaling operation is not generally applicable to transforming between coordinate frames where we keep all data in units of meters according to REP 103. So we limit the system to homogeneous transformations.

Since your scaling operation is semantically significant you should do that before or after you operate on the data and then make sure that the data is then observably different between the two states.

To your direct question if you want to send a transform with scale you can create a message to capture any sort of transform representation that you want and send it through a topic.