Nomenclature of tf_echo and tf_listener

[lepalom]

I have a question about nomenclature that it is getting me crazy. Probably is because English is not my language.

The tool tf2_echo help says:

tf2_echo source_frame target_frame [echo_rate]

This will echo the transform from the coordinate frame of the source_frame to the coordinate frame of the target_frame. Note: This is the transform to get data from target_frame into the source_frame.

For me is clear, if I have a frame, for instance, "grasp_01", and another frame "base", and I execute:

ros2 run tf2_ros tf2_echo base grasp_01

I would obtain the transformation that comes from base to grasp_01. Easily, I can see the distance between the origin of "base" to the origin of "grasp_01" expressed in the "base" frame.

But, in the documentation, for example the turtle, the sentence says:

Let’s look at the transform of the turtle2 frame with respect to turtle1 frame which is equivalent to: ros2 run tf2_ros tf2_echo turtle2 turtle1

To me is the opposite, I got the transformation from turtle2 to turtle1, that is the same the frame turtle1 w.r.t. turtle2, not the opposite as claims the documentation.

Also, I got another issue, the tf_buffer class has the method

lookupTransform (const std::string &target_frame, const std::string &source_frame, const tf2::TimePoint &time, const tf2::Duration timeout) const override

where you gets the transform between two frames by frame ID. With:

• target_frame = The frame to which data should be transformed
• source_frame = The frame where the data originated ...

If I create a listener in C++, and I call the function with:

tfs = tf_buffer_->lookupTransform("base", "grasp_01", tf2::TimePointZero);

I would obtain the same value as using tf2_echo. So, please, could you explain what should I understand as source_frame, target_frame in the ROS context?