how to add linear(x,y) twist with linear(x), angular(z) one

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Hello,

• I receive the target_speed of an object moving in front of my robot. This speed is described with linear_xand linear_y fields.

• My differential robot may be moving at the time I receive the target_speed, but I only now the current speed of my robot in the form linear_x, angular_z

• I have access to the target_distance if nedded

=> How to compute the target_speedin the world fixed frame ?

EG : if my target does not move but my robot is rotating clockwise, I now compute a positive linear_y, which is true in the robot frame, but not in the odom frame.

NB: I read a lot about this, tried some things without clearly understanding the maths behind... I also tried to attach a frame to my target so I can use lookup_twistor something like that... but with no luck until now.

Thanks to all of you