imu orientation meaning and lookupTransform direction ?

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Hello,

• Q1:what is the imu orientation meaning?
• I think imu's orientation meaning is similar as odom's orientation.In odom 's orientation, it means that from base_link_0(odom frame,this moment odom is launched ) to base_link_t (base_link frame in time t ) rotation.So,i think imu's orientation means that from imu_t(imu_frame in time t) to imu_0(this moment imu is launched),just like the this picture picture, my thinking is right??
• Q2: Does tf::TransformListener::lookupTransform(const std::string& target_frame, const std::string& source_frame, const ros::Time& time, StampedTransform& transform)function means that : transform variable represents from target_frame to source_frame R-T ，namely ,target_frame is reference frame,we start target_frame ??
• Q3: When i see robot_pose_ekf code: imu measurement line 230, base_imu_offset line247 , imu measurement' 248, add measurement line261, If my Q1 and Q2 understanding are all right, i think line247,should be robot_state_.lookupTransform( imu->header.frame_id,base_footprint_frame_, imu_stamp_, base_imu_offset);,it should be lookupTransform: from imu_t---->base_t, Where do I understand the mistake？？
• Thanks in advance a lot !!!