ros2-tf-clock

# ROS 2 Time and TF2 ## Clocks | Source | When | |--------|------| | `RCL_SYSTEM_TIME` | Default wall clock | | `RCL_ROS_TIME` | `/clock` when `use_sim_time:=true` | | `RCL_STEADY_TIME` | Monotonic; **not** for TF / message stamps | Set **every** node (and launch) `use_sim_time: true` for sim **and** bag replay with `--clock`. One node on wall time + one on ROS time → TF "extrapolation into the future/past". `rclcpp::Node::now()` follows the node clock. `std::chrono::system_clock::now()` does **not**. Timers: `create_timer` (ROS time) vs `create_wall_timer` (wall). Rate loops in sim must use ROS timers or they race the bag. ## TF2 lookups ```cpp tf_buffer.lookupTransform(target, source, tf2::TimePointZero); // latest tf_buffer.lookupTransform(target, source, stamp, timeout); ``` | Error | Cause | |-------|-------| | `Lookup would require extrapolation` | Stamp newer/older than buffer; clock mismatch; publisher died | | `Frame not in tree` | Wrong frame_id / namespace; static TF not latched (QoS) | | `Connectivity` | Unconnected trees (`base_link` vs `odom` missing) | Rules: - Stamp outgoing msgs with **the same clock** as TF (`node->now()` or the sensor stamp you are expressing). - `tf2_ros::TransformBroadcaster` / static broadcaster: static TF needs **TRANSIENT_LOCAL** (see `ros2-qos-dds`). - `TimePointZero` / `rclcpp::Time(0)` = latest — OK for teleop, **wrong** for aligning a sensor frame to a specific scan stamp. - Buffer cache time must exceed your worst sensor delay (default ~10 s). ## Bag replay `ros2 bag play --clock` **and** `use_sim_time:=true` on consumers. Without `--clock`, `/clock` is absent and ROS time stays 0 or wall. See `ros2-bag-replay`.