ROS學習（一）：tf and time

深刻理解：
This tutorial will teach you how to get a transform at a specific time.

報錯

監聽TF變換

    rate = rospy.Rate(10.0)
    while not rospy.is_shutdown():
        try:
            (trans,rot) = listener.lookupTransform('/turtle2', '/turtle1', rospy.Time(0))
        except (tf.LookupException, tf.ConnectivityException, tf.ExtrapolationException):
            continue

如果改成下面這個樣子：

        try:
             now = rospy.Time.now()
            (trans,rot) = listener.lookupTransform("/turtle2", "/carrot1", now)
        except (tf.LookupException, tf.ConnectivityException):

報錯：

Traceback (most recent call last):
  File "~/ros/pkgs/wg-ros-pkg-trunk/sandbox/learning_tf/nodes/turtle_tf_listener.py", line 25, in <module>
    (trans,rot) = listener.lookupTransform('/turtle2', '/carrot1', now)
tf.ExtrapolationException: Extrapolation Too Far in the future: target_time is 1253830476.460, but the closest tf  data is at 1253830476.435 which is 0.024 seconds away.Extrapolation Too Far in the future: target_time is 1253830476.460, but the closest tf  data is at 1253830476.459 which is 0.001 seconds away.Extrapolation Too Far from single value: target_time is 1253830476.460, but the closest tf  data is at 1253830476.459 which is 0.001 seconds away. See http://pr.willowgarage.com/pr-docs/ros-packages/tf/html/faq.html for more info. When trying to transform between /carrot1 and /turtle2. See http://www.ros.org/wiki/tf#Frequently_Asked_Questions

或者

Traceback (most recent call last):
  File "/home/rosGreat/ROS_tutorial/learning_tf/nodes/turtle_tf_listener.py", line 28, in <module>
    (trans,rot) = listener.lookupTransform('/turtle2', '/carrot1', now)
tf.ExtrapolationException: Lookup would require extrapolation into the future.  Requested time 1319591145.491288900 but the latest data is at time 1319591145.490932941, when looking up transform from frame [/carrot1] to frame [/turtle2]

這是爲什麼呢?
每個監聽器都有一個緩衝區，用來存儲來自不同tf廣播器的所有座標轉換。當廣播發送一個轉換時，在轉換進入緩衝區之前需要一段時間(通常是幾毫秒，usually a couple of milliseconds)。因此，當您在“now”時間請求幀轉換時，您應該等待幾毫秒，等待信息到達。

等待變換

*tf provides a nice tool that will wait until a transform becomes available. Let’s look at what the code would look like:
*

    listener.waitForTransform("/turtle2", "/carrot1", rospy.Time(), rospy.Duration(4.0))
    while not rospy.is_shutdown():
        try:
            now = rospy.Time.now()
            listener.waitForTransform("/turtle2", "/carrot1", now, rospy.Duration(4.0))
            (trans,rot) = listener.lookupTransform("/turtle2", "/carrot1", now)

waitForTransform會阻塞幾秒鐘，直到得到變換信息或者超時。
爲什麼調用兩次？
第一次是爲了等待/turtle2座標的廣播，The first waitForTransform() will wait until the /turtle2 frame is broadcast on tf before trying to waitForTransform() at time now

不翻譯了：
So, you noticed there is no noticeable difference in the turtle behavior. That is because the actual timing difference is only a few milliseconds. But then why did we make this change from Time(0) to now()? Just to teach you about the tf buffer and the time delays that are associated with it. For real tf use cases, it is often perfectly fine to use Time(0).

Time travel

怎樣讓烏龜二走到烏龜一5秒之前的地方？

        try:
            now = rospy.Time.now() - rospy.Duration(5.0)
            listener.waitForTransform("/turtle2", "/turtle1", now, rospy.Duration(1.0))
            (trans, rot) = listener.lookupTransform("/turtle2", "/turtle1", now)
        except (tf.Exception, tf.LookupException, tf.ConnectivityException):

高級api：

        try:
            now = rospy.Time.now()
            past = now - rospy.Duration(5.0)
            listener.waitForTransformFull("/turtle2", now,
                                      "/turtle1", past,
                                      "/world", rospy.Duration(1.0))
            (trans, rot) = listener.lookupTransformFull("/turtle2", now,
                                      "/turtle1", past,
                                      "/world")