Understanding the URDF Tutorial

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I have problem to figure out what the tutorial on this page is proposing to explain.

In order:

• Under "Completing the Kinematics" it states:

So, if you look at joint2, you see it rotates around the positive Y-axis. So, simple add the following xml to the joint element:

<axis xyz="0 1 0" />

But if you look in the code you will see:

  <joint name="joint2" type="continuous">
    <parent link="link1"/>
    <child link="link3"/>
    <origin xyz="-2 5 0" rpy="0 0 1.57" />
    <axis xyz="-0.707 0.707 0" />
  </joint>

Not really what it stated before.

• Wachting the figure above it not really clear on which axis joints are orientated. To me, it looks like joint1, joint2, joint3 are rotating on an axis that is going inside the figure. In every 3 cases in Z. This is much more clear if one reads the above code, all three links are in the same plane, meaning that the only possible rotation is on Z (which is entering or escaping from the figure's plane). So to me should be: <axis xyz="0 0 1" /> for all three joints;

• It is to me not clear how it gets the following axis rotations:

  <axis xyz="-0.9 0.15 0"/> <axis xyz="-0.707 0.707 0"/> <axis xyz="0.707 -0.707 0"/>

which is the reference frame? How should I get those values?

• It seems to me that there is no "world frame" in the example. Does it mean, that the first link is always considered fixed and unmovable?

Pleae help me to understand the example, because I searched in internet but I was not able to find anything better the ROS' Turorials

Thanx in advance