Urdf rgba displays strangely in rviz

edit

I am doing the ROS urdf tutorials. Upon reaching the materials section I noticed that the alpha for rgba materials property is acting strange. For me if a=1.0 it is 50% transparent and if a=2.0 it is no longer transparent. This is bizarre to me as logically 0->1 should be the transparency range as with the rgb portion of rgba.

Below is the urdf I used and a screenshot of the display on rviz. Anyone have an explanation?

<?xml version="1.0"?>
<robot name="myfirst">

  <material name="blue">
    <color rgba="0 0 0.8 2"/>
  </material>

  <material name="white">
    <color rgba="1 1 1 1"/>
  </material>

  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
      <material name="blue"/>
    </visual>
  </link>

  <link name ="right_leg">
    <visual>
      <geometry>
        <box size="0.6 0.1 0.2"/>
      </geometry>
      <origin rpy="0 1.57075 0" xyz="0 0 -0.3"/>
      <material name="white"/>
    </visual>
  </link>

  <joint name="base_to_right_leg" type="fixed">
    <parent link="base_link"/>
    <child link="right_leg"/>
    <origin xyz="0 -0.22 0.25"/>
  </joint>

  <link name ="left_leg">
    <visual>
      <geometry>
        <box size="0.6 0.1 0.2"/>
      </geometry>
      <origin rpy="0 1.57075 0" xyz="0 0 -0.3"/>
      <material name="white"/>
    </visual>
  </link>

  <joint name="base_to_left_leg" type="fixed">
    <parent link="base_link"/>
    <child link="left_leg"/>
    <origin xyz="0 0.22 0.25"/>
  </joint>

</robot>