Xacro convert property to integer

edit

I'm using ROS Indigo and I want to create a robot with parametrizable camera resolution using just one urdf.xacro file. The camera width gets passed into the myrobot.urdf.xacro file via the prefix parameter when I spawn the robot:

<param name="robot_description" command="$(find xacro)/xacro.py '$(find simulator)/urdf/myrobot.urdf.xacro' prefix:=$(arg camera_resolution)" />

In the myrobot.urdf.xacro I then create the properties for a 4:3 aspect ratio:

<xacro:property name="cam_width" value="${arg prefix}" />
<xacro:property name="cam_height" value="${cam_width * 0.75}" />

Later I pass the values into the <sensor> field:

<sensor type="camera" name="camera_link_sensor">
  <update_rate>10.0</update_rate>
  <camera name="head">
    <horizontal_fov>${M_PI/2}</horizontal_fov>
    <image>
      <width>${cam_width}</width>
      <height>${cam_height}</height>
      <format>R8G8B8</format>
    </image>
...
  </camera>
    <plugin name="gazebo_ros_prosilica" filename="libgazebo_ros_camera.so">
...
    </plugin>
</sensor>

So far so good. Now when I input a camera_resolution parameter of 1024, the cam_width get's correctly set to 1024, but for the height I just get the default value with an error:

Error [Param.cc:181] Unable to set value [768.0] for key[height]

I investigated the error and it seems, the sensor field can't handle float values, but xacro automatically creates cam_height as float. If I manually insert the value 768.0 the same error message appears, but if I manually insert 768, the parameter get's set correctly.

Now to my question: is there a way to manually convert the cam_height property to an integer? I know with ROS Jade arbitrary python expressions will get supported by xacro, so I could use the floor() function, but is there any other way apart from switching to ROS Jade? I already tried stuff like cam_height / 1 without success.

Alternatively, maybe someone has a better idea how I could realize the parametrizable camera resolution.

Best regards, Malefitz