How to read an array from a ROS1 launch file into a C++ vector variable

I was having problems reading an array of doubles in a ROS launch file into a vector variable in a C++ ROS1 program node. After some digging around, I found I was doing it the wrong way; instead of using the <param> tag in the ROS launch file, I should be using the <rosparam> tag. 

The example ROS launch file listing shows the correct way to enter an array, e.g. [0.01, 0.1, 0.2] with the <rosparam> tag:

?xml version="1.0" encoding="UTF-8"?>
<launch>
  <node pkg="learning_tutorial" type="my_node" name="my_node">
        <param name="my_string_param" value="hello" />
        <rosparam param="my_array_param">[0.01, 0.1, 0.2]</rosparam>
  </node>
</launch>

Then in the ROS C++ code, I could do the following to read the array:

// ...etc...

using namespace ros;
using namespace std;

vector<double> myArrayParam;
NodeHandle nh;

// Read the my_array_param from the launch file into the myArrayParam variable
nh.param<vector<double>> ( "my_array_param", myArrayParam, { 1, 2, 3});

// Just print out the array parameter
ROS_INFO ( "My array: %f, %f, %f", myArrayParam[0], myArrayParam[1], myArrayParam[2]);
 
// ...etc...

Hope this helps somebody.

Using GIMP to copy an image layer into a channel

In GIMP, it is possible to copy a colored image layer into a single grayscale channel. Here's how to do the job.

Create an empty channel

1. In GIMP, open up the image file.
   
   
   
   
2. Click the Channels tab on the bottom right.
   
   
   
   
3. Then click the Create a new channel icon.
   
   The New Channel dialog box appears.
   
   
   
4. Optional. In the Channel name field, type in a name, e.g. flats. In the Fill opacity field, slide to 50.0.
    
   
5. Click OK.
   
   A new channel is created.
   
   

Copy and paste the colored image into the channel

1. Click the Layers tab. Click the image layer, e.g. bored-tiger.jpg to make sure it is active.
   
   
   
   
   
2. In the menu, choose Select | All. Then choose Edit | Copy.
   
   The layer is copied to the clipboard.
   
   
3. Click the Channels tab. Click the previously created channel, e.g. flats.
   
   
   
   
4. In the menu, select Edit | Paste in Place. 
   
   
   
   
5. Then click the Layers tab.
   
   A Floating Selection (Pasted Layer) is visible in the Layers list.
   
   
   
   
6. Click the green Anchor the floating layer icon.
   
   The pasted layer is added to the active channel.
   
   
7. Optional. Click the Channels tab.
   
   Note the channel thumbnail has been updated with a grayscale image of the pasted layer.