Monday, December 12, 2022

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"?>
  <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>

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.

Monday, December 5, 2022

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.

Monday, November 28, 2022

Using GIMP's color channels to remove blue guide lines from inked line art

Penciled comic book art typically have blue guide lines (and text) as shown in the screen shot below. 

The sketch can be downloaded from this site if you want to practice with it. There are a number of ways to remove the blue lines from the image. I will be using the Color channels panel to remove the guide lines.

The steps:

Remove pixels from the Red and Green channels

  1. In GIMP, open up the image. Click the Channels tab on the bottom right.

  2. Click the Blue channel to deselect it.

  3. In the Tool palette, click the Fill icon.

  4. Toggle on the FG color fill. Make sure the foreground color is black since we want to remove the Red and Green channels data.

  5. Then, click anywhere on the canvas.

    The background becomes blue.

Create the line art in the Red and Green channels

  1.  In the Channels tab, press the mouse right click button on the Blue channel.

    A popup menu appears.

  2. Choose Channel to Selection.

    The blue channel's non-zero pixels are selected.

  3. In the Tool box, click the Fill icon. Toggle on the BG color fill option. (Note: the background color should be white). Then click anywhere in the selection on the canvas.

    The background becomes white.

  4. In the Channels tab, click the Blue channel to select it again.  Then in the menu, choose Select | None to clear the selection.

Use Threshold to clean up the line work

  1. In the menu, select Colors | Threshold.

    The Threshold dialog box appears.

  2.  Drag the black triangle until you are satisfied with the contrast between the line work and the background.

  3. Save your work.

Monday, November 7, 2022

Shell script to batch bulk convert *.flac files to *.mp3

I have many music files in flac format and I wanted to convert them to a more compressed mp3 format with ffmpeg on Ubuntu so I can upload them to a storage limited portable music player. To ease the conversion task, I decided to write this simple shell script to do the job. In brief, the script will do the following:

  • find all the files with the extension .flac in the current directory
  • replace the file name extension .flac with the .mp3 extension
  • create a temporary script that calls the ffmpeg command to convert
  • run the temporary script

The listing of the shell script is shown below.  

# Define the internal field separator as a newline

# Find all the *.flac files in the current directory and perform the conversion
for f in `find . -name "*.flac" `;
	# Use the input flac file name prefix and replace the .flac extension with a .mp3 extension
	f=$(echo $f | cut -c 3-)
	outfile=$(basename $f .flac)
	echo "Convert $f->$outfile..."
	# Form the ffmpeg command to convert the input flac file to mp3
	cmd="ffmpeg -hide_banner -i \"$f\" -ab 320k -map_metadata 0 -id3v2_version 3 \"$outfile\" "
	# Create a temporary shell script for running the conversion
	echo $cmd > /tmp/
	# Run the conversion to mp3
	bash /tmp/
	# Clean up
	rm /tmp/

To use this shell script, you can do the following:

  1. Save the code listing above to a file e.g. in a directory, e.g. /path/to

  2. Open up a Linux Terminal.

  3. In the Terminal, type in the command to change directory to the location of the flac files, e.g. /path/to/music/

    $ cd /path/to/music

  4. At the prompt, type in the command to run the shell script.

    $ bash /path/to/

    The flac files are converted to mp3 files.

Monday, October 31, 2022

Simple C++ example to send serial AT commands to and receive data from a modem

I tried using many C/C++ libraries trying to coax a 5G modem to respond to my input AT commands for a long time but I was not successful. The command I was trying to send was the Quectel modem command to query for PDN channels:


After a while, I figured out I had to simulate a keyboard Enter press in code to actually tell the modem the command is complete. So all I had to do was append the carriage return (\r) and new line (\n) characters to the AT command string, e.g:

string cmd = "AT+CGDCONT?\r\n";

A working C++ code example is shown below:

#include <string>
#include <iostream>
#include <cstdio>
#include <unistd.h>

// Using header only library from 
#include "Serial.h"

using namespace std;

int main ( int argc, char** argv) {

        string commPort = "/dev/ttyUSB2";
        unsigned int baud = 115200;
        serial::Serial serial; ( commPort, baud);

        if ( !serial.isOpen()) {
                cout << "comm port is not open" << endl;
                return 1;

        // A modem AT query command
        string cmd = "AT+CGDCONT?\r\n";
        vector<uint8_t> cmdVec (cmd.begin(), cmd.end());

        // send command to the modem
        size_t bytesSent = serial.transmitAsync(cmdVec);
        cout << "Bytes sent " << bytesSent << endl;

        int received_bytes = -1;

        while (received_bytes != 0 ) {
                // read one byte from the modem and timeout if the
                // there is no response in more than 1 sec.
                future<vector<uint8_t>> future = serial.receiveAsync(1, 1000);
                vector<uint8_t> const received_data = future.get();
                received_bytes = received_data.size();

                string str(received_data.begin(), received_data.end());
                cout << "[" << received_data[0] << "] " << endl;
        // Close the serial port

        cout << "End of process" << endl;

        return 0;


The example prints out the data sent by the modem to the calling program, as shown below:

Note: this example is using the modern serial header only C++ library from this site:

Monday, October 24, 2022

QGIS unable to edit SpatiaLite database layer workaround

Recently I upgraded to QGIS 3.26 on Ubuntu 22.04 and found that I was unable to edit my existing SpatiaLite database layers, i.e. the Toggle Editing tool bar icon could not be enabled. The screenshot below illustrates the problem.

While I have not found the cause of the problem, I found a simple workaround to the issue: just create a new SpatiaLite database and import the layers from the old database. The newly imported database layers can be enabled for editing. 


Create a new SpatiaLite database

  1. In the Browser pane of QGIS, press the right mouse button on the SpatiaLite node.

    A pop up menu appear.

  2. Choose Create New Database.

  3. In the Name field, type in a new SpatiaLite database file name, e.g. new_rail.sqlite. Click Save.

    The database is created.

Import layers from the old SpatiaLite database to the new SpatiaLite database

  1. If the layer e.g. node from the old SpatiaLite database e.g. rail.sqlite is not displayed in QGIS, then add the layer to the map window.

  2. In the QGIS menu, select Database | DB Manager.

    The DB Manager dialog box appears.

  3. Expand the SpatiaLite Providers tree node and double click on the newly created SpatiaLite database, e.g. new_rail.sqlite.

  4. Click Import Layer/File.

    The Import vector layer dialog box appears.

  5. In the Input drop down list, choose the layer from the old database, e.g. nodes. In the Output Table field, type in a new table name, e.g. nodes. Click OK.

    The old layer is imported into the new database.

Displaying the newly imported Spatialite layer

  1. In QGIS, select Layer | Add Layer | Add SpatiaLite Layer.

  2. In the Connections drop down list, select the new SpatiaLite database, e.g. new_rail.sqlite. Click Connect.

  3. In the table list, select the new layer nodes. Click Add.

    The new nodes layer is displayed in the map window.

  4. Select the new layer nodes in the Legend pane.

    The Toggle Editing icon is enabled now.

Monday, October 17, 2022

Auto mount an SD card upon insert on Ubuntu Server

I have a headless Raspberry Pi board running Ubuntu 20.04.x server with a USB SD card reader. I wanted the system to automatically mount an SD card to a fixed mount point, e.g. /media/ubuntu/sdcard/ upon card insertion. By default, Ubuntu Server doesn't mount the SD card so I had to do some setup and configuration, as illustrate in the steps below. I had to install some prerequisite software package (udevil) and create a systemd service.

Install udevil package and create mount directory

  1. Open up a Terminal and type in the following command to install udevil.

    $ sudo apt install udevil

  2. Then, create a SD card directory mount point, e.g. /media/ubuntu/sdcard/ with the following command:

    $ mkdir -p /media/ubuntu/sdcard


Add a mount rule to the fstab file

  1. Using a text editor, e.g. vi, open up the system file /etc/fstab.

    $ sudo vi /etc/fstab

  2. Append the following line:

    /dev/sda1 /media/ubuntu/sdcard auto rw,user,exec,umask=000  0  2

    An example fstab file is shown below:
LABEL=writable  /        ext4   defaults        0 1
LABEL=system-boot       /boot/firmware  vfat    defaults        0       1
/dev/sda1       /media/ubuntu/sdcard auto rw,user,exec,umask=000        0       2

Create the devmon Systemd service

  1. In the Terminal, change directory to /etc/systemd/system/.

    $ cd /etc/systemd/system

  2. Using a text editor, e.g. vi, create a file e.g. devmon.service. Enter the following and save the file.

    Description=Systemd service for running devmon

  3. In the Terminal, type in the following to generate the service:

    $ sudo systemctl daemon-reload
    $ sudo systemctl enable devmon

  4. Then either reboot the board or run the following command to start the service:

    $ sudo systemctl start devmon

 Monitor the service

  1. In a Terminal, type in the following to monitor the newly create devmon service when an SD card is inserted or unmounted.

    $ journalctl -u devmon -f

 Note: The SD card should be unmounted properly using the following command(s):

$ udevil umount /media/ubuntu/sdcard

- or -

$ sudo umount /media/ubuntu/sdcard

Monday, September 19, 2022

Run Quectel's Connection Manager as a systemd service on Ubuntu

I found myself in possession of a modem by Quectel ( to hook up to a Raspberry Pi running Ubuntu 20.04. I wanted to auto run Quectel's connection manager executable quectel-CM on system start up but found limited information available online from Quectel. So I had to roll up my sleeves and make my own systemd service for that purpose.

This post outlines the steps I took:

Download and compile the quectel-CM executable

  1. Using an Internet browser, download the latest Quectel LTE, 5G Linux USB driver, e.g. from
  2.  Extract into a folder e.g. /home/ubuntu/Downloads/quectel-CM/.

  3. In a Terminal, change directory to the extracted folder.

    $ cd ~/Downloads/quectel-CM/

  4. Use the make command to compile the connection manager executable.

    $ make

    The quectel-CM executable is compiled.

Place the quectel-CM exe to the run time directory

  1. In a Terminal, type in the following commands to change directory to the extracted directory.

    $ cd /home/ubuntu/Downloads/quectel-CM/

  2. Copy the executable to /usr/local/bin/.

    $ sudo cp quectel-CM /usr/local/bin

Create a systemd service file

  1.  In the directory /etc/systemd/system, use a text editor to create a service file e.g. quectelcm.service.

    $ sudo vi /etc/systemd/system/quectelcm.service

  2. Type in the following lines, save and exit the file:
    Description=Systemd service for running Quectel's Connection Manager quectel-CM executable

  3. Give the service file the following executable permission:

    $ sudo chmod 664 /etc/systemd/system/quectelcm.service

 Create the service

  1. In the Terminal, type in the following commands:

    $ sudo systemctl daemon-reload
    $ sudo systemctl enable quectelcm

    The quectelcm service is created.


Running and monitoring the service

  1. You can either reboot the Raspberry Pi or type in the following command to start the service in a Terminal.

    $ sudo systemctl start quectelcm

  2. To monitor the quectelcm service, use the journalctl command:

    $ journalctl -u quectelcm -f

Note: In order for quectel-CM to request and set the machine's network address, the software prerequisites net-tools and udhcpc must be installed on the Raspberry Pi.

Monday, July 11, 2022

Blender: how to make an edge flush with another edge by snapping and scaling

Making an plane edge geometry flush with another plane geometry is a typical CAD task. In Blender 3D, it is possible to do the same using simple Move and Scale commands, but with the right settings. 

The screenshot below shows 2 plane objects in the top view. This post shows the steps to adjust the bottom plane to snap nicely to the plane on the left.

Set to Snap to Vertex

  1.  In the Blender 3D Viewport menu bar, click the Snap icon and choose Vertex as shown below.

Snap the vertex of one plane to the vertex of the other plane

  1. In the Blender 3D Viewport, make sure the Edit Mode is enabled and Select Mode is Vertex.

  2. In the Blender Toolbar, select the Move icon. Then click on the bottom vertex of the plane to be moved.

  3. On the keyboard, press down CTRL and drag the bottom vertex to snap to the vertex of the other plane.


 Scale the edge vertices by axis to zero

  1. In the Blender 3D Viewport menu, set the Transform Pivot Point to Active Element as shown below.

  2. On the keyboard, press ALT-a to deselect all selection.

  3. In the Toolbar, click the Select Box icon.

  4. Click the top vertex of the plane to be scaled.

  5. Press SHIFT and click the bottom vertex.

    Note: since the Transform Pivot Point is set to Active Element, the scaling will be relative to the last selected vertex.

  6. Press S.

    The plane scales dynamically around the last selected vertex as the cursor is moved.

  7. Limit the direction of scaling to the X-axis (in this example) which is perpendicular to the matching edges. Press X.

    The plane can only scale along the X axis.

  8. Press 0.

    The planes are snapped nicely to one another.

Monday, June 27, 2022

WebApp for Singapore Weather forecasts rewritten using Flutter

I thought the old Singapore Weather Nowcast WebApp at this site written using the AngularJS framework and Bootstrap was a good candidate for a Flutter app. So I rolled up the sleeves and rewrote it using Flutter. Now it has Material design and this is how it looks and works.

4 day outlooks

For the Flutter version, I included additional data from the National Environment Agency (NEA)'s 4 day outlooks with temperature, wind and humidity values.

2 hour nowcasts

The 2 hour weather nowcast is now displayed in a responsive 2 pane display.

On small screens, a tab bar will appear displaying the nowcast list and map on separate tabs. 


24 hour forecasts

The new 24 hour forecasts are separated by 3 tabs as shown in the screenshot below. Clicking each tab will show the forecast for that time period. And like the 2 hour nowcast display, the 24 hour forecasts display is also responsive to screen sizes.

To try out the new Flutter WebApp, visit this web site:

Monday, May 16, 2022

Convert Geoids from BYN to GTX format with this WebApp

This WebApp came about because I wanted to use some Geoid files for some vertical datum corrections with Proj (which uses .gtx format) but could only find Geoid files in Natural Resources Canada's .byn format.

  1. To convert Geoid BYN files to GTX, open up a browser to the following url:

    The WebApp is loaded.

  2. Click the Add File button.

    The File Upload dialog appears.

  3. Browse and select a .byn file e.g. EGM96.byn. Click Open.

    The Geoid file is displayed in a list and the Convert button is enabled.
    Note: the Geoid .byn is loaded locally to the browser and not transferred to some server on the Internet.

  4. Click the Convert button.

    The Geoid file is converted into .gtx format and the Process log and Save As icons are enabled.

  5. Optional. To view the process log, click the Show Process Log icon.

    The process log messages are displayed.

  6. To save the converted .gtx Geoid locally, click the Save Converted File icon.

    The Geoid file is saved.

  7. Optional. If necessary, you can use a GIS software like QGIS to display the converted .gtx file, as shown below.