Speeding up JPEG capture using the Raspberry Pi’s camera module and raspistill


The Raspberry Pi with camera module is nice way of generating image and video data. However, I don’t know if this is a problem for everyone but I was only getting 2fps max out of raspistill in timelapse mode, even with a much smaller interval set. After some investigation, it seemed to be related to shutter delay. There is a way of speeding this up using the code below:

        MMAL_PARAMETER_ZEROSHUTTERLAG_T shutter_config =
            { MMAL_PARAMETER_ZERO_SHUTTER_LAG, sizeof(shutter_config) },
            .zero_shutter_lag_mode = 1,
            .concurrent_capture = 1
        mmal_port_parameter_set(camera->control, &shutter_config.hdr);

Add this code segment into the create_camera_component() function. The result is more like 8fps with my moderately overclocked Pi. Better than 2fps at least!

Using the scp command to copy files between Linux systems

scp is very handy for moving files between Linux systems, especially from desktops to embedded systems and vice versa. However, the syntax is always a challenge. For example, to copy a file from a remote system:

scp <username>@<machine>:<full path to file> .

would copy the specified file to the local directory. So for example:

scp pi@ fred

would copy some file called file into the local directory as a file called fred. The order can be reversed to send the file the other way.

It’s the ‘:’ that causes all the trouble…

Using Qt on the Raspberry Pi running Raspian

The Raspberry Pi Raspbian distribution is a pretty nice way to go. I am currently working on some stuff using Qt and the Raspberry Pi camera and decided that the easiest thing was to get things going natively. Raspbian comes with Qt4.8.2 so the easy way to get started was to do:

sudo apt-get install libqt4-dev qtcreator

Unfortunately, QtCreator does not find the toolchain for some reason. This has to be configured by hand. Go to Tools > Options and select Build & Run. Then, select the Tool Chains tab and click on the Add button. Select the GCC option. This will add some stuff to the dialog. The compiler path should be set to /usr/bin/g++ and the debugger to /usr/bin/gdb. This should complete setting up the tool chain.

Another problem I had was that it wanted to do a remote deployment. To change this, click on the Projects icon in the toolbar on the left of the QtCreator window and select the tab Run Settings. The deployment method should be set to No deployment. Do this by clicking on the Add button and selecting Deploy configuration. Click the Add button for the Run configuration and select Custom executable. On the Executable: line below, enter the relative path to the executable.

Then everything should pretty much work!

Using Git and GitHub with Embedded Linux Systems

I often need to clone a private Git repo from GitHub onto an embedded Linux system for native compilation. The only problem is that, to clone a private repo from GitHub, there must be a public key registered for each system. To do this for every embedded system is a total pain. However, a Linux machine that has the repo on it already can act as a local server very easily.

On the embedded system, clone the repo using this command:

git clone ssh://user@

For example,

git clone ssh://richard@

would clone the repo veryinterestingrepo from the directory /home/richard on the machine at using user name richard. At some point Git will ask for the password for user richard. Once this is entered, the clone should work as expected. Pulls and pushes will work as normal as they use the remote from which the repo was cloned.

Grabbing a screenshot from an Android device via adb

This blog entry has a very neat way of grabbing screenshots from Android mobile devices. It boils down to a one liner (reproduced here just in case that blog vanishes for some reason and so it’s easy to find):

adb shell screencap -p | sed 's/\r$//' > screen.png

on Linux and

adb shell screencap -p | perl -pe 's/x0Dx0A/x0A/g' > screen.png

on the Mac.

Works perfectly. Thanks!

Finding PCM audio devices in C/C++ on the BeagleBone Black (and other embedded Linux systems)

Finding PCM audio devices using the ALSA interface from software isn’t always completely trivial. On some builds for the BeagleBone Black, PCM audio devices appear as /dev/audio, /dev/audio1 etc. But this doesn’t seem to be generally true. Plus, this doesn’t definitively identify which device is which. A good way to find out is to open the pseudo file /proc/asound/pcm as it lists all the PCM devices in the system. To see what’s currently active, just cat the pseudo file:

UbuntuDev$ cat /proc/asound/pcm
 00-00: ALC889 Analog : ALC889 Analog : playback 1 : capture 1
 00-01: ALC889 Digital : ALC889 Digital : playback 1
 00-02: ALC889 Analog : ALC889 Analog : capture 2
 00-03: HDMI 0 : HDMI 0 : playback 1
 01-00: USB Audio : USB Audio : capture 1
 02-03: HDMI 0 : HDMI 0 : playback 1
 02-07: HDMI 0 : HDMI 0 : playback 1
 02-08: HDMI 0 : HDMI 0 : playback 1
 02-09: HDMI 0 : HDMI 0 : playback 1

The first two numbers are the plughw card and device numbers. So, if you wanted to connect to the USB audio source for example, you could do:

snd_pcm_open (&m_handle, "plughw:1,0", SND_PCM_STREAM_CAPTURE, 0);