Logitech, Inc. Webcam C270 on Raspberry Pi

Just a small note. I tried to get my Logitech, Inc. Webcam C270 running on an raspberry pi v1 running raspbian wheezy and ran into some problems.

The webcam can be used on linux with the uvcvideo kernel module but using uvccapture did not work for me.

$ dmesg
uvcvideo: Found UVC 1.00 device <unnamed> (046d:0825)
input: UVC Camera (046d:0825) as /devices/.../input/input0
usbcore: registered new interface driver uvcvideo

Running uvccapture:

$ uvccapture
ERROR opening V4L interface
: No such file or directory

Mplayer didn’t work either with various settings – producing green only images.

mplayer tv:// -tv driver=v4l2:device=/dev/video0 -vo png -frames 1

Then I found out there is a limited set of formats your camera can deliver and how to display:

$  v4l2-ctl --list-formats -d /dev/video0
        Index       : 0
        Type        : Video Capture
        Pixel Format: 'YUYV'
        Name        : YUV 4:2:2 (YUYV)

        Index       : 1
        Type        : Video Capture
        Pixel Format: 'MJPG' (compressed)
        Name        : MJPEG

Seems like I can capture images in YUYV Format and MJPEG directly. This led me to vgrabbj which seems to be a highly flexible image grabber. The following command works for me:

$  vgrabbj -q 100 -i vga -o jpg -f /tmp/snap.jpg -e -d /dev/video0

So, give vgrabbj a try.

Simple caldav4j example in java and clojure

I was trying to access a caldav calendar through java and clojure to build a small application. But it wasn’t too easy.

caldav4j seemed to be very confusing and not so well maintained. There is no quick start guide and you are directed to have a look at the unit tests. But they are a compilation of interconnected classes and their hierarchies including a lot of deprecated classes being used – pretty confusing for a newcomer.

I had also problems using the alternative ical4j and it’s caldav connector. After some debugging it seemed incompatible with zimbra because of some strange filter constraints.

This might all be due to the fact that caldav is a complex protocol and I accepted to dig deeper into caldav4j.

Enough of the yacking, I traced down the necessary wire to build a basic example and hope it’ll help others looking for a quick start.

Java: https://github.com/dedeibel/list-events-caldav4j-example

Clojure: https://github.com/dedeibel/list-events-caldav4j-example-clojure

I tested it with zimbra. For google calendar a little more work might be required. The key might be in the GoogleCalDavDialect.java class.


    HttpClient httpClient = new HttpClient();
    // I tried it with zimbra - but I had no luck using google calendar
    httpClient.getHostConfiguration().setHost("CALDAVHOST", 443, "https");
    String username = "username";
    UsernamePasswordCredentials httpCredentials = new UsernamePasswordCredentials(username, "secret");
    httpClient.getState().setCredentials(AuthScope.ANY, httpCredentials);
    // Need a proxy?
    //httpClient.getHostConfiguration().setProxy("phost", 8080);

    CalDAVCollection collection = new CalDAVCollection(
        "/dav/"+ username +"/Calendar",
        (HostConfiguration) httpClient.getHostConfiguration().clone(),
        new CalDAV4JMethodFactory(),

    GenerateQuery gq=new GenerateQuery();
    // TODO you might want to adjust the date
    gq.setFilter("VEVENT [20131001T000000Z;20131010T000000Z] : STATUS!=CANCELLED");
    // Get the raw caldav query
    // System.out.println("Query: "+ gq.prettyPrint());
    CalendarQuery calendarQuery = gq.generate();
    List<Calendar>calendars = collection.queryCalendars(httpClient, calendarQuery);

    for (Calendar calendar : calendars) {
      ComponentList componentList = calendar.getComponents().getComponents(Component.VEVENT);
      Iterator<VEvent> eventIterator = componentList.iterator();
      while (eventIterator.hasNext()) {
        VEvent ve = eventIterator.next();
        System.out.println("Event: "+ ve.toString());


defn- create-query []
  (let [gq (GenerateQuery.)]
    ; Date Format yyyyMMdd
    (.setFilter gq "VEVENT [20131001T000000Z;20131010T000000Z] : STATUS!=CANCELLED")
    (.generate gq)))

(defn -main []
  (let [http-client (initialize-http-client)
        collection-path (:collection-path configuration)
        host-configuration (.clone (.getHostConfiguration http-client))
        method-factory (CalDAV4JMethodFactory.)
        collection (CalDAVCollection. collection-path host-configuration method-factory CalDAVConstants/PROC_ID_DEFAULT)
        calendars (.queryCalendars collection http-client (create-query))
    (doseq [cal calendars
            event (-> cal (.getComponents) (.getComponents Component/VEVENT))]
      (println "Event: " event)
      (println "\n\n"))))

Clojure core.logic cheatsheets

I am currently having a look at clojure core.logic and was missing some kind of reference documentation. There are a lot of examples and tutorials but I never had an overview about what’s available.

In a google groups discussion the idea of a reference documentation like clojure cheat sheets came up. https://groups.google.com/forum/#!topic/clojure/ScJa_o4lP2k

So I decided to create one, view it directly: cheatsheet on github

Have a look at the github project: https://github.com/dedeibel/clojure-core-logic-cheatsheets

Thanks to the original author of the cheatsheet generator Michael Fogus and Andy Fingerhut.

Thanks goes to xavriley who created a cool version of clojuredocs for core.logic which is where the cheatsheet entries link to.

How to install oracle java 7 update 11 (jdk-7u11) on debian linux

Currently you can’t install the new java sdk 7 update 11 from oracle using java_package. Usually you can simply follow this instruction: Installing oracle jre / sdk on debian.

Unfortunately the error is: No matching plugin was found.

But you can modify the script /usr/share/java-package/oracle-j2sdk.sh to accept the new package:

<       "jdk-7u"[0-9]"-linux-i586.tar.gz") # SUPPORTED
<           j2se_version=1.7.0+update${archive_name:6:1}${revision}
>       "jdk-7u"[0-9][0-9]"-linux-i586.tar.gz") # SUPPORTED
>           j2se_version=1.7.0+update${archive_name:6:2}${revision}
<       "jdk-7u"[0-9]"-linux-x64.tar.gz") # SUPPORTED
<           j2se_version=1.7.0+update${archive_name:6:1}${revision}
>       "jdk-7u"[0-9][0-9]"-linux-x64.tar.gz") # SUPPORTED
>           j2se_version=1.7.0+update${archive_name:6:2}${revision}

Worked for me:

The Debian package has been created in the current directory. You can
install the package as root (e.g. dpkg -i oracle-j2sdk1.7_1.7.0+update11_amd64.deb).

(#:/tmp)- dpkg -i oracle-j2sdk1.7_1.7.0+update11_amd64.deb
(Reading database … 225711 files and directories currently installed.)
Preparing to replace oracle-j2sdk1.7 1.7.0+update7 (using oracle-j2sdk1.7_1.7.0+update11_amd64.deb) …
Unpacking replacement oracle-j2sdk1.7 …
Setting up oracle-j2sdk1.7 (1.7.0+update11) …

(#:/tmp)- java -version
java version „1.7.0_11“
Java(TM) SE Runtime Environment (build 1.7.0_11-b21)
Java HotSpot(TM) 64-Bit Server VM (build 23.6-b04, mixed mode)

There is also a bug report: Bug#698108: jdk7u11 not supported

Edit: also works for java 7 update 12 (jdk-7u12), java 7 update 13 (jdk-7u13), java 7 update 14 (jdk-7u14) and java 7 update 15 (jdk-7u15)

Edit: Oh boy, too many updates recently.

Created a shortcut cheat sheet for the clojure foreplay vim plugin

In Meikel Brandmeyer’s post „On the state of VimClojure“ he mentioned the alternative „foreplay“ to his vimclojure repl solution which is using ng-server.

I tried the plugin and liked it. It has a more robust feel and very easy setup – none required to be precise. You can simply start your nrepl using lein repl and the plugin automagically talks to it.

There are quite some useful shortcuts you should know, therefore I created a foreplay-cheat-sheet (PDF) which might be helpful at the beginning.

Maybe the more often required commands should be on top, but I guess you’ll learn those quite fast anyway.

I created it manually from the foreplay vim documentation, so the content is actually from Tim Pope. The office document is available here foreplay-cheat-sheet.odt


Simple xmpp standup meeting notification for the team

I guess you all know it – when it is standup time some notice it immediately while others are still on the phone or some still digging in the code. It does help to have a calendar notification but everyone has to enable it etc.

At my work we are all using the company wide jabber service and it’s a great tool I think. With these few steps you can build a simple notifier for your team.

First you’ll have to install sendxmpp. For debian:
% apt-get install sendxmpp

The usage is easy and you can write a simple script file and place it in your PATH.
% cat bin/standupnote


echo "Reminder: Standup Meeting! :-)" | sendxmpp $MEMBERS

Your credentials have to be added to added to the ~/.sendxmpprc file. Don’t forget to chmod 600 it!
echo "user@jabber.example.com password componentname" > ~/.sendxmpprc && chmod 600 ~/.sendxmpprc

And finally you should install a cronjob to run the script at standup time. Simply run as user and edit the config file:
% crontab -e
# m h dom mon dow command
45 10 * * 1,2,3,4,5 /home/fred/bin/standupnote

Yes, our meeting is not until 10:45 … :-\

Clojure store persistent data structures gotcha – load-file size limit

In clojure – the data structure and their default representation are suited as readable and portable format to persist data. But there is a small pitfall in using it.

Data Structure example:

  :request-time 2636,
  :status 200,
    "server" "Server",
    "content-encoding" "gzip",

Be aware that this function is intended to load code only. If your data structures or a string in them grow bigger than around 65,535 it crashes.

Exception similar to:

java.lang.ClassFormatError: Unknown constant tag 49 in class file parse$eval13

Please use read-string instead.

Example: (read-string (slurp "data.clj"))

Source: Rich Hickey on google groups Google Groups