The current answer is incomplete. Installing `libv4l-dev` creates a `/usr/include/linux/videodev2.h` but doesn&#39;t solve the stated problem of not being able to find `linux/videodev.h`. The library does ship header files for compatibility, but fails to put them where applications will look for them.

    sudo apt-get install libv4l-dev
    cd /usr/include/linux
    sudo ln -s ../libv4l1-videodev.h videodev.h

This provides a `linux/videodev.h`, and of the right version (1).

    sudo apt-get install libv4l-dev

**Editing for RH based systems** :

On a Fedora 16 to install pygame 1.9.1 (in a virtualenv):

    sudo yum install libv4l-devel
    sudo ln -s /usr/include/libv4l1-videodev.h   /usr/include/linux/videodev.h 


v4l support has been dropped in recent kernel versions (including the one shipped with Ubuntu 11.04).

**EDIT**: Your question is connected to a [recent message][1] that was sent to the OpenCV users group, which has instructions to compile OpenCV 2.2 in Ubuntu 11.04. Your approach is *not ideal*.


  [1]: http://tech.groups.yahoo.com/group/OpenCV/message/79758

The patch is here: https://code.ros.org/trac/opencv/attachment/ticket/862/OpenCV-2.2-nov4l1.patch

By adding `#ifdef HAVE_CAMV4L` around

    #include &lt;linux/videodev.h&gt;
in `OpenCV-2.2.0/modules/highgui/src/cap_v4l.cpp` and removing `|| defined (HAVE_CAMV4L2)` from line `174` allowed me to compile.

for CMake remove/disable `with_libv4l` `with_v4l` variables if you do not need this lib.

I am learning Scala now and I want to write some silly little app like a console Twitter client, or whatever. The question is, how to structure application on disk and logically. I know python, and there I would just create some files with classes and then import them in the main module like `import util.ssh` or `from tweets import Retweet` (strongly hoping you wouldn&#39;t mind that names, they are just for reference). But how **should** I do this stuff using Scala? Also, I have not much experience with JVM and Java, so I am a complete newbie here.

Scala application structure

I&#39;ve been experimenting with a Chrome Extension, and I want to make one similar to my Google Voice Extension where the icon shows a little blue &quot;1&quot; next to the icon when I receive an event, Is this a whole separate icon? and then they just use the &quot;setIcon&quot; method? Like this

    chrome.browserAction.setIcon({path:&quot;icon.png&quot;}); 

Here&#39;s a visual example of how it looks:
![enter image description here][1]


The gmail one seems to go all the up to 500! They can&#39;t have an icon for each number, or do they?? Is there something I can use thats made to do this stuff? I already have my icon, if I have to make a unique one I may just make like 10 and then have a &quot;10+&quot; icon. I am connecting it to an API which could have a lot of events.

Has anybody had this issue? How did they get around it?

Any advice or suggestions would help!

Thanks!


  [1]: http://i.stack.imgur.com/GwPl7.png

Google Chrome Extension Numbers on the Icon

I tried to install OpenCV2.2 on Ubuntu 11.04. But OpenCV compilation fails stating an error related to linux/videodev.h file. 
File available in /user/includes/linux is named videodev2.h.

    /home/user/OpenCV-2.2.0/modules/highgui/src/cap_v4l.cpp:217:28: fatal error:    linux/videodev.h: No such file or directory
    compilation terminated.
    make[2]: *** [modules/highgui/CMakeFiles/opencv_highgui.dir/src/cap_v4l.o] Error 1
    make[1]: *** [modules/highgui/CMakeFiles/opencv_highgui.dir/all] Error 2
    make: *** [all] Error 2

Is there a solution for this ?

thank you.


linux/videodev.h : no such file or directory  - OpenCV on ubuntu 11.04

I tried to install OpenCV2.2 on Ubuntu 11.04. But OpenCV compilation fails stating an error related to linux/videodev.h file.
File available in /user/includes/linux is named videodev2.h.
<pre><code class="hljs language-bash">/home/user/OpenCV-2.2.0/modules/highgui/src/cap_v4l.cpp:217:28: fatal error: linux/videodev.h: No such file or directory
compilation terminated.
make[2]: *** [modules/highgui/CMakeFiles/opencv_highgui.dir/src/cap_v4l.o] Error 1
make[1]: *** [modules/highgui/CMakeFiles/opencv_highgui.dir/all] Error 2
make: *** [all] Error 2
</code></pre>
Is there a solution for this ?
thank you.

Can lambda expressions be used *as* class template parameters?  (Note this is a very different question than [this one][1], which asks if a lambda expression itself can be templated.)

I&#39;m asking if you can do something like:

    template &lt;class Functor&gt; 
    struct Foo { };
    // ...
    Foo&lt;decltype([]()-&gt;void { })&gt; foo;

This would be useful in cases where, for example, a class template has various parameters like `equal_to` or something, which are usually implemented as one-liner functors.  For example, suppose I want to instantiate a hash table which uses my own custom equality comparison function.  I&#39;d like to be able to say something like:

    typedef std::unordered_map&lt;
      std::string,
      std::string,
      std::hash&lt;std::string&gt;,
      decltype([](const std::string&amp; s1, const std::string&amp; s2)-&gt;bool 
        { /* Custom implementation of equal_to */ })
      &gt; map_type;

But I tested this on GCC 4.4 and 4.6, and it doesn&#39;t work, apparently because the anonymous type created by a lambda expression doesn&#39;t have a default constructor.  (I recall a similar issue with `boost::bind`.)  Is there some reason the draft standard doesn&#39;t allow this, or am I wrong and it is allowed but GCC is just behind in their implementation?

  [1]: https://stackoverflow.com/questions/4124579/c0x-lambdas-with-template-parameters-closed



Lambda expressions as class template parameters

In all our c++ courses, all the teachers always put `using namespace std;` right after the `#include`s in their `.h` files. This seems to me to be dangerous since then by including that header in another program I will get the namespace imported into my program, maybe without realizing, intending or wanting it (header inclusion can be very deeply nested).

So my question is double: Am I right that `using namespace` should not be used in header files, and/or is there some way to undo it, something like:

    //header.h
    using namespace std {
    .
    .
    .
    }

One more question along the same lines: Should a header file `#include` all the headers that it&#39;s corresponding `.cpp` file needs, only those that are needed for the header definitions and let the `.cpp` file `#include` the rest, or none and declare everything it needs as `extern`?  
The reasoning behind the question is the same as above: I don&#39;t want surprises when including `.h` files.

Also, if I am right, is this a common mistake? I mean in real-world programming and in &quot;real&quot; projects out there.

Thank you.

&quot;using namespace&quot; in c++ headers

So I can do something like

    #ifdef MSVC
    //do compiler specific code here
    #endif

Is there a preprocessor define that is defined if the compiler is MSVC?

Is there is any difference between using a `std::tuple` and a data-only `struct`?

    typedef std::tuple&lt;int, double, bool&gt; foo_t;

    struct bar_t {
        int id;
        double value;
        bool dirty;
    }

From what I have found online, I found that there are two major differences: the `struct` is more readable, while the `tuple` has many generic functions that can be used.
Should there be any significant performance difference?
Also, is the data layout compatible with each other (interchangeably casted)?

C++ Tuple vs Struct

I know that you can use `const_cast` to cast a `const` to a non-`const`.

But what should you use if you want to cast non-`const` to `const`?

casting non const to const in c++

My program generates a series of windows using the following code:

    def display(img, name, fun):
        global clicked
        
        cv.NamedWindow(name, 1)
        cv.ShowImage(name, img)
        cv.SetMouseCallback(name, fun, img)
    
        while cv.WaitKey(33) == -1:
            if clicked == 1:
                clicked = 0
                cv.ShowImage(name, img)
            
        cv.DestroyWindow(name)

I press &quot;q&quot; within the gui window to close it. However, the code continues to the next call of the display function and displays a second gui window while not closing the first. I&#39;m using a Mac with OpenCV 2.1, running the program in Terminal. How can I close the gui windows? Thanks.

DestroyWindow does not close window on Mac using Python and OpenCV

What is the best way to detect the corners of an invoice/receipt/sheet-of-paper in a photo? This is to be used for subsequent perspective correction, before OCR.

My current approach has been:
-----------------

RGB &gt; Gray &gt; Canny Edge Detection with thresholding &gt; Dilate(1) &gt; Remove small objects(6) &gt; clear boarder objects &gt; pick larges blog based on Convex Area. &gt; [corner detection - Not implemented]

I can&#39;t help but think there must be a more robust &#39;intelligent&#39;/statistical approach to handle this type of segmentation. I don&#39;t have a lot of training examples, but I could probably get 100 images together.

Broader context:
-----------------
I&#39;m using matlab to prototype, and planning to implement the system in OpenCV and Tesserect-OCR. This is the first of a number of image processing problems I need to solve for this specific application. So I&#39;m looking to roll my own solution and re-familiarize myself with image processing algorithms. 

Here are some sample image that I&#39;d like the algorithm to handle: If you&#39;d like to take up the challenge the large images are at http://madteckhead.com/tmp 

[![case 1][1]][1]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0773_sml.jpg))&lt;/sub&gt;  

[![case 2][2]][2]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0774_sml.jpg))&lt;/sub&gt;  

[![case 3][3]][3]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0775_sml.jpg))&lt;/sub&gt;  

[![case 4][4]][4]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0776_sml.jpg))&lt;/sub&gt;  


In the best case this gives:
-----------------

[![case 1 - canny][5]][5]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0773_canny.jpg))&lt;/sub&gt;  

[![case 1 - post canny][6]][6]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0773_postcanny.jpg))&lt;/sub&gt;  

[![case 1 - largest blog][7]][7]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0773_blob.jpg))&lt;/sub&gt;  


However it fails easily on other cases: 
-----------------

[![case 2 - canny][8]][8]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0774_canny.jpg))&lt;/sub&gt;  

[![case 2 - post canny][9]][9]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0774_postcanny.jpg))&lt;/sub&gt;  

[![case 2 - largest blog][10]][10]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0774_blob.jpg))&lt;/sub&gt;  


Thanks in advance for all the great ideas! I love SO!

EDIT: Hough Transform Progress
-----------------
Q: What algorithm would cluster the hough lines to find corners?
Following advice from answers I was able to use the Hough Transform, pick lines, and filter them. My current approach is rather crude. I&#39;ve made the assumption the invoice will always be less than 15deg out of alignment with the image. I end up with reasonable results for lines if this is the case (see below). But am not entirely sure of a suitable algorithm to cluster the lines (or vote) to extrapolate for the corners. The Hough lines are not continuous. And in the noisy images, there can be parallel lines so some form or distance from line origin metrics are required. Any ideas?

![case 1][14]
[![case 2][11]][11]
[![case 3][12]][12]
[![case 4][13]][13]  
&lt;sub&gt;(source: [madteckhead.com](http://madteckhead.com/tmp/IMG_0776_hough.jpg))&lt;/sub&gt;  


 [1]: https://i.stack.imgur.com/z13MW.jpg
 [2]: https://i.stack.imgur.com/6GOPm.jpg
 [3]: https://i.stack.imgur.com/y6Nq8.jpg
 [4]: https://i.stack.imgur.com/Mzaud.jpg
 [5]: https://i.stack.imgur.com/817rw.jpg
 [6]: https://i.stack.imgur.com/NFior.jpg
 [7]: https://i.stack.imgur.com/suaqb.jpg
 [8]: https://i.stack.imgur.com/nKNwp.jpg
 [9]: https://i.stack.imgur.com/ZqznM.jpg
 [10]: https://i.stack.imgur.com/X8hba.jpg
 [11]: https://i.stack.imgur.com/NYGGm.jpg
 [12]: https://i.stack.imgur.com/toesV.jpg
 [13]: https://i.stack.imgur.com/Cyq6f.jpg
 [14]: https://web.archive.org/web/20130913020727/http://madteckhead.com/tmp/IMG_0773_hough.jpg

Algorithm to detect corners of paper sheet in photo

Does anyone know the difference between feature detection and descriptor extraction in OpenCV 2.3? 

I understand that the latter is required for matching using DescriptorMatcher. If that&#39;s the case, what is FeatureDetection used for?

What is the difference between feature detection and descriptor extraction?

I am building a project with OpenCV in C#. It requires a dll file called `cvextern.dll`. but, when adding this file as a reference, this message appears :-

 **a reference &quot;cvextern.dll&quot; can&#39;t be added, Please make sure that the file is accessible and that it is a valid assembly or COM component.**

I get tired from searching, I spent the past 2 days in searching for a solution for that problem 

How do I resolve &quot;Please make sure that the file is accessible and that it is a valid assembly or COM component&quot;?

I&#180;m trying to find the corners on a image, I don&#180;t need the contours, only the 4 corners. I will change the perspective using 4 corners.

I&#180;m using Opencv, but I need to know the steps to find the corners and what function I will use.

My images will be like this:(without red points, I will paint the points after)
![enter image description here][1]


EDITED:

After suggested steps, I writed the code: (Note: I&#180;m not using pure OpenCv, I&#180;m using javaCV, but the logic it&#180;s the same).

	// Load two images and allocate other structures (I&#180;m using other image)
        IplImage colored = cvLoadImage(
                &quot;res/scanteste.jpg&quot;,
                CV_LOAD_IMAGE_UNCHANGED);

![enter image description here][2]

        IplImage gray = cvCreateImage(cvGetSize(colored), IPL_DEPTH_8U, 1);
        IplImage smooth = cvCreateImage(cvGetSize(colored), IPL_DEPTH_8U, 1);

        //Step 1 - Convert from RGB to grayscale (cvCvtColor)
        cvCvtColor(colored, gray, CV_RGB2GRAY);


![enter image description here][3]


       
        //2 Smooth (cvSmooth)
        cvSmooth( gray, smooth, CV_BLUR, 9, 9, 2, 2); 
  
![enter image description here][4]

        //3 - cvThreshold  - What values?
        cvThreshold(gray,gray, 155, 255, CV_THRESH_BINARY);

![enter image description here][5]

        //4 - Detect edges (cvCanny) -What values?
    	int N = 7;
    	int aperature_size = N;
    	double lowThresh = 20;
    	double highThresh = 40;    	
        cvCanny( gray, gray, lowThresh*N*N, highThresh*N*N, aperature_size );	




![enter image description here][6]


        //5 - Find contours (cvFindContours)
        int total = 0;
        CvSeq contour2 = new CvSeq(null);
        CvMemStorage storage2 = cvCreateMemStorage(0);
        CvMemStorage storageHull = cvCreateMemStorage(0);
        total = cvFindContours(gray, storage2, contour2, Loader.sizeof(CvContour.class), CV_RETR_CCOMP, CV_CHAIN_APPROX_NONE);
        if(total &gt; 1){
              while (contour2 != null &amp;&amp; !contour2.isNull()) {
                  if (contour2.elem_size() &gt; 0) {
                	//6 - Approximate contours with linear features (cvApproxPoly)
                      CvSeq points = cvApproxPoly(contour2,Loader.sizeof(CvContour.class), storage2, CV_POLY_APPROX_DP,cvContourPerimeter(contour2)*0.005, 0);
                      cvDrawContours(gray, points,CvScalar.BLUE, CvScalar.BLUE, -1, 1, CV_AA);

                  }
                  contour2 = contour2.h_next();
              }

        } 

![enter image description here][7]


So, I want to find the cornes, but I don&#180;t know how to use corners function like cvCornerHarris and others.


  [1]: http://i.stack.imgur.com/9CtnW.jpg
  [2]: http://i.stack.imgur.com/ag0Sa.jpg
  [3]: http://i.stack.imgur.com/xCFht.png
  [4]: http://i.stack.imgur.com/sgBAi.png
  [5]: http://i.stack.imgur.com/79WA4.png
  [6]: http://i.stack.imgur.com/01VZ0.png
  [7]: http://i.stack.imgur.com/Zj35W.png

How to find corners on a Image using OpenCv

I&#39;m trying to install the Rails environments on Ubuntu 11.04. When I launch the command `rvm install 1.9.2 --with-openssl-dir=/usr/local` the following error is received:

    curl : (1) Protocol https not supported or disabled in libcurl

How can this be resolved?



curl : (1) Protocol https not supported or disabled in libcurl

I am trying to edit sources.list using vi editor but getting the following error while saving the file:

    /etc/apt/sources.list&quot; E212: Can&#39;t open file for writing

/etc/apt/sources.list&quot; E212: Can&#39;t open file for writing

I want to run some Java programs in the background when the system boots in Ubuntu. I have tried to add a script in /etc/init.d directory but failed to start a program. i.e programs are not started. What should I do for that?

How to run a script at the start up of Ubuntu?

I&#39;m trying to install Hadoop on Ubuntu 11.10. I set the `JAVA_HOME` variable in the file `conf/hadoop-env.sh` to: 

    # export JAVA_HOME=/usr/lib/jvm/java-1.6.0-openjdk

and then I execute these commands (Standalone Operation):

    $ mkdir input 
    $ cp conf/*.xml input 
    $ bin/hadoop jar hadoop-examples-*.jar grep input output &#39;dfs[a-z.]+&#39; 
    $ cat output/*

but I have the following error when executing the third command:

&gt; ERROR : JAVA_HOME is not set

Is the `JAVA_HOME` variable not set correctly?

Hadoop: &#171;ERROR : JAVA_HOME is not set&#187;

One of the problems, I face with supervisord is that when I have a command which in turn spawns another process, supervisord is not able to kill it. 

For example I have a java process which when runs normally is like

     $ zkServer.sh start-foreground
     $ ps -eaf | grep zk
     user 30404 28280  0 09:21 pts/2    00:00:00 bash zkServer.sh start-foreground
     user 30413 30404 76 09:21 pts/2    00:00:10 java -Dzookeeper.something..something

The supervisord config file looks like:

    [program:zookeeper]
    command=zkServer.sh start-foreground
    autorestart=true
    stopsignal=KILL

These kind of processes which have multiple childs are not well handled by supervisord when it comes to stopping them from `supervisorctl`. So when I run this from the supervisord and try to stop it from supervisorctl, only the top level process gets killed but not the actual java process.


Content Type	Original Author	Original Content on Stackoverflow
Question	Niroshan	View Question on Stackoverflow
Solution 1 - C++	bukzor	View Answer on Stackoverflow
Solution 2 - C++	Alberto	View Answer on Stackoverflow
Solution 3 - C++	karlphillip	View Answer on Stackoverflow
Solution 4 - C++	sujit	View Answer on Stackoverflow
Solution 5 - C++	Vyacheslav	View Answer on Stackoverflow

linux/videodev.h : no such file or directory - OpenCV on ubuntu 11.04

C++ Problem Overview

C++ Solutions

Solution 1 - C++

Solution 2 - C++

Solution 3 - C++

Solution 4 - C++

Solution 5 - C++

Google Chrome Extension Numbers on the Icon

Scala application structure

Attributions