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OpenCV Beginners/Newbie Frequently Asked Questions

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OpenCV Beginners/Newbie Frequently Asked Questions

General Questions

How can I get acquainted with OpenCV fast?

Look at samples.
Look at some web guides
Using C/C++ interpreter Ch to run OpenCV
- http://www.softintegration.com/products/thirdparty/opencv/demos/
Within Visual Studio you may load OpenCV workspace:
- opencv.dsw for Microsoft Visual Studio 6.0
- opencv.sln for Miscrosoft Visual Studio .NET 2003
- cbuilderx/opencv.bpgr for Borland C++ BuilderX
- select cvsample project, build it and run. Look into the code and modify it as you wish.
Also, scan through reference manuals - they contain some example code as well.
Search OpenCV archives at http://groups.yahoo.com/group/OpenCV for the topic you are interesting in.
Create a new project from scratch or modify existing cvsample. There are application wizards for Microsoft Visual Studio that create OpenCV-aware projects; look for them at http://groups.yahoo.com/group/OpenCV (Files section - you have to be registered OpenCV@yahoogroups.com user)

Where do I submit Bug reports for the computer vision library?

Send email to OpenCV@yahoogroups.com Subject: BUG <....your title...>

How do I send bug reports on the Intel® Ingegrated Performance Primitives

Send email to developer_support@intel.com

How do I join the OpenCV forum?

Send email to OpenCV-subscribe@yahoogroups.com , after you are a member and select your logon, you can read the web group at http://groups.yahoo.com/group/OpenCV. This mailing list is mirrored at gmane: http://news.gmane.org/gmane.comp.lib.opencv so you can read it with a usenet client.

How do I modify the web group so that I don't receive tons of email's?

To get the messages real time, or once a day as a daily digest, you can go to http://groups.yahoo.com/mygroups and choose your setting from the pull down list to the right of OpenCV.

Ok, I found the group completely useless for me. How can I unsubscribe?

Mail to OpenCV-unsubscribe@yahoogroups.com with subject [OpenCV] and arbitrary message contents.

When using OpenCV and IPL simultaneously, I get compiler errors. How to resolve this problem?

To be completely independent from IPL, OpenCV duplicates declarations of IplImage and few other structures and constants if it is not told explicitly that IPL is present. Defining HAVE_IPL before including OpenCV headers or putting "#include <ipl.h>" before OpenCV headers resolves the conflict.

Does OpenCV work on other processors?

Yes, OpenCV itself is open source and it is quite portable, especially across 32-bit platforms. On the other hand, OpenCV can run much faster on Intel processors because of IPP.

Windows® OS related Qs:

When I try to build one of the apps, I get an error, streams.h not found.

You need DirectShow SDK that is a part of Microsoft Platform SDK (note: It is no longer found in the DirectX SDK).

Download DirectX SDK from msdn.microsoft.com/directx/ (It's huge, but you can download it by parts). If it doesn't work for you, consider HighGUI that can capture video using VFW or MIL
Install it TOGETHER WITH SAMPLES.
Open <DirectXSDKInstallFolder>\samples\C++\DirectShow\BaseClasses\baseclasses.{dsw|sln}. If there is no such file, it is that you either didn't install samples or the path has changed, in the latter case search for streams.h and open a workspace file located in the same folder.
Build the library in both Release in Debug configurations.
Copy the built libraries (in DirectX 9.x they are called strmbase.lib and strmbasd.lib) to <DirectXSDKInstallFolder>\lib.
In Developer Studio add the following paths:
- <DirectXSDKInstallFolder>\include <DirectXSDKInstallFolder>\samples\C++\DirectShow\BaseClasses to the includes' search path (at Tools->Options->Directories->Include files in case of Developer Studio 6.0) Add <DirectXSDKInstallFolder>\lib to the libraries' search path (at Tools->Options->Directories->Library files in case of Developer Studio 6.0) NOTE: PUT THE ADDED LINES ON THE VERY TOP OF THE LISTS, OTHERWISE YOU WILL STILL GET COMPILER OR LINKER ERRORS. This is necessary, because Developer Studio 6.0 includes some older DirectX headers and libs that conflict with new DirectX SDK versions.
- (This solution works prior to DirectX 9 SDK. Now BaseClasses is included in the Platform SDK and don't forget to add strmiids.lib to linker options, otherwise you will get some unresolved symbol errors)
Enjoy!

After installing DirectX SDK I'm still getting linker error about undefined or redefined "TransInPlace" filter class constructors etc.

Read the instructions from the previous answer, especially about the order of search directories.

When I use try to use cvcam, it just crashes

Make sure, you registered ProxyTrans.ax and SyncFilter.ax

How to register *.ax (DirectShow filter)?

Open the file (within explorer) using regsvr32.exe (under Win2000 it is done by Open with->Choose Program...->Browse...->c:\windows\system32\regsvr32.exe (path may be different). You may remember association to save clicks later.

Filter couldn't be registered (regsvr32 reports an error)

The most probable reason is that the filter requires some DLLs that are not in the path. In case of OpenCV make sure <OpenInstallFolder>\bin is in the path

cxcore096d.dll or cxcored.lib seem to be missing

cxcore096d.dll means debug version of cxcore DLL and cxcored.lib is the import library for cxcore096d.dll. Open OpenCV workspace, select "cxcore" as active project and select "Win32 Debug" configuration. Build the library and you will get bin\cxcore096d.dll and lib\cxcored.lib files. The same should be repeat for all the other OpenCV components - name of binary, ending with d means Debug version that are not shipped.

When compiling HighGUI I get the error message "mil.h is not found"

mil.h is a part of Matrox Imaging Library (MIL) that is usually supplied with Matrox (or compatible) framegrabbers, such as Meteor, Meteor II etc.

If you have such a framegrabber and MIL installed, add mil\include and mil\lib to the search paths within Developer Studio (submenu Tools->Options->Directories).
If you do not have MIL, just ignore the error. The file mil.h is only required to build MIL-aware version of Highgui "Win32 MIL Debug" or "Win32 MIL Release". Select "Win32 Debug" or "Win32 Release" configuration of highgui (submenu Build->Set Active Configuration...) instead - these versions of highgui can still be used to grab video via VFW interface, work with AVIs and still images.

How can I debug DirectShow filter?

Open workspace with the filter (e.g. opencv.dsw),
select the filter as active project and build it in debug configuration,
switch to explorer for a minute to register debug version of the filter (e.g. regsvr32 camshiftd.ax) (it needs to be done only when debug/release version are switched - not every time when filter is recompiled, because registry stores only the filter name),
get back to Developer Studio and start debugging session (F5). It will ask, what application do you want to run to debug the module. You may choose camshiftdemo to debug camshift.ax and DirectX SDK tool graphedit to debug arbitrary DirectShow filter.
Within graphedit build filter graph (e.g. camera->camshift->renderer)
Save the graph (you may just load it next time)
Set the breakpoint inside ::Transform method of the filter or in other location.
Run the filter and ... have fun

How can I create DeveloperStudio project to start playing with OpenCV

(note: this is a lengthy answer)

To create your own OpenCV-based project in Developer Studio from scratch do the following:

Within Developer Studio create new application:
1. select from menu "File"->"New..."->"Projects" tab. Choose "Win32 Application" or "Win32 console application" - the latter is the easier variant and the both sample projects have this type.
2. type the project name and choose location
3. you may create own workspace for the project ("Create new workspace") or include the new project into the currently loaded workspace ("Add to current workspace").
4. click "next" button
5. choose "An empty project", click "Finish", "OK". After the above steps done Developer Studio will create the project folder (by default it has the same name as the project), <project name>.dsp file and, optionally, <project name>.dsw,.ncb ... files if you create own workspace.
Add a file to the project:
1. select from menu "File"->"New..."->"Files" tab.
2. choose "C++ Source File", type file name and press "OK"
3. add OpenCV-related #include directives:
  - #include "cv.h" /* #inlcude "cvaux.h" // experimental stuff (if need) */ #include "highgui.h"
  - Or, you may copy some existing file (say, opencv\samples\c\morphology.c) to the project folder, open it and add to the project (right click in editor view -> "Insert File into Project" -> <your project name> ).
Customize project settings:
1. Activate project setting dialog by choosing menu item "Project"->"Settings...".
2. Select your project in the right pane.
3. Tune settings, common to both Release and Debug configurations:
  - Select "Settings For:"->"All Configurations"
  - Choose "C/C++" tab -> "Preprocessor" category -> "Additional Include Directories:". Add comma-separated relative (to the .dsp file) or absolute paths to opencv\cxcore\include, opencv\cv\include, opencv\otherlibs\highgui and, optionally, opencv\cvaux\include.
  - Choose "Link" tab -> "Input" category -> "Additional library path:". Add the paths to all neccessary import libraries (cxcore[d].lib cv[d].lib hihghui[d].lib cvaux[d].lib)
4. Tune settings for "Debug" configuration
  - Select "Settings For:"->"Win32 Debug".
  - Choose "Link" tab -> "General" category -> "Object/library modules". Add space-separated cvd.lib, highguid.lib, cvauxd.lib (optionally)
  - You may also want to change location and name of output file. For example, if you want the output .exe file to be put into the project folder, rather than Debug/ subfolder, you may type ./<exe-name>d.exe in "Link" tab -> "General" category -> "Output file name:".
5. Tune settings for "Release" configuration
  - Select "Settings For:"->"Win32 Release".
  - Choose "Link" tab -> "General" category -> "Object/library modules". Add space-separated cv.lib, highgui.lib, cvaux.lib (optionally)
  - Optionally, you may change name of the .exe file: type ./<exe-name>.exe in "Link" tab -> "General" category -> "Output file name:".
Add dependency projects into workspace:
1. Choose from menu: "Project" -> "Insert project into workspace".
2. Select opencv\cv\make\cv.dsp.
3. Do the same for opencv\cvaux\make\cvaux.dsp, opencv\otherlibs\highgui\highgui.dsp.
4. Set dependencies:
  - Choose from menu: "Project" -> "Dependencies..."
  - For "cv" choose "cxcore",
  - For "cvaux" choose "cv", "cxcore",
  - for "highgui" choose "cxcore",
  - for your project choose all: "cxcore", "cv", "cvaux", "highgui". The dependencies customization allows to automatically build debug versions of opencv libraries and rebuild the binaries if the sources are changed somehow.
That's it. Now compile and run everything.

Linux Related Qs:

TODO

Technical Questions on Library use:

How to access image pixels

(The coordinates are 0-based and counted from image origin, either top-left (img->origin=IPL_ORIGIN_TL) or bottom-left (img->origin=IPL_ORIGIN_BL)

Suppose, we have 8-bit 1-channel image I (IplImage* img):

I(x,y) ~ ((uchar*)(img->imageData + img->widthStep*y))[x]

Suppose, we have 8-bit 3-channel image I (IplImage* img):

I(x,y)blue ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3]
I(x,y)green ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3+1]
I(x,y)red ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3+2]

e.g. increasing brightness of point (100,100) by 30 can be done this way:

CvPoint pt = {100,100};
((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3] += 30;
((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3+1] += 30;
((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3+2] += 30;

or more efficiently

CvPoint pt = {100,100};
uchar* temp_ptr = &((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3];
temp_ptr[0] += 30;
temp_ptr[1] += 30;
temp_ptr[2] += 30;

Suppose, we have 32-bit floating point, 1-channel image I (IplImage* img):

I(x,y) ~ ((float*)(img->imageData + img->widthStep*y))[x]

Now, the general case: suppose, we have N-channel image of type T:

I(x,y)c ~ ((T*)(img->imageData + img->widthStep*y))[x*N + c] '' or you may use macro CV_IMAGE_ELEM( image_header, elemtype, y, x_Nc ) '' I(x,y)c ~ CV_IMAGE_ELEM( img, T, y, x*N + c )

There are functions that work with arbitrary (up to 4-channel) images and matrices (cvGet2D, cvSet2D), but they are pretty slow.

How to access matrix elements?

The technique is very similar. (In the samples below i - 0-based row index, j - 0-based column index)

Suppose, we have 32-bit floating point real matrix M (CvMat* mat):

M(i,j) = ((float*)(mat->data.ptr + mat->step*i))[j]

Suppose, we have 64-bit floating point complex matrix M (CvMat* mat):

Re M(i,j) = ((double*)(mat->data.ptr + mat->step*i))[j*2]
Im M(i,j) = ((double*)(mat->data.ptr + mat->step*i))[j*2+1]

For single-channel matrices there is a macro CV_MAT_ELEM( matrix, elemtype, row, col ), i.e. for 32-bit floating point real matrix

M(i,j) ~ CV_MAT_ELEM( mat, float, i, j )

e.g. here is initialization of 3x3 identity matrix:

CV_MAT_ELEM( mat, float, 0, 0 ) = 1.f;
CV_MAT_ELEM( mat, float, 0, 1 ) = 0.f;
CV_MAT_ELEM( mat, float, 0, 2 ) = 0.f;
CV_MAT_ELEM( mat, float, 1, 0 ) = 0.f;
CV_MAT_ELEM( mat, float, 1, 1 ) = 1.f;
CV_MAT_ELEM( mat, float, 1, 2 ) = 0.f;
CV_MAT_ELEM( mat, float, 2, 0 ) = 0.f;
CV_MAT_ELEM( mat, float, 2, 1 ) = 0.f;
CV_MAT_ELEM( mat, float, 2, 2 ) = 1.f;

How do I apply a normalized/modified histogram back to an image?

This is called "back project". See "CalcBackProject" in CvReference : cv_imgproc_histograms.

How to process my data with OpenCV

Suppose, you have 300x200 32-bit floating point image/array, that resides in 60000-element array.

int cols = 300, rows = 200;
float* myarr = new float[rows*cols];

// step 1) initializing CvMat header
CvMat mat = cvMat( rows, cols,
                   CV_32FC1, // 32-bit floating-point, single channel type
                   myarr // user data pointer (no data is copied)
                   );
// step 2) using cv functions, e.g. calculating l2 (Frobenius) norm
double norm = cvNorm( &mat, 0, CV_L2 );

...
delete myarr;

Other scenaria are described in the reference manual. See cvCreateMatHeader, cvInitMatHeader, cvCreateImageHeader, cvSetData etc.

How to load and display image

/* usage: prog <image_name> */
#include "cv.h"
#include "highgui.h"

int main( int argc, char** argv )
{
    IplImage* img;
    if( argc == 2 && (img = cvLoadImage( argv[1], 1)) != 0 )
    {
        cvNamedWindow( "Image view", 1 );
        cvShowImage( "Image view", img );
        cvWaitKey(0); // very important, contains event processing loop inside
        cvDestroyWindow( "Image view" );
        cvReleaseImage( &img );
        return 0;
    }
    return -1;
}

How to find and process contours

Look at squares demo : http://www710.univ-lyon1.fr/~bouakaz/OpenCV-0.9.5/samples/c/squares.c Note: You might recieve an error as "too many arguments to function ‘char* cvGetSeqElem(const CvSeq*, int)’" If so, just edit the file and on lines 120,121,122 and 133 you can find the function cvGetSeqElem;

just delete the third argument '0' whereever the function is used i.e. change (CvPoint*)cvGetSeqElem( result, i,0), to (CvPoint*)cvGetSeqElem( result, i).

How to use 2 cameras (multiple cameras) with cvCam library

First, init the cvcam library and get the number of cams by:

int ncams = cvcamGetCamerasCount( );    //returns the number of available cameras in the system

Show dialog to choose which cameras in use

int* out; int nselected = cvcamSelectCamera(&out);

Get the selected cams and enable them.

int cam1 = out[0]; 
int cam2 = out[1];

cvcamSetProperty(cam1, CVCAM_PROP_ENABLE, CVCAMTRUE); 
cvcamSetProperty(cam1, CVCAM_PROP_RENDER, CVCAMTRUE);  //We'll render stream from this source
cvNamedWindow("Cam1", 1); 
cvcamWindow MyWin1 = (cvcamWindow)cvGetWindowHandle("Cam1");
cvcamSetProperty(cam1, CVCAM_PROP_WINDOW, &MyWin1);   // Selects a window for  video rendering 

//Same code for camera 2
cvcamSetProperty(cam2, CVCAM_PROP_ENABLE, CVCAMTRUE); 
cvcamSetProperty(cam2, CVCAM_PROP_RENDER, CVCAMTRUE); 
cvNamedWindow("Cam2", 1); 
cvcamWindow MyWin2 = (cvcamWindow)cvGetWindowHandle("Cam2");
cvcamSetProperty(cam2, CVCAM_PROP_WINDOW, &MyWin1);   

//If you want to open the property dialog for setting the video format parameters, uncomment this line
//cvcamGetProperty(cam1, CVCAM_VIDEOFORMAT, NULL);
//cvcamGetProperty(cam2, CVCAM_VIDEOFORMAT, NULL);

Enable the stereo mode (2 cameras working at the same time)

cvcamSetProperty(cam1, CVCAM_STEREO_CALLBACK , stereocallback); //stereocallback is the function running to process every frames

cvcamInit(); 
cvcamStart();

//Your app is working 
while (1) 
{ 
     int key = cvWaitKey(5); 
     if (key == 27) break; 
}

cvcamStop( ); 
cvcamExit( );

Define the stereocallback function outside of the function above.

void stereocallback(IplImage* image1, IplImage* image2) {

//Process 2 images here

}

Getting started with Python

Where can I get help?

There are quite a few samples included in the opencv source tarball, you should read those.

I'm used to Matlab, is there a way to use matlab syntax?

Yes, there is a file matlab_syntax.py that comes with the opencv python bindings. It contains some of the most used matlab functions.

How do I convert an OpenCV image to a numpy array?

There is a file adaptors.py that helps you convert PIL images and Numpy arrays to and from OpenCV images. You can find it in the source tarball.

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