Saving an image in OpenCV

Question

I am new to OpenCV  and trying to capture an image  and then save it to a file  I am posting the code for your reference  below   The jpg file is being saved  but it is black      Capture the Image from the webcam CvCapture  pCapturedImage   cvCreateCameraCapture 0       Get the frame IplImage  pSaveImg   cvQueryFrame pCapturedImage       Save the frame into a file cvSaveImage  test jpg    pSaveImg      A JPG FILE IS BEING SAVED                                        OF 6KB   BUT IT IS BLACK   All of the functions are succesful  I have tried the above code in both XP and Vista - the result is a black image on both  Please let me know what I am missing out

User · Answer

I suggest you run OpenCV sanity check  Its a serie of small executables located in the bin directory of opencv   It will check if your camera is ok

User · Answer

If you use C    it is best to use C   interface   using namespace cv     Capture the Image from the webcam VideoCapture cap 0       Get the frame Mat save img  cap  gt  gt  save img   if save img empty        std  cerr  lt  lt   Something is wrong with the webcam  could not get frame    lt  lt  std  endl       Save the frame into a file imwrite  test jpg   save img      A JPG FILE IS BEING SAVED

User · Answer

From my experiences the first few frames that are captured when using   frame   cvQueryFrame  capture     Tend to be blank  You may want to wait a short while about 3 seconds  and then try to capture the image

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Sorry to bring up an old post  but I wanted to provide another answer for anyone that comes across this thread   I had the same problem  No matter what I did  the image just looked like it was complete black  I tried making multiple consecutive calls to cvQueryFrame and noticed that when I made 5 or more  I could see the image  So I started removing the calls one by one to see where the  breaking point  was  What I ended up finding was that the image got darker and darker as I removed each call  Making just a single call provided an image that was almost completely black  but if I looked very closely  I could make out my image   I tried 10 consecutive calls to test my theory  and sure enough  I was given a very bright image  considering that I m in a dimly lit room  Hopefully  this was the same problem you were encountering   I don t know much about imaging  but it looks like multiple consecutive calls to cvQueryFrame increases the length of exposure for the camera  This definitely fixes the problem  though it doesn t seem like the most elegant solution  I m going to see if I can find a parameter that will increase the exposure  or perhaps some other parameter that will brighten up my images   Good luck

User · Answer

SayntLewis is right  When you activate the camera with cvCaptureFromCAM  the auto white balance is not yet adjusted  it s a slow process   so you may get a mostly white or mostly black  depends on the camera and your lightning conditions  on the first frames  The same happens when there s a sudden change of the lightning  of the scene  Just wait some time after opening the camera  flush the buffer and you re ready to go

User · Answer

On OSX  saving video frames and still images only worked for me when I gave a full path to cvSaveImage   cvSaveImage   Users nicc image jpg  img

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hopefully this will save images form your webcam    include  lt opencv2 core core hpp gt   include  lt opencv2 highgui highgui hpp gt   include  lt opencv2 imgproc imgproc hpp gt   include  lt iostream gt   using namespace std  using namespace cv   int main        VideoCapture cap 0       Mat save img      cap  gt  gt  save img      char Esc   0      while  Esc    27  amp  amp  cap isOpened                  bool Frame   cap read save img               if   Frame    save img empty                     cout  lt  lt   error  frame not read from webcam n                 break                                                          namedWindow  save img   CV WINDOW NORMAL         imshow  imgOriginal   save img                   Esc   waitKey 1     imwrite  test jpg  save img

User · Answer

i think  simply camera not initialize in first frame  Try to save image after 10 frames

User · Answer

I use the following code to capture images   CvCapture  capture   cvCaptureFromCAM  CV CAP ANY    if  capture  error  char   No Capture    IplImage  img cvQueryFrame capture     I know this works for sure

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I had similar problem with my Microsoft WebCam  I looked in the image aquisition toolbox in Matlab and found that the maximum supported resolution is 640 480    I just changed the code in openCV and added  cvSetCaptureProperty capture  CV CAP PROP FRAME WIDTH  352    cvSetCaptureProperty capture  CV CAP PROP FRAME HEIGHT  288     before cvQueryFrame function which was the next supported resolution and changed skipped some initial frames before saving the image and finally got it working   I am sharing my working Code   include  cv h    include  highgui h    include  lt stdio h gt      using namespace cv  using namespace std   int main      CvCapture  capture   cvCaptureFromCAM  CV CAP ANY    cvSetCaptureProperty capture  CV CAP PROP FRAME WIDTH  352    cvSetCaptureProperty capture  CV CAP PROP FRAME HEIGHT  288        Get one frame IplImage  frame   for  int i   0  i  lt  25  i       frame   cvQueryFrame  capture        printf   Image captured  n         IplImage  RGB frame   frame    cvCvtColor frame RGB frame CV YCrCb2BGR     cvWaitKey 1000   cvSaveImage  test jpg   frame     cvSaveImage  cam jpg   RGB frame    printf   Image Saved  n        cvWaitKey 10       Release the capture device housekeeping cvReleaseCapture   amp capture      cvDestroyWindow   mywindow     return 0      My Suggestions    Dont grab frame with maximum resolution Skip some frames for correct camera initialisation

User · Answer

In my experience OpenCV writes a black image when SaveImage is given a matrix with bit depth different from 8 bit  In fact  this is sort of documented      Only 8-bit single-channel or 3-channel    with    BGR    channel order  images can   be saved using this function  If the   format  depth or channel order is   different  use cvCvtScale and   cvCvtColor to convert it before   saving  or use universal cvSave to   save the image to XML or YAML format    In your case you may first investigate what kind of image is captured  change capture properties  I suppose CV CAP PROP CONVERT RGB might be important  or convert it manually afterwards   This is an example how to convert to 8-bit representation with OpenCV  cc here is an original matrix of type CV 32FC1  cc8u is its scaled version which is actually written by SaveImage     I want to save cc here cc8u   CreateMat cc rows  cc cols  CV 8U  ccmin ccmax minij maxij   MinMaxLoc cc  ccscale  ccshift   255 0  ccmax-ccmin   -ccmin CvtScale cc  cc8u  ccscale  ccshift  SaveImage  cc png   cc8u     sorry  this is Python code  but it should be easy to translate it to C C

User · Answer

sorry if this is too obvious  Are you sure the webcam is properly seen and detected by OpenCV in other words  do you get an image when you redirect the captured frame to a  highGui  window  For instance like so    frame   cvQueryFrame  capture     cvNamedWindow   myWindow   CV WINDOW AUTOSIZE     cvShowImage   myWindow   frame

User · Answer

I had the same problem on Windows Vista  I just added this code before cvQueryFrame   cvSetCaptureProperty capture  CV CAP PROP FRAME WIDTH  720   cvSetCaptureProperty capture  CV CAP PROP FRAME HEIGHT  480

User · Answer

I know the problem  You just put a dot after  test jpg    cvSaveImage  test jpg    pSaveImg    I may be wrong but I think its not good

User · Answer

Sometimes the first call to cvQueryFrame   returns an empty image  Try   IplImage  pSaveImg   cvQueryFrame pCapturedImage   pSaveImg   cvQueryFrame pCapturedImage     If that does not work  try to select capture device automatically   CvCapture  pCapturedImage   cvCreateCameraCapture -1     Or you may try to select other capture devices where n 1 2 3     CvCapture  pCapturedImage   cvCreateCameraCapture n     PS  Also I believe there is a misunderstanding about captured image looking at your variable name  The variable pCapturedImage is not an Image it is a Capture  You can always  read  an image from capture

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