c++ - opencv: single pixal assignment confusion -

i working opencv , c++ project , found weird thing: when try access single pixal value in iplimage , assign other value, run result can operate part of whole image.

relevant code:

iplimage* output_frame = cvcreateimage(size, ipl_depth_8u, 1); iplimage* current_frame = cvcreateimage(size, ipl_depth_8u, 1); while((current_frame = cvqueryframe(video_gray)) != 0 ) {   (int row=0;row<height;row++)   {          uchar* ptr_current_frame = (uchar *)(current_frame->imagedata+current_frame->widthstep*row);         uchar* ptr_output_frame = (uchar *)(output_frame->imagedata+output_frame->widthstep*row);         (int cols=0;cols<width;cols++)         {             //other codes...             ptr_output_frame[cols]=ptr_current_frame[cols];         }   } } 

the result left part of image copied output_frame. , when run following code:

iplimage* output_frame = cvcreateimage(size, ipl_depth_8u, 1); iplimage* current_frame = cvcreateimage(size, ipl_depth_8u, 1); while((current_frame = cvqueryframe(video_gray)) != 0 ) {   (int row=0;row<height;row++)   {          uchar* ptr_current_frame = (uchar *)current_frame->imagedata+current_frame->width*row;         uchar* ptr_output_frame = (uchar *)output_frame->imagedata+output_frame->width*row;         (int cols=0;cols<width;cols++)         {             //other codes...             ptr_output_frame[cols]=ptr_current_frame[cols];         }   } } 

i got upside part of image in output_frame.

i cannot the whole image copy in output_frame either way. me this? lot!

[updates] 05/16/2015

i have found out output_frame->widthstep different current->widthstep after current_frame executed in following code:

 current_frame = cvqueryframe(video_gray); 

it makes sense why first part of code wouldnt work. still dont know why second part of code doesnt work. have whole codes updated here , hope can me make right. appreciate help.

cvcapture* video_gray = cvcreatefilecapture("test_gray.avi"); const double fps = cvgetcaptureproperty(video_gray, cv_cap_prop_fps);  const int width = (int)cvgetcaptureproperty(video_gray, cv_cap_prop_frame_width); const int height = (int)cvgetcaptureproperty(video_gray, cv_cap_prop_frame_height); const cvsize size = cvsize(width, height); iplimage* current_frame = cvcreateimage(size, ipl_depth_8u, 1); iplimage* output_frame=cvcreateimage(size, ipl_depth_8u, 1); int flag = 0; cvnamedwindow("test",1); cvnamedwindow("test2",1); while((current_frame = cvqueryframe(video_gray)) != 0 ) {     cout<<flag++<<endl;     if(flag<500) continue;     (int row=0;row<height;row++)     {         uchar* ptr_current_frame = (uchar *)(current_frame->imagedata+current_frame->widthstep*row);         uchar* ptr_output_frame = (uchar *)(output_frame->imagedata+output_frame->widthstep*row);         (int cols=0;cols<width;cols++)         {             ptr_output_frame[cols]= ptr_current_frame[cols];         }     }     cvshowimage("test",output_frame);     cvshowimage("test2",current_frame);     cvwaitkey(10); } 

you don't handle number of channels...

please try

iplimage* output_frame = cvcreateimage(size, ipl_depth_8u, 1); iplimage* current_frame = cvcreateimage(size, ipl_depth_8u, 1); while((current_frame = cvqueryframe(video_gray)) != 0 ) {   (int row=0;row<height;row++)   {      uchar* ptr_current_frame = (uchar *)(current_frame->imagedata+current_frame->widthstep*row);     uchar* ptr_output_frame = (uchar *)(output_frame->imagedata+output_frame->widthstep*row);     (int cols=0;cols<width;cols++)     {         //other codes...         // here insead take care of channels too. write ne channel output         ptr_output_frame[cols]=ptr_current_frame[cols*current_frame->nchannels];     }   } } 

but should try switch c++ api: try this:

cv::videocapture video = cv::videocapture("test_gray.avi");  int width = ... int height = ...  cv::mat image_captured; cv::mat image_gray; // if wanted cv::mat image_output1 = cv::mat(height, width, cv_8uc1); cv::mat image_output2 = cv::mat(height, width, cv_8uc1);  while(video.read(image_captured)) {     // if want convert image      if(image_captured.channels() > 1)     {         cv::cvtcolor(image_captured, image_gray, cv_bgr2gray);     }     else     {         image_gray = image_captured;     }      [...]     for(int j=0; j<height; ++j)     {         for(int i=0; i<width; ++i)         {             image_output1.at<uchar>(j,i) = image_gray.at<uchar>(j,i); // read single channel image             image_output1.at<uchar>(j,i) = image_captured.at<cv::vec3b>(j,i)[0]; // read first channel of multi-channel image         }     }          }      cv::imshow("output1", image_output1);     cv::imshow("output2", image_output2);     cv::imshow("input", image_capured);      cv::waitkey(0); } 

much easier use, , can still make more efficient using row-pointers in each iteration etc