c++ - countNonZero function gives an assertion error in openCV -

i tried horizontal projection using countnonzero() function below.

mat src = imread(input_file, cv_load_image_color); mat binaryimage = src.clone(); cvtcolor(src, src, cv_bgr2gray);  mat horizontal = mat::zeros(1,binaryimage.cols, cv_8uc1);  (int = 0; i<binaryimage.cols; i++) {     mat roi = binaryimage(rect(0, 0, 1, binaryimage.rows));      horizontal.at<int>(0,i) = countnonzero(roi);     cout << "col no:" << << " >>" << horizontal.at<int>(0, i); } 

but error occured in line of calling countonzero() function. error follows.

    opencv error: assertion failed (src.channels() == 1 && func != 0) in cv::countno     nzero, file c:\builds\2_4_packslave-win32-vc12-shared\opencv\modules\core\src\st     at.cpp, line 549 

can please point out mistake?

assertion src.channels() == 1 means image should have 1 channel, i.e. has gray, not colored. calling countnonzero on roi, subimage of binaryimage, clone of src, colored.

i suppose wanted write cvtcolor(binaryimage, binaryimage, cv_bgr2gray);. in case makes sense. however, not see using src anywhere again, perhaps not need intermediate image. in case do, not call "binary", since "binary" in computer vision stands black-or-white image, 2 colors. image "gray", since has shades of black , white.

concerning original task, miki right, should use cv::reduce it. gave example on how use it.