Sum of each column opencv

Question

In Matlab, If A is a matrix, sum(A) treats the columns of A as vectors, returning a row vector of the sums of each column.

sum(Image); How could it be done with OpenCV?

Answer 1

Using cvReduce has worked for me. For example, if you need to store the column-wise sum of a matrix as a row matrix you could do this:

CvMat * MyMat = cvCreateMat(height, width, CV_64FC1);
// Fill in MyMat with some data...

CvMat * ColSum = cvCreateMat(1, MyMat->width, CV_64FC1);
cvReduce(MyMat, ColSum, 0, CV_REDUCE_SUM);

More information is available in the OpenCV documentation.

Answer 2

EDIT after 3 years:

The proper function for this is cv::reduce.

Reduces a matrix to a vector.

The function reduce reduces the matrix to a vector by treating the matrix rows/columns as a set of 1D vectors and performing the specified operation on the vectors until a single row/column is obtained. For example, the function can be used to compute horizontal and vertical projections of a raster image. In case of REDUCE_MAX and REDUCE_MIN , the output image should have the same type as the source one. In case of REDUCE_SUM and REDUCE_AVG , the output may have a larger element bit-depth to preserve accuracy. And multi-channel arrays are also supported in these two reduction modes.

OLD: I've used ROI method: move ROI of height of the image and width 1 from left to right and calculate means.

 Mat src = imread(filename, 0);     
 vector<int> graph( src.cols );
 for (int c=0; c<src.cols-1; c++)
 {
     Mat roi = src( Rect( c,0,1,src.rows ) );
     graph[c] = int(mean(roi)[0]);
 }

 Mat mgraph(  260, src.cols+10, CV_8UC3); 
 for (int c=0; c<src.cols-1; c++)
 {
     line( mgraph, Point(c+5,0), Point(c+5,graph[c]), Scalar(255,0,0), 1, CV_AA);    
 }

 imshow("mgraph", mgraph);
 imshow("source", src);

EDIT: Just out of curiosity, I've tried resize to height 1 and the result was almost the same:

 Mat test;
 cv::resize(src,test,Size( src.cols,1 ));
 Mat mgraph1(  260, src.cols+10, CV_8UC3); 
 for(int c=0; c<test.cols; c++) 
 {
        graph[c] = test.at<uchar>(0,c);
 }
 for (int c=0; c<src.cols-1; c++)
 {
     line( mgraph1, Point(c+5,0), Point(c+5,graph[c]), Scalar(255,255,0), 1, CV_AA);     
 }
 imshow("mgraph1", mgraph1);