countNonZero function gives an assertion error in openCV

Question

I tried to get horizontal projection using countNonZero() function as below.

Mat src = imread(INPUT_FILE, CV_LOAD_IMAGE_COLOR);
Mat binaryImage = src.clone()         


        

Answer 1

BTW, you can compute horizontal projection using reduce giving as argument CV_REDUCE_SUM.

A minimal example:

Mat1b mat(4, 4, uchar(0));
mat(0,0) = uchar(1);
mat(0,1) = uchar(1);
mat(1,1) = uchar(1);

// mat is: 
//
// 1100
// 0100
// 0000
// 0000

// Horizontal projection, result would be a column matrix
Mat1i reducedHor;
cv::reduce(mat, reducedHor, 1, CV_REDUCE_SUM);

// reducedHor is:
//
// 2
// 1
// 0
// 0

// Vertical projection, result would be a row matrix
Mat1i reducedVer;
cv::reduce(mat, reducedVer, 0, CV_REDUCE_SUM);

// reducedVer is:
//
// 1200


// Summary
//
// 1100 > 2
// 0100 > 1
// 0000 > 0
// 0000 > 0
// 
// vvvv
// 1200

You can use this with your images like this:

// RGB image
Mat3b img = imread("path_to_image");

// Gray image, contains values in [0,255]
Mat1b gray;
cvtColor(img, gray, CV_BGR2GRAY);

// Binary image, contains only 0,1 values
// The sum of pixel values will equal the count of non-zero pixels
Mat1b binary;
threshold(gray, binary, 1, 1, THRESH_BINARY);

// Horizontal projection
Mat1i reducedHor;
cv::reduce(binary, reducedHor, 1, CV_REDUCE_SUM);

// Vertical projection
Mat1i reducedVer;
cv::reduce(binary, reducedVer, 0, CV_REDUCE_SUM);