OpenCV error: Image step is wrong (The matrix is not continuous)

Question

When starting my programm through command line, have such problem: OpenCV Error: Image step is wrong (The matrix is not continuous, thus its number of rows can not be changed) un cv::Mat::reshape, file C:\builds\2_4_PackSlave-win64-vc12-shared\opencv\modules\core\src\matrix.cpp, line 802.

Code of the program:

#include "opencv2/core/core.hpp"
#include "opencv2/contrib/contrib.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/objdetect/objdetect.hpp"

#include <iostream>
#include <fstream>
#include <sstream>

using namespace cv;
using namespace std;

static void read_csv(const string& filename, vector<Mat>& images, vector<int>& labels, char separator = ';') {
    std::ifstream file(filename.c_str(), ifstream::in);
    if (!file) {
        string error_message = "No valid input file was given, please check the given filename.";
        CV_Error(CV_StsBadArg, error_message);
    }
    string line, path, classlabel;
    while (getline(file, line)) {
        stringstream liness(line);
        getline(liness, path, separator);
        getline(liness, classlabel);
        if (!path.empty() && !classlabel.empty()) {
            images.push_back(imread(path, 0));
            labels.push_back(atoi(classlabel.c_str()));
        }
    }
}

int main(int argc, const char *argv[]) {
    // Check for valid command line arguments, print usage
    // if no arguments were given.
    if (argc != 4) {
        cout << "usage: " << argv[0] << " </path/to/haar_cascade> </path/to/csv.ext> </path/to/device id>" << endl;
        cout << "\t </path/to/haar_cascade> -- Path to the Haar Cascade for face detection." << endl;
        cout << "\t </path/to/csv.ext> -- Path to the CSV file with the face database." << endl;
        cout << "\t <device id> -- The webcam device id to grab frames from." << endl;
        exit(1);
    }
    // Get the path to your CSV:
    string fn_haar = string(argv[1]);
    string fn_csv = string(argv[2]);
    int deviceId = atoi(argv[3]);
    // These vectors hold the images and corresponding labels:
    vector<Mat> images;
    vector<int> labels;
    // Read in the data (fails if no valid input filename is given, but you'll get an error message):
    try {
        read_csv(fn_csv, images, labels);
    }
    catch (cv::Exception& e) {
        cerr << "Error opening file \"" << fn_csv << "\". Reason: " << e.msg << endl;
        // nothing more we can do
        exit(1);
    }
    // Get the height from the first image. We'll need this
    // later in code to reshape the images to their original
    // size AND we need to reshape incoming faces to this size:
    int im_width = images[0].cols;
    int im_height = images[0].rows;
    // Create a FaceRecognizer and train it on the given images:
    Ptr<FaceRecognizer> model = createFisherFaceRecognizer();
    model->train(images, labels);
    // That's it for learning the Face Recognition model. You now
    // need to create the classifier for the task of Face Detection.
    // We are going to use the haar cascade you have specified in the
    // command line arguments:
    //
    CascadeClassifier haar_cascade;
    haar_cascade.load(fn_haar);
    // Get a handle to the Video device:
    VideoCapture cap(deviceId);
    // Check if we can use this device at all:
    if (!cap.isOpened()) {
        cerr << "Capture Device ID " << deviceId << "cannot be opened." << endl;
        return -1;
    }
    // Holds the current frame from the Video device:
    Mat frame;
    for (;;) {
        cap >> frame;
        // Clone the current frame:
        Mat original = frame.clone();
        // Convert the current frame to grayscale:
        Mat gray;
        cvtColor(original, gray, CV_BGR2GRAY);
        // Find the faces in the frame:
        vector< Rect_<int> > faces;
        haar_cascade.detectMultiScale(gray, faces);
        // At this point you have the position of the faces in
        // faces. Now we'll get the faces, make a prediction and
        // annotate it in the video. Cool or what?
        for (int i = 0; i < faces.size(); i++) {
            // Process face by face:
            Rect face_i = faces[i];
            // Crop the face from the image. So simple with OpenCV C++:
            Mat face = gray(face_i);
            // Resizing the face is necessary for Eigenfaces and Fisherfaces. You can easily
            // verify this, by reading through the face recognition tutorial coming with OpenCV.
            // Resizing IS NOT NEEDED for Local Binary Patterns Histograms, so preparing the
            // input data really depends on the algorithm used.
            //
            // I strongly encourage you to play around with the algorithms. See which work best
            // in your scenario, LBPH should always be a contender for robust face recognition.
            //
            // Since I am showing the Fisherfaces algorithm here, I also show how to resize the
            // face you have just found:
            Mat face_resized;
            cv::resize(face, face_resized, Size(im_width, im_height), 1.0, 1.0, INTER_CUBIC);
            // Now perform the prediction, see how easy that is:
            int prediction = model->predict(face_resized);
            // And finally write all we've found out to the original image!
            // First of all draw a green rectangle around the detected face:
            rectangle(original, face_i, CV_RGB(0, 255, 0), 1);
            // Create the text we will annotate the box with:
            string box_text = format("Prediction = %d", prediction);
            // Calculate the position for annotated text (make sure we don't
            // put illegal values in there):
            int pos_x = std::max(face_i.tl().x - 10, 0);
            int pos_y = std::max(face_i.tl().y - 10, 0);
            // And now put it into the image:
            putText(original, box_text, Point(pos_x, pos_y), FONT_HERSHEY_PLAIN, 1.0, CV_RGB(0, 255, 0), 2.0);
        }
        // Show the result:
        imshow("face_recognizer", original);
        // And display it:
        char key = (char)waitKey(20);
        // Exit this loop on escape:
        if (key == 27)
            break;
    }
    return 0;
}

What I must to do?

Answer 1

I think that the problem could be in Mat face = gray(face_i).
Read the comments for clarification.

 Rect face_i = faces[i];
 // This operation makes a new header for the specified sub-array of
 // *this, thus it is a 0(1) operation, that is, no matrix data is 
 // copied. So matrix elements are no longer stored continuously without
 // gaps at the end of each row.
 Mat face = gray(face_i);
 ...
 Mat face_resized;

 // Here new memory for face_resized should be allocated, but I'm not sure. 
 // If not then it is the reason of the error, because in this case 
 // face_resized will contain not continuous data (=face).
 cv::resize(face, face_resized, Size(im_width, im_height), 1.0, 1.0, INTER_CUBIC);
 ...
 // here reshape(face_resized) will be called and will throw the error if 
 // matrix is not continuous
 int prediction = model->predict(face_resized);

Answer 2

the FisherFaceRecognizer (Eigen, too) tries to 'flatten' the images to a single row (reshape()) for training and testing.

this does not work, if the Mat is non-continuous (because it's either padded or a submat/roi only).

( then again, 'fileNotFound' counts as 'non-continuous', too ;] )

if your images are e.g. .bmp , there's a high chance, that some image-editor padded your images, so the row-size is a factor of 4.

maybe you can get away with batch converting your imgs to .png or .pgm externally

else resizing your train images after loading them will help (anything, that makes a copy of it)

or, change this line in the loading code:

images.push_back(imread(path, 0));

to:

Mat m = imread(path, 1);
Mat m2;
cvtColor(m,m2,CV_BGR_GRAY);
images.push_back(m2);

Answer 3

It's a slash/anti-slash problem in your csv file. For example mine was like this:

sujets\s1/1.pgm;0
sujets\s1/10.pgm;0
...
sujets\s1/9.pgm;0
sujets\s2/1.pgm;1
sujets\s2/10.pgm;1
...
sujets\s2/9.pgm;1
sujets\s3/1.pgm;2
sujets\s3/10.pgm;2
...
sujets\s3/9.pgm;2
sujets\s4/1.pgm;3
sujets\s4/10.pgm;3
...
sujets\s4/9.pgm;3

Changing for this:

sujets/s1/1.pgm;0
sujets/s1/10.pgm;0
...
sujets/s1/9.pgm;0
sujets/s2/1.pgm;1
sujets/s2/10.pgm;1
...
sujets/s2/9.pgm;1
sujets/s3/1.pgm;2
sujets/s3/10.pgm;2
...
sujets/s3/9.pgm;2
sujets/s4/1.pgm;3
sujets/s4/10.pgm;3
...
sujets/s4/9.pgm;3

did the tricks