how to check whether two matrices are identical in OpenCV

Question

I have two instances of cv::Mat : m1 and m2. They are of the same numeric type and sizes. Is there any function in OpenCV that returns whether the matrices are identical (ha

Answer 1

I use this:

bool areEqual(const cv::Mat& a, const cv::Mat& b) {
    cv::Mat temp;
    cv::bitwise_xor(a,b,temp); //It vectorizes well with SSE/NEON
    return !(cv::countNonZero(temp) );
}

If you have to do this operation many times, you can make this into a class, have temp as member and prevent the image to be allocated every time. Detail: Make temp mutable so that areEqual can be a const method.

Note though that cv::countNonZero only works with cv::Mat of one channel. It's overkill, but in that case one could use cv::split to split each channel into separate images and do cv::countNonZero on them.

Answer 2

As mentioned by Acme, you can use cv::compare although it is not as clean as you might hope.
In the following example, cv::compare is called by using the != operator:

// Get a matrix with non-zero values at points where the 
// two matrices have different values
cv::Mat diff = a != b;
// Equal if no elements disagree
bool eq = cv::countNonZero(diff) == 0;

Presumably it would be quicker to just iterate through comparing the elements though? If you know the type you could use the STL equal function:

bool eq = std::equal(a.begin<uchar>(), a.end<uchar>(), b.begin<uchar>());

Answer 3

For multichannel images, you could use cv::Mat::reshape to create a single channel image without any extra overhead.

An update of Antonio's answer would be

bool areEqual(const cv::Mat& a, const cv::Mat& b)
{
    cv::Mat temp;
    cv::bitwise_xor(a,b,temp);
    return !(cv::countNonZero(temp.reshape(1)));
}

Answer 4

Use cv::compare combined with cv::countNonZero.

An SO question that might help you further OpenCV compare two images and get different pixels

Answer 5

As mentioned by Acme and Tim, you can use cv::compare. This is the code I use to compare my cv::Mat:

 bool matIsEqual(const cv::Mat mat1, const cv::Mat mat2){
    // treat two empty mat as identical as well
    if (mat1.empty() && mat2.empty()) {
        return true;
    }
    // if dimensionality of two mat is not identical, these two mat is not identical
    if (mat1.cols != mat2.cols || mat1.rows != mat2.rows || mat1.dims != mat2.dims) {
        return false;
    }
    cv::Mat diff;
    cv::compare(mat1, mat2, diff, cv::CMP_NE);
    int nz = cv::countNonZero(diff);
    return nz==0;
}

It is important to stand out that the function cv::countNonZero only works with cv::Mat of one channel, so if you need to compare two cv::Mat images, you need first to convert your cv::Mat in this way:

Mat gray1, gray2;
cvtColor(InputMat1, gray1, CV_BGR2GRAY);
cvtColor(InputMat2, gray2, CV_BGR2GRAY);

where InputMat1 and InputMat2 are the cv::Mat you want to compare. After that you can call the function:

bool equal = matsEqual(gray1, gray2);

I took this code of this site: OpenCV: compare whether two Mat is identical

I hope this help you.

Answer 6

The following will work also for multi-channel matrices:

bool isEqual = (sum(img1 != img2) == Scalar(0,0,0,0));

Since sum accepts matrices with 1 to 4 channels, and returns a Scalar, where the element at [0] is the result of the sum for first channel, and so on.