Compute the similarity rate between two Images with opencv/c++ - c++

Question

I'm using OpenCV/C++ to compute the similarity rate between two images. I want to tell the user how much % image A looks like image B.
Let's take a look at the code below :
double getSimilarityRate(const cv::Mat A, const cv::Mat B){
double cpt = 0.0;
cv::Mat imgGray1, imgGray2;
cv::cvtColor(A, imgGray1, CV_BGR2GRAY);
cv::cvtColor(B, imgGray2, CV_BGR2GRAY);
imgGray1 = imgGray1 > 128;
imgGray2 = imgGray2 > 128;
double total = imgGray1.cols * imgGray1.rows;
if(imgGray1.rows > 0 && imgGray1.rows == B.rows && imgGray1.cols > 0 && imgGray1.cols == B.cols){
for(int rows = 0; rows < imgGray1.rows; rows++){
for(int cols = 0; cols < imgGray1.cols; cols++){
if(imgGray1.at<int>(rows, cols) == imgGray2.at<int>(rows,cols)) cpt ++;
}
}
}else{
std::cout << "No similartity between the two images ... [EXIT]" << std::endl;
exit(0);
}
double rate = cpt / total;
return rate * 100.0;
}
int main(void)
{
/* ------------------------------------------ # ALGO GETSIMILARITY BETWEEN 2 IMAGES # -------------------------------------- */
double rate;
string fileNameImage1("C:\\Users\\hugoo\\Documents\\Prog\\NexterMU\\Qt\\OpenCV\\DetectionShapeProgram\\mire.jpg");
cv::Mat image1 = imread(fileNameImage1);
string fileNameImage2("C:\\Users\\hugoo\\Documents\\Prog\\NexterMU\\Qt\\OpenCV\\DetectionShapeProgram\\mire.jpg");
cv::Mat image2 = imread(fileNameImage2);
if(image1.empty() || image2.empty()){
std::cout << "Images couldn't be loaded" << std::endl;
exit(-1);
}
rate = getSimilarityRate(image1, image2) ;
First I convert the matrices from BGR to GRAY. So I have only one channel remaining. (Much more easier to compare).
cv::Mat imgGray1, imgGray2;
cv::cvtColor(A, imgGray1, CV_BGR2GRAY);
cv::cvtColor(B, imgGray2, CV_BGR2GRAY);
Then I make them binary (255 or 0 --> pixel's White or Black) :
imgGray1 = imgGray1 > 128;
imgGray2 = imgGray2 > 128;
In my for loops I pass through each pixel and compare him to other one in the second image.
If it matches I increase a variable (cpt ++).
I compute the rate and turn it to a %, with :
double rate = cpt / total;
return rate * 100.0;
The thing is it doesn't seem to compute correctly, because it doesn't return me the rate value in the console...
I think the problem comes from the at() function maybe I don't use it properly.

Answer 1

I suspect imgGray1.at<int>(rows, cols) should be imgGray1.at<uchar>(rows, cols) instead.
Currently .at() function call has int as a template argument, but typically cv::Mat consist of uchar elements. Are you pretty sure that your image has int elements? If it does consist of uchar elements, then using int template argument will result in accessing memory beyond what corresponds to the image (basically all pointer offsets would now be 4x as large as they should be).
More generally, if you use cv::Mat::at(), you need to use different template arguments depending on the output of cv::Mat::type():
8-bit 3-channel image (CV_8UC3) --> .at<cv::Vec3b>(row, column)
8-bit 1-channel image (CV_8UC1) --> .at<uchar>(row, column)
32-bit 3-channel image (CV_32FC3) --> .at<cv::Vec3f>(row, column)
32-bit 1-channel image (CV_32FC1) --> .at<float>(row, column)
For this reason, if a function should support arbitrary cv::Mat's, one either needs to write a bunch of if-else clauses, or to avoid .at() altogether. In your situation, since imgGray1 and imgGray2 are "binarized", I wonder if rate can be calculated using cv::norm, possibly like so:
// NORM_INF counts the number of non-equal elements.
int num_non_equal = cv::norm(imgGray1, imgGray2, NORM_INF);
double rate = 1.0 - num_non_equal / static_cast<double>(total);