How to identify two similar images?

We can use Opencv to find the similar images,below is a simple java code

package com.company;

import org.opencv.core.*;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;
import org.opencv.objdetect.CascadeClassifier;

import java.util.Arrays;

public class FaceCompareMain {

static CascadeClassifier faceDetector;

static {
System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
faceDetector = new CascadeClassifier(
"D:\\ib\\face-detact\\src\\com\\company\\haarcascade_frontalface_alt.xml");
}

public static void main(String[] args) {
String basePicPath = "D:\\ib\\face-detact\\src\\pics\\";
double compareHist = compare_image(basePicPath + "image_1.png", basePicPath + "image_2.png");
System.out.println(compareHist);
if (compareHist > 0.72) {
System.out.println("matched!");
} else {
System.out.println("not match!");
}
}

public static double compare_image(String img_1, String img_2) {
Mat mat_1 = conv_Mat(img_1);
Mat mat_2 = conv_Mat(img_2);

Mat hist_1 = new Mat();
Mat hist_2 = new Mat();

MatOfFloat ranges = new MatOfFloat(0f, 256f);

MatOfInt histSize = new MatOfInt(1000);

Imgproc.calcHist(Arrays.asList(mat_1), new MatOfInt(0), new Mat(), hist_1, histSize, ranges);
Imgproc.calcHist(Arrays.asList(mat_2), new MatOfInt(0), new Mat(), hist_2, histSize, ranges);

double res = Imgproc.compareHist(hist_1, hist_2, Imgproc.CV_COMP_CORREL);
return res;
}

// "D:\\ib\\face-detact\\src\\com\\company\\a1.jpg"
private static Mat conv_Mat(String img_1) {
Mat image0 = Imgcodecs.imread(img_1);

Mat image = new Mat();
Imgproc.cvtColor(image0, image, Imgproc.COLOR_BGR2GRAY);

MatOfRect faceDetections = new MatOfRect();
faceDetector.detectMultiScale(image, faceDetections);

for (Rect rect : faceDetections.toArray()) {
Mat mat = new Mat(image, rect);
return mat;
}
return null;
}

}
The sample similar images：
image1:https://image.emojipng.com/5/5.png
image2:https://image.emojipng.com/247/502247.png