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opencv3/C++ PHash算法图像检索详解

2019-12-12 12:10 197 查看

PHash算法即感知哈希算法/Perceptual Hash algorithm，计算基于低频的均值哈希．对每张图像生成一个指纹字符串，通过对该字符串比较可以判断图像间的相似度．

PHash算法原理

将图像转为灰度图，然后将图片大小调整为32*32像素并通过DCT变换，取左上角的8*8像素区域。然后计算这64个像素的灰度值的均值。将每个像素的灰度值与均值对比，大于均值记为1，小于均值记为0，得到64位哈希值。

PHash算法实现

将图片转为灰度值

将图片尺寸缩小为32*32

resize(src, src, Size(32, 32));

DCT变换

Mat srcDCT;
dct(src, srcDCT);

计算DCT左上角8*8像素区域均值，求hash值

double sum = 0;
for (int i = 0; i < 8; i++)
for (int j = 0; j < 8; j++)
sum += srcDCT.at<float>(i,j);

double average = sum/64;
Mat phashcode= Mat::zeros(Size(8, 8), CV_8U);
for (int i = 0; i < 8; i++)
for (int j = 0; j < 8; j++)
phashcode.at<char>(i,j) = srcDCT.at<float>(i,j) > average ? 1:0;

hash值匹配

int d = 0;
for (int n = 0; n < srchash.size[1]; n++)
if (srchash.at<uchar>(0,n) != dsthash.at<uchar>(0,n)) d++;

即，计算两幅图哈希值之间的汉明距离，汉明距离越大，两图片越不相似。

OpenCV实现

如图在下图中对比各个图像与图person.jpg的汉明距离，以此衡量两图之间的额相似度。

#include <iostream>
#include <stdio.h>
#include <fstream>
#include <io.h>
#include <string>
#include <opencv2\opencv.hpp>
#include <opencv2\core\core.hpp>
#include <opencv2\core\mat.hpp>
using namespace std;
using namespace cv;
int fingerprint(Mat src, Mat* hash);

int main()
{
Mat src = imread("E:\\image\\image\\image\\person.jpg", 0);
if(src.empty())
{
cout << "the image is not exist" << endl;
return -1;
}
Mat srchash, dsthash;
fingerprint(src, &srchash);
for(int i = 1; i <= 8; i++)
{
string path0 = "E:\\image\\image\\image\\person";
string number;
stringstream ss;
ss << i;
ss >> number;
string path = "E:\\image\\image\\image\\person" + number +".jpg";
Mat dst = imread(path, 0);
if(dst.empty())
{
cout << "the image is not exist" << endl;
return -1;
}
fingerprint(dst, &dsthash);
int d = 0;
for (int n = 0; n < srchash.size[1]; n++)
if (srchash.at<uchar>(0,n) != dsthash.at<uchar>(0,n)) d++;

cout <<"person" << i <<" distance= " <<d<<"\n";
}

system("pause");
return 0;
}

int fingerprint(Mat src, Mat* hash)
{resize(src, src, Size(32, 32));
src.convertTo(src, CV_32F);Mat srcDCT;
dct(src, srcDCT);
srcDCT = abs(srcDCT);double sum = 0;
for (int i = 0; i < 8; i++)
for (int j = 0; j < 8; j++)
sum += srcDCT.at<float>(i,j);

double average = sum/64;
Mat phashcode= Mat::zeros(Size(8, 8), CV_8U);
for (int i = 0; i < 8; i++)
for (int j = 0; j < 8; j++)
phashcode.at<char>(i,j) = srcDCT.at<float>(i,j) > average ? 1:0;
*hash = phashcode.reshape(0,1).clone();
return 0;
}

输出汉明距离：

可以看出若将阈值设置为20则可将后三张其他图片筛选掉。

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标签：  opencv3 C++ PHash 图像检索
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